100 questions
Which part of a plant cell captures light energy for photosynthesis?
- a.Cell wall
- b.Ribosome
- c.Nucleus
- d.Chloroplast✓
Chloroplasts contain chlorophyll, the pigment that absorbs sunlight to power photosynthesis. The nucleus stores genetic material, and ribosomes build proteins. Photosynthesis converts light energy into chemical energy stored in sugars.
What is the chemical formula for water?
- a.H2O✓
- b.O2
- c.CO2
- d.NaCl
Water is made of two hydrogen atoms and one oxygen atom, giving the formula H2O. CO2 is carbon dioxide and NaCl is table salt. Chemical formulas show the types and numbers of atoms in a molecule.
In a food chain, organisms that make their own food using sunlight are called—
- a.decomposers
- b.producers✓
- c.predators
- d.consumers
Producers, such as green plants, make their own food through photosynthesis and form the base of a food chain. Consumers eat other organisms, and decomposers break down dead matter. Energy flows from producers to consumers.
Which gas do humans need to breathe in to survive?
- a.Nitrogen
- b.Carbon dioxide
- c.Hydrogen
- d.Oxygen✓
Humans inhale oxygen, which cells use to release energy from food through respiration. We exhale carbon dioxide as a waste product. Although nitrogen is the most abundant gas in air, the body does not use it for respiration.
A scientist records that a substance boils at 100°C at sea level. Boiling point is an example of a—
- a.chemical formula
- b.chemical change
- c.living factor
- d.physical property✓
Boiling point is a physical property because it can be measured without changing the substance's identity. A chemical change would produce a new substance. Physical properties include color, density, and melting point.
Which of the following best describes the function of the heart?
- a.It produces digestive enzymes.
- b.It filters waste from the blood.
- c.It pumps blood around the body.✓
- d.It controls breathing rate.
The heart is a muscular pump, and circulating blood is the whole of its job: it moves oxygen and nutrients out to the tissues and carries waste away. Filtering waste from the blood is the kidneys' work. Producing digestive enzymes is done by the pancreas and the glands of the gut. And breathing rate is set by the brainstem, not by the heart, although the two adjust together during exercise.
What force pulls objects toward the center of the Earth?
- a.Gravity✓
- b.Friction
- c.Electricity
- d.Magnetism
Gravity is the force that attracts objects toward Earth's center, giving them weight. Friction resists motion between surfaces, and magnetism acts on certain metals. Gravity keeps us and objects on the ground.
A trait passed from parents to offspring through genes is called—
- a.an instinct
- b.a learned behavior
- c.an inherited trait✓
- d.an adaptation
Inherited traits, such as eye color, are passed genetically from parents to offspring. Learned behaviors are acquired through experience, not genes. Genes carry the instructions that determine inherited characteristics.
During the water cycle, what process turns liquid water into water vapor?
- a.Precipitation
- b.Condensation
- c.Evaporation✓
- d.Collection
Evaporation is the change of liquid water into vapor, usually driven by heat from the sun. Condensation is the reverse, forming clouds, and precipitation returns water as rain or snow. Evaporation moves water into the atmosphere.
Which of these is a renewable energy source?
- a.Natural gas
- b.Solar power✓
- c.Petroleum
- d.Coal
Solar power comes from sunlight, which is replenished continuously, making it renewable. Coal, natural gas, and petroleum are fossil fuels that take millions of years to form and can be used up. Renewable sources do not run out on a human timescale.
The smallest unit of life capable of carrying out all life processes is the—
- a.atom
- b.cell✓
- c.molecule
- d.organ
The cell is the basic structural and functional unit of all living things. Atoms and molecules are smaller but are not themselves alive. Organs are made of many cells working together.
A study finds that a group of plants given fertilizer grew taller than plants without it. In this experiment, fertilizer is the—
- a.conclusion
- b.control group
- c.independent variable✓
- d.dependent variable
The independent variable is the factor the scientist deliberately changes; here it is the fertilizer. The plant height that responds is the dependent variable. Well-designed experiments change only one variable at a time.
Which planet is closest to the Sun?
- a.Jupiter
- b.Venus
- c.Earth
- d.Mercury✓
Mercury is the innermost planet, orbiting nearer the Sun than any other. Venus is the second planet out and Earth the third, which is why both are common wrong picks. Jupiter is the fifth planet and far beyond all of them. Distance from the Sun drives a planet's temperature and the length of its year.
When wood burns, it turns to ash and gas. This is an example of a—
- a.chemical change✓
- b.reversible change
- c.physical change
- d.change of state
Burning is a chemical change because new substances, such as ash and gases, form and cannot easily be turned back into wood. Physical changes, like melting, do not create new substances. Chemical changes rearrange atoms into different compounds.
What do we call the variety of living organisms in an ecosystem?
- a.Community
- b.Climate
- c.Population
- d.Biodiversity✓
Biodiversity is the range of different species living in an area. A community is all the populations living together in that area — a related idea, but it names the assemblage rather than its variety. A population is a single species, so it cannot describe variety at all. And climate describes long-term weather patterns, not living things. High biodiversity tends to make an ecosystem more stable.
The process by which a species changes over many generations in response to its environment is called—
- a.condensation
- b.circulation
- c.digestion
- d.evolution✓
Evolution is the gradual change in inherited traits of a population over generations, often through natural selection. It explains how species adapt to their environments over long periods. Individuals do not evolve; populations do.
Which type of blood vessel carries blood away from the heart?
- a.Capillaries
- b.Nerves
- c.Veins
- d.Arteries✓
An artery is defined by direction: it carries blood away from the heart. Note that this is about direction and not oxygen — the pulmonary arteries carry oxygen-poor blood from the heart to the lungs, which is the exception that shows what the definition really rests on. Veins run the other way, returning blood to the heart. Capillaries are the fine vessels between the two where gases and nutrients are exchanged with the tissues. Nerves carry electrical signals and are not blood vessels at all.
A graph shows a town's average temperature rising steadily each decade. This trend best supports the idea of—
- a.climate change✓
- b.seasonal change
- c.daily weather
- d.an eclipse
A steady rise held across decades is a climate signal, because climate is precisely the long-run pattern rather than any single reading. Daily weather is the wrong timescale — it swings far more than this in a single afternoon. Seasonal change is also the wrong timescale, and it repeats each year instead of trending in one direction. And an eclipse is a brief astronomical event with no bearing on a decades-long temperature record.
Which of the following is a chemical property of a substance?
- a.Its density
- b.Its ability to rust✓
- c.Its hardness
- d.Its boiling point
Rusting is iron reacting with oxygen to form a new substance, so the ability to rust describes how the material behaves chemically. Density, hardness and boiling point can all be measured without turning the substance into anything else, which is exactly what makes them physical properties. The dividing line is whether observing the property destroys the original substance: weigh it, scratch it or boil it and you still have the same material, but rust it and you do not.
In the human body, which system is responsible for breaking down food into nutrients?
- a.Respiratory system
- b.Nervous system
- c.Digestive system✓
- d.Skeletal system
The digestive system breaks food down into nutrients the body can absorb and use. The respiratory system handles gas exchange, and the nervous system sends signals. Digestion begins in the mouth and continues through the stomach and intestines.
Which of these best explains why ice floats on water?
- a.Ice is less dense than water.✓
- b.Ice contains more salt than water.
- c.Ice has more mass than water.
- d.Ice is warmer than water.
Water is unusual: as it freezes its molecules lock into an open lattice that takes up more room than the liquid, so a given mass of ice occupies more volume and is therefore less dense. Anything less dense than the fluid around it floats. Ice does not contain more salt than the water it forms from — freezing tends to exclude salt. Having more mass is irrelevant without knowing volume, since density is mass per unit volume. And ice is colder than liquid water, not warmer.
Bacteria that break down dead plants and animals are known as—
- a.producers
- b.herbivores
- c.decomposers✓
- d.predators
Decomposers, including many bacteria and fungi, break down dead organisms and return nutrients to the soil. This recycling supports new plant growth. Without decomposers, dead matter and nutrients would accumulate unused.
A meteorologist predicts rain based on falling air pressure. Air pressure is measured with a—
- a.thermometer
- b.barometer✓
- c.telescope
- d.anemometer
A barometer measures air pressure, which helps forecast weather changes. A thermometer measures temperature and an anemometer measures wind speed. Falling air pressure often signals approaching storms.
Which statement correctly describes an atom?
- a.It is the smallest unit of a compound.
- b.It is a particle found only in metals.
- c.It is the smallest living thing.
- d.It is the smallest unit of an element.✓
An atom is the smallest particle of an element that still has that element's properties. The smallest unit of a compound is a molecule, not an atom, which is the near-miss worth learning. Atoms are not confined to metals — every element, including the gases you are breathing, is made of them. And the smallest living thing is a cell, which is built from enormous numbers of atoms and is a different order of thing entirely.
The energy stored in food is a form of—
- a.chemical energy✓
- b.light energy
- c.sound energy
- d.kinetic energy
Food stores chemical energy in the bonds of its molecules, which the body releases during digestion and respiration. Kinetic energy is energy of motion. Cells convert this chemical energy into usable forms to power life processes.
Which action would most reduce the amount of waste sent to landfills?
- a.Sending yard waste to the landfill
- b.Recycling paper, glass, and plastic✓
- c.Buying products with extra packaging
- d.Using more disposable items
Recycling diverts paper, glass and plastic out of the waste stream and back into manufacturing, so the material never reaches the landfill at all. Sending yard waste there does the opposite: it is organic matter that could be composted, and it adds directly to landfill volume. Buying products with extra packaging increases waste before the item is even used. And using more disposable items replaces things that would have been reused, which raises the volume thrown away.
A scientist repeats an experiment several times and gets similar results. This repetition mainly helps to—
- a.change the independent variable
- b.make the results more reliable✓
- c.prove the hypothesis is a law
- d.eliminate the need for a control
Repeating an experiment and getting consistent results makes the findings more reliable and trustworthy. Repetition helps rule out chance errors. Reliable, reproducible data is a cornerstone of the scientific method.
Which cell structure releases the energy stored in food molecules?
- a.The cell membrane
- b.The vacuole
- c.The mitochondrion✓
- d.The ribosome
Mitochondria carry out cellular respiration, breaking down sugars in the presence of oxygen and capturing the released energy in a form the cell can spend. The cell membrane controls what crosses into and out of the cell. A vacuole is a storage compartment, largest in plant cells. And ribosomes assemble proteins, a job that consumes energy rather than supplying it.
The instructions a cell uses to build its proteins are carried by—
- a.glucose
- b.DNA✓
- c.cholesterol
- d.hemoglobin
DNA stores the sequence information that specifies each protein, which is why it is copied before a cell divides and passed to both daughter cells. Glucose is a fuel molecule and carries no information. Hemoglobin is itself a protein, one of the products rather than the instructions. And cholesterol is a lipid that helps form membranes.
How do the cells produced by meiosis differ from those produced by mitosis?
- a.They contain no genetic material at all
- b.They carry twice the number of chromosomes
- c.They are unable to divide ever again
- d.They carry half the number of chromosomes✓
Meiosis halves the chromosome number to make sex cells, so that fertilization restores the full set rather than doubling it each generation. Doubling is exactly what meiosis prevents. The cells are not dead ends, since a fertilized egg divides many times. And they certainly contain genetic material; half a set is still a set.
An allele is described as dominant when it—
- a.shows in the trait even with one copy present✓
- b.appears only when two copies are present
- c.is found more often in a population
- d.changes into a different allele over time
A dominant allele expresses itself in the observable trait whenever it is present, so a single copy is enough to mask a recessive partner. Needing two copies is the definition of recessive, which is the contrast being drawn. Dominance says nothing about how common an allele is; rare alleles can be dominant. And alleles do not convert into one another, though mutation can create new ones.
Two parents are each heterozygous for a trait, carrying one dominant and one recessive allele. What fraction of their offspring is expected to show the recessive trait?
- a.One in four✓
- b.One in two
- c.None at all
- d.Three in four
A Punnett square for this cross gives one homozygous dominant, two heterozygous and one homozygous recessive, so a quarter of the offspring are expected to show the recessive form. One in two would be the result of crossing a heterozygote with a homozygous recessive parent. Three in four is the fraction expected to show the dominant trait. And a recessive trait can certainly appear, since both parents carry the allele without showing it.
The difference between an organism's genotype and its phenotype is that the phenotype is—
- a.the trait that can actually be observed✓
- b.the number of chromosomes in its cells
- c.the species to which it belongs
- d.the set of alleles it happens to carry
Phenotype names what shows: the height, the color, the enzyme activity you can measure. The alleles themselves are the genotype, the underlying combination that the phenotype partly reflects. Chromosome number is a feature of the karyotype rather than either term. And species membership is a classification, not an individual's expressed trait.
Natural selection acts on a population when—
- a.the environment stays exactly the same across generations
- b.organisms decide which traits to pass on
- c.every individual remakes itself to suit the habitat it lives in
- d.individuals with certain traits leave more offspring✓
Selection works through differential reproduction: heritable traits that help their bearers survive and breed become more common in later generations. Individuals do not remake themselves to fit their surroundings; the variation is already present and is sorted. A wholly unchanging environment gives selection less to act on rather than more. And no organism chooses what it transmits.
The similar bone arrangement in a human arm, a whale flipper and a bat wing is evidence that these animals—
- a.use their limbs for the same task
- b.share a common ancestor✓
- c.belong to the same living species
- d.live in the same kind of habitat
The same underlying skeletal plan put to very different uses is what biologists call a homologous structure, and shared ancestry is the explanation for the shared plan. The three animals live in quite different habitats, which is part of the point. Their limbs do entirely different jobs, so function cannot explain the resemblance. And they are unmistakably separate species.
A bacterial population survives repeated treatment with the same antibiotic and becomes harder to kill. This is best explained as—
- a.the antibiotic itself growing weaker with repeated use
- b.selection of resistant individuals already present✓
- c.bacteria gradually learning how to avoid the medicine
- d.patients becoming immune to the drug
Chance variation means a few bacteria can survive the drug, and when the rest are killed those few multiply, so resistance spreads through the population. Bacteria have no capacity to learn anything. The chemical itself does not weaken; the population changes. And it is the bacteria, not the patient, that become resistant, which is why the distinction matters clinically.
The role of an enzyme in a cell is to—
- a.carry oxygen in the blood
- b.speed up a chemical reaction✓
- c.store genetic information
- d.form the wall of the cell
Enzymes are catalysts: they lower the energy needed for a specific reaction so it proceeds fast enough to sustain life, and they emerge unchanged to do it again. Genetic information is stored in DNA. Oxygen transport is the work of hemoglobin. And cell walls are built from carbohydrates such as cellulose in plants.
Sweating when body temperature rises is an example of—
- a.metabolism
- b.photosynthesis
- c.homeostasis✓
- d.reproduction
Homeostasis is the maintenance of steady internal conditions, and evaporating water off the skin is one of the body's ways of pushing temperature back toward its set point. Metabolism is the sum of chemical reactions in the body, which generates the heat rather than regulating it. Reproduction produces new organisms. And photosynthesis occurs in plants and some microbes, not in people.
In the human lungs, oxygen passes into the blood across the walls of—
- a.the diaphragm
- b.the alveoli✓
- c.the trachea
- d.the bronchi
The alveoli are the tiny sacs at the ends of the airways, thin-walled and richly supplied with capillaries, and gas exchange happens there. The trachea and the bronchi are conducting tubes that carry air toward them without exchanging much of anything. And the diaphragm is the muscle whose contraction draws air in.
Most absorption of nutrients from digested food takes place in—
- a.the large intestine
- b.the esophagus
- c.the small intestine✓
- d.the stomach
The small intestine is lined with folds and tiny projections that give it an enormous surface area, and it is where the products of digestion cross into the blood. The stomach mainly stores food and begins protein digestion with acid and enzymes. The esophagus only transports food downward. And the large intestine chiefly reclaims water and salts from what is left.
Signals travel through the human nervous system along—
- a.tendons
- b.alveoli
- c.neurons✓
- d.platelets
A neuron is the cell built for the job, receiving a signal at one end and passing it along its fiber to the next cell. Tendons are connective tissue joining muscle to bone. Platelets are blood cell fragments involved in clotting. And alveoli are air sacs in the lungs.
A vaccine protects against a disease by—
- a.prompting the immune system to prepare defenses✓
- b.blocking the germ from entering the body
- c.killing the germ once the first symptoms have appeared
- d.replacing the gene that causes the illness in a patient
A vaccine presents the immune system with a harmless version or piece of a pathogen so that antibodies and memory cells are ready before a real exposure. Killing germs after symptoms begin is what a treatment such as an antibiotic attempts. Gene replacement is gene therapy, a different technology for different diseases. And a vaccine does not form a barrier; the germ can still enter and is then met by a prepared defense.
Why are antibiotics ineffective against a viral illness such as influenza?
- a.Antibiotics act on features that viruses lack✓
- b.Antibiotics only work on animal cells
- c.Viruses are too small for any medicine
- d.Viruses reproduce faster than any drug acts
Antibiotics attack structures and processes specific to bacteria, such as the bacterial cell wall or bacterial ribosomes, and a virus has neither, which is why the drug has nothing to act on. Size is not the obstacle; antiviral medicines do exist. Antibiotics are not aimed at animal cells at all, which is what makes them safe to take. And speed of reproduction is not the reason the drug fails.
In photosynthesis, a plant uses carbon dioxide, water and light energy to produce—
- a.sugar and oxygen✓
- b.starch and hydrogen
- c.protein and nitrogen
- d.carbon dioxide and water
Photosynthesis builds a sugar from carbon dioxide and water and releases oxygen as a by-product, which is the source of nearly all the oxygen in the air. Protein synthesis requires nitrogen taken up separately as nitrate and is a different process. Plants do store starch, but it is made later from sugar, and hydrogen gas is not released. And carbon dioxide and water are the inputs, not the products.
Cellular respiration in human cells consumes glucose and oxygen and releases—
- a.sugar, oxygen and stored light energy
- b.nitrogen, water and heat only
- c.protein, salt and electrical charge
- d.carbon dioxide, water and usable energy✓
Respiration is the reverse direction from photosynthesis: the carbon in glucose leaves as carbon dioxide, the hydrogen ends up in water, and the energy released is captured for the cell's work with some lost as heat. Producing sugar and oxygen is what photosynthesis does. Protein and salt are not products of this reaction. And nitrogen plays no part in it.
In an energy pyramid for an ecosystem, the amount of energy available at each higher level—
- a.stays about the same
- b.doubles at every step
- c.decreases sharply✓
- d.increases sharply
Most of the energy an organism takes in is spent staying alive and lost as heat, so only a small fraction passes to whatever eats it, which is why food chains are short and top predators are few. Energy cannot increase as it moves up, since nothing is added. Staying constant would violate that same loss. And doubling is the opposite of what happens.
Decomposers such as fungi and bacteria are important to an ecosystem because they—
- a.control the size of predator groups
- b.add oxygen to the atmosphere
- c.return nutrients to the soil✓
- d.produce food from sunlight
Decomposers break down dead tissue and waste, releasing nitrogen, phosphorus and other elements in forms that plants can take up again, which is what keeps the nutrient cycle turning. Making food from sunlight is the producers' role. Predator numbers are regulated by prey supply and competition. And oxygen is added by photosynthesis, while decomposition generally consumes it.
The carrying capacity of a habitat is—
- a.the total area the habitat covers
- b.the number of different species it currently holds
- c.the rate at which its population grows each year
- d.the largest population it can support over time✓
Carrying capacity is set by the limiting resources — food, water, space, nesting sites — and a population that overshoots it falls back. The number of species present is richness, a measure of variety rather than of a ceiling. Growth rate describes how fast a population changes, not where it levels off. And area alone says nothing about how much life the ground can support.
A graph shows a rabbit population falling steeply one year. What is the most likely effect on the fox population that preys on those rabbits?
- a.It falls in the following period✓
- b.It rises immediately and then levels
- c.It is unaffected by the change
- d.It rises in the following period
Predators depend on their prey for food, so fewer rabbits means fewer foxes surviving and breeding, and the fox decline lags the rabbit decline by a season or more. A rise would require more food, not less. Being unaffected would mean the foxes had another equally good food source, which the question does not offer. And an immediate rise gets the direction and the timing both wrong.
An introduced species often spreads rapidly in its new range because—
- a.it reproduces more slowly than natives
- b.the new habitat has no resources for natives
- c.it needs fewer nutrients than any native
- d.local predators and diseases do not control it✓
A species arriving without the predators, parasites and diseases that held it in check at home can multiply faster than native competitors, which is what makes some introductions invasive. Slow reproduction would hinder the spread rather than help it. Lower nutrient needs may sometimes help but is not the general reason. And the habitat plainly does have resources, which is why natives were living there.
A bee gathers nectar from a flower and carries pollen to the next blossom. This relationship is—
- a.parasitism, because the bee harms the plant
- b.competition, because both want the same food
- c.mutualism, because both species benefit✓
- d.predation, because the bee consumes the plant
Each partner gains: the bee gets food and the plant gets its pollen delivered, which is the defining feature of mutualism. Parasitism would require the bee to live at the flower's expense, and pollination does the opposite. The two are not competing, since they are not after the same resource. And the bee does not kill or consume the plant, so this is not predation.
A food web is a better model than a single food chain because it shows—
- a.the seasonal migration of animals
- b.the exact number of each species
- c.the many feeding links in a community✓
- d.how long each organism lives
Most animals eat several things and are eaten by several others, so a web captures the branching connections that a single line cannot. Lifespan is not part of either model. Population sizes are shown in other diagrams, such as a pyramid of numbers. And migration is a behavior that neither a chain nor a web is built to display.
A person whose food supplies more energy than the body uses over a long period will most likely—
- a.lose weight through faster digestion
- b.store the excess as body fat✓
- c.convert the excess into vitamins
- d.excrete all of the excess energy
Energy that is taken in and not spent is stored, mostly as fat, which is the physical meaning of a sustained surplus. The body does not simply discard surplus energy; that is the whole reason a surplus matters. Vitamins are nutrients obtained from food and cannot be manufactured out of excess calories. And digestion does not speed up enough to make a surplus disappear.
Protein in the diet is used mainly to—
- a.carry oxygen to the lungs
- b.insulate the body against cold
- c.supply the fastest source of energy
- d.build and repair body tissue✓
Dietary protein is broken into amino acids that the body reassembles into its own proteins, from muscle fiber to enzymes, which is why needs rise during growth and healing. Oxygen is carried from the lungs by hemoglobin, not to them by protein in food. Insulation is largely the work of fat. And the quickest energy comes from carbohydrate.
Red blood cells are specialized to—
- a.produce digestive enzymes
- b.fight off invading bacteria and viruses
- c.help the blood form clots after an injury
- d.carry oxygen to the body's tissues✓
A red blood cell is packed with hemoglobin, which binds oxygen in the lungs and releases it where oxygen is scarce, and it even gives up its nucleus to make room. Fighting bacteria is the work of white blood cells. Clotting depends on platelets and plasma proteins. And digestive enzymes come from the pancreas and the gut lining.
The cell membrane contributes to the life of a cell by—
- a.copying the cell's genetic material
- b.controlling which substances enter and leave✓
- c.assembling amino acids into proteins
- d.converting sunlight into chemical energy
The membrane is selectively permeable, admitting some molecules, pumping others out and excluding the rest, which is how a cell keeps its interior different from its surroundings. Copying DNA happens in the nucleus. Capturing sunlight is the chloroplast's job in plants. And protein assembly takes place at the ribosomes.
A book rests on a table and stays there. Which statement best explains why, according to Newton's first law?
- a.No net force is acting on it✓
- b.Its mass has become zero
- c.The table exerts no force at all
- d.Gravity has stopped pulling on it
An object at rest stays at rest unless a net force acts, and here gravity pulling down is exactly balanced by the table pushing up, so the forces sum to zero. Gravity has not stopped; it is being opposed. The table is certainly pushing, and that push is half the balance. And the book's mass is unchanged, which is why gravity still acts on it.
A swimmer pushes backward against the water and moves forward. This illustrates Newton's third law because—
- a.the swimmer's mass increases in water
- b.the water pushes forward with equal force✓
- c.force and motion are unrelated in fluids
- d.water offers no resistance to motion
Forces come in pairs: the swimmer's push on the water is matched by the water's push on the swimmer, equal in size and opposite in direction, and that second force is what drives her forward. Water plainly does resist motion, which is why swimming is work. Mass does not change in water, though buoyancy changes apparent weight. And force and motion are tightly related here, as the third law says.
A net force of 20 newtons acts on a 4-kilogram cart. What is the cart's acceleration?
- a.24 m/s²
- b.0.2 m/s²
- c.5 m/s²✓
- d.80 m/s²
Newton's second law gives acceleration as force divided by mass, so 20 N ÷ 4 kg = 5 m/s². Multiplying instead of dividing gives 80 and reverses the relationship. Adding the two numbers gives 24, which mixes incompatible units. And dividing the mass by the force gives 0.2, the reciprocal of the right calculation.
Velocity differs from speed because velocity also states—
- a.the distance that was covered
- b.the direction of the motion✓
- c.the time the trip required
- d.the mass of the moving object
Speed is how fast something moves; velocity is how fast and which way, which is why two cars at the same speed in opposite directions have different velocities. Distance and time are the inputs that give speed in the first place. And mass appears in momentum and in force but not in either speed or velocity.
Acceleration is defined as the rate at which—
- a.force is applied
- b.mass is gained
- c.distance increases
- d.velocity changes✓
Acceleration measures how quickly velocity changes, which means an object speeding up, slowing down or merely turning is accelerating. Distance increasing over time is speed, one step back from acceleration. Force is what causes the acceleration rather than the quantity being measured. And mass is not changing in these problems at all.
A worker pushes a crate 3 meters along the floor with a steady force of 10 newtons in the direction of motion. How much work is done?
- a.3.3 joules
- b.13 joules
- c.300 joules
- d.30 joules✓
Work is force times the distance moved in the direction of the force, so 10 N × 3 m = 30 J. Adding the numbers gives 13 and ignores what work means. Dividing gives 3.3 and inverts the relationship. And 300 would require ten times the distance or the force.
A book sitting on a high shelf has energy because of its position. This energy is—
- a.thermal energy
- b.chemical energy stored in molecular bonds
- c.gravitational potential energy✓
- d.the kinetic energy of a moving body
Energy stored by virtue of position in a gravitational field is potential energy, and it converts to motion if the book falls. Kinetic energy belongs to objects that are actually moving, and this one is not. Chemical energy is stored in the bonds of molecules, as in fuel or food. And thermal energy is the energy of random particle motion, measured as temperature.
A swinging pendulum slows and finally stops. The law of conservation of energy says that its energy—
- a.was never present in the system
- b.turned into additional mass
- c.was destroyed by the friction
- d.changed into heat and sound✓
Energy is not lost but transferred, and friction at the pivot and drag through the air move it into heat and a little sound in the surroundings. Saying friction destroyed it contradicts the law being asked about. The pendulum plainly had energy while it swung. And converting to mass is a relativistic effect utterly negligible here.
In a sealed container, 10 grams of one substance react completely with 4 grams of another. What is the total mass of the products?
- a.6 grams
- b.10 grams
- c.14 grams✓
- d.It cannot be determined
Mass is conserved in a chemical reaction, so the products of a sealed reaction weigh what the reactants weighed: 14 grams. Subtracting gives 6 and treats the reaction as a loss. Reporting 10 keeps only the larger reactant. And the answer is entirely determinable, which is the point of the conservation law.
In the equation 2H2 + O2 -> 2H2O, the coefficient 2 in front of H2O tells you—
- a.how fast the reaction will proceed
- b.how many atoms are in one molecule
- c.how much heat the reaction gives off
- d.how many molecules of water are formed✓
A coefficient counts whole units of the substance, so two water molecules are produced for every oxygen molecule consumed. Atoms within a single molecule are counted by the subscripts, not the coefficient. Heat released is written separately as an energy term or measured in the laboratory. And the rate of reaction depends on conditions the equation does not record.
In an atom, the protons and neutrons are found—
- a.bonded to neighboring atoms
- b.orbiting outside the electron cloud
- c.spread evenly through the whole atom
- d.packed together in the nucleus✓
Almost all of an atom's mass sits in a tiny central nucleus made of protons and neutrons, with electrons occupying the far larger volume around it. An even spread through the atom was an early model that scattering experiments ruled out. Nothing orbits outside the electrons; they are the outermost part. And bonding involves electrons, not the nucleus.
The atomic number of an element equals the number of—
- a.bonds the atom can form
- b.particles in the nucleus
- c.protons in the nucleus✓
- d.neutrons in the nucleus
Atomic number counts protons, and it is what identifies the element: change it and you have a different element. Neutron number varies among isotopes of the same element, so it cannot be the identifier. Protons plus neutrons together give the mass number instead. And bonding capacity follows from the electron arrangement.
Two atoms are isotopes of the same element. They have—
- a.the same neutrons but different protons
- b.no neutrons in either nucleus
- c.different numbers of both particles
- d.the same protons but different neutrons✓
Isotopes share the proton count that defines the element and differ in neutrons, which makes them differ in mass while behaving almost identically in chemical reactions. Differing in protons would make them different elements altogether. Differing in both would likewise change the element. And only the lightest hydrogen isotope has no neutron, which is a special case rather than the definition.
An atom becomes an ion when it—
- a.joins with an identical atom
- b.is heated above its boiling point
- c.gains or loses one or more electrons✓
- d.gains or loses one or more protons
An ion is a charged atom, and charge appears when the number of electrons no longer balances the number of protons. Changing the proton count would change the element, which is a nuclear event rather than ionization. Heating can eventually strip electrons, but boiling alone does not make ions. And two identical atoms joining form a molecule, which is neutral.
Elements in the same column of the periodic table tend to—
- a.contain the same number of protons
- b.have the same atomic mass
- c.react in similar ways✓
- d.exist only as gases
A column, or group, collects elements with the same number of outer electrons, and since bonding is done by those electrons the group members behave alike chemically. Atomic mass rises steadily down a column rather than repeating. Only one group is entirely gaseous, so that cannot be a general rule. And identical proton counts would make them the same element.
A solution with a pH of 3 is—
- a.basic
- b.neutral
- c.saturated
- d.acidic✓
The pH scale runs from strongly acidic to strongly basic with 7 as neutral, so a reading of 3 lies well on the acidic side. Basic solutions read above 7. Neutral is 7 exactly, as pure water is at room temperature. And saturation describes how much solute a solution holds, which pH does not measure.
When an acid and a base are mixed in the right proportions, the products are—
- a.an alcohol and heat
- b.a salt and water✓
- c.two new acids
- d.a metal and a gas
Neutralization pairs the acid's hydrogen ions with the base's hydroxide ions to make water, leaving the remaining ions as a dissolved salt. No metal is produced, although many salts contain a metal ion that was already present. Alcohols come from quite different reactions, and heat is released but is not a product in the chemical sense. And the acidity is consumed rather than doubled.
Stirring sugar into warm water until it disappears is a physical change because—
- a.the sugar keeps its chemical identity✓
- b.the mixture cannot be separated
- c.the water molecules break apart
- d.a new compound has been created
The sugar is dispersed among the water molecules but is still sugar, which is why evaporating the water recovers it unchanged. No new compound formed, and that is exactly what distinguishes this from a chemical change. Water molecules are not broken apart by dissolving anything. And the mixture can be separated, by evaporation or distillation.
Compared with a liquid, the particles of a gas—
- a.spread out to fill their container✓
- b.are locked into a fixed pattern
- c.move more slowly at the same heat
- d.occupy a fixed volume of their own
Gas particles are far apart and move freely, so a gas expands until it meets the walls of whatever holds it. A fixed repeating pattern describes a solid, at the other end of the scale. At the same temperature gas particles move faster than those in a liquid, not slower. And having a definite volume is a property of solids and liquids, not gases.
Ice at 0 °C is heated and begins to melt. While melting continues, the temperature of the ice-water mixture—
- a.falls below 0 °C
- b.rises then falls back
- c.rises steadily
- d.stays at 0 °C✓
Energy added during a change of state goes into breaking the bonds that hold the solid together rather than into faster particle motion, so the thermometer holds steady until the last ice is gone. A steady rise is what happens before melting starts and after it finishes. Falling below freezing would require removing heat. And there is no reason for the temperature to reverse.
An object placed in water floats. This tells you that the object's density is—
- a.less than the density of water✓
- b.unrelated to the density of water
- c.greater than the density of water
- d.equal to the density of water
Floating means the object displaces a weight of water greater than its own, which happens when it is less dense than the fluid. Greater density is what makes something sink. Equal density leaves an object suspended, neither rising nor settling. And the comparison is the whole explanation, so the two densities are anything but unrelated.
A metal spoon left in a pot of hot soup becomes hot along its handle. The heat travels mainly by—
- a.convection
- b.conduction✓
- c.evaporation
- d.radiation
Conduction moves energy through direct contact as fast-moving particles jostle their neighbors along the solid metal. Convection needs a fluid to circulate, which the spoon is not. Radiation would warm the handle without contact and is a minor effect here. And evaporation removes energy from the soup's surface rather than carrying it up the handle.
Copper is used for household wiring because it is—
- a.a good insulator of electricity
- b.lighter than every other metal
- c.a good conductor of electricity✓
- d.unable to carry a magnetic field
Copper's loosely held outer electrons move easily, so current passes with little energy wasted as heat, which is what a wire is for. An insulator is the opposite and is used for the covering, not the core. Copper is not especially light; aluminum is lighter and is sometimes used for that reason. And its magnetic behavior is not why it was chosen.
Three bulbs are wired in a single series loop. If one bulb burns out, the others—
- a.flicker but stay lit
- b.go out as well✓
- c.shine more brightly
- d.stay lit as before
A series circuit offers current only one path, so a break anywhere stops the current everywhere and every bulb goes dark. Staying lit is what happens in a parallel circuit, where each branch is independent. Brightening would require more current, and there is none. And flickering implies an intermittent connection rather than a burnt-out filament.
A 12-volt battery drives a current through a 3-ohm resistor. What is the current?
- a.4 amperes✓
- b.15 amperes
- c.36 amperes
- d.0.25 amperes
Ohm's law gives current as voltage divided by resistance, so 12 V ÷ 3 Ω = 4 A. Multiplying gives 36 and confuses the relationship. Adding gives 15 and mixes units that cannot be added. And dividing resistance by voltage gives 0.25, the reciprocal of the correct result.
Two bar magnets are brought together north pole to north pole. They will—
- a.push each other apart✓
- b.pull each other together
- c.swap their poles
- d.lose their magnetism
Like poles repel, so two north poles pressed toward each other resist, and releasing them lets them fly apart. Attraction happens between a north pole and a south pole. Magnetism can be lost by heating or hammering but not by this. And a magnet's poles do not trade places because another magnet is nearby.
For waves traveling at the same speed, a wave with a higher frequency has—
- a.a greater amplitude
- b.a shorter wavelength✓
- c.a longer wavelength
- d.a lower speed
Speed equals frequency times wavelength, so at fixed speed the two must trade off and more waves per second means each is shorter. A longer wavelength accompanies a lower frequency. Amplitude is the height of the wave and varies independently of frequency. And the speed was given as unchanged.
A ringing bell is sealed in a jar and the air is pumped out. The sound fades away because sound—
- a.needs matter to travel through✓
- b.changes into light in a vacuum
- c.is absorbed by the glass jar
- d.travels faster in empty space
Sound is a mechanical wave: it propagates as particles of a medium push on their neighbors, and with no air left there is nothing to carry it. It does not travel faster in a vacuum; it does not travel at all. The glass was there before the air was removed and did not silence the bell. And sound does not convert into light.
A straw in a glass of water looks bent at the surface. This happens because light—
- a.is absorbed by the clear water
- b.changes direction entering the water✓
- c.bounces off the water's surface
- d.travels in a curved path in water
Light slows and bends as it crosses from water into air, so the rays reaching your eye arrive from a different apparent direction and the straw appears displaced. Bouncing off the surface is reflection, which produces an image on the water rather than a bend in the straw. Clear water absorbs very little visible light. And within the water the light travels in straight lines; only the boundary bends it.
Igneous rock forms when—
- a.heat and pressure reshape an existing rock
- b.flowing water wears away a rock face
- c.molten rock cools and hardens✓
- d.layers of sediment are pressed together
Igneous rock is solidified melt, whether it cools slowly underground into coarse crystals or quickly at the surface into fine ones. Compacted sediment produces sedimentary rock instead. Heat and pressure altering an existing rock without melting it produce metamorphic rock. And water wearing away rock is erosion, which makes sediment rather than rock.
In an undisturbed sequence of sedimentary layers, the oldest layer is normally—
- a.impossible to locate
- b.at the bottom✓
- c.in the middle
- d.at the top
Sediment settles from above, so each new layer rests on older material and the sequence reads oldest upward, a rule geologists call superposition. The topmost layer is the youngest. The middle is intermediate in age rather than oldest. And the age order is entirely readable so long as the layers have not been overturned by later folding.
Most of the world's earthquakes occur—
- a.near the center of the largest continents
- b.in the deepest parts of the ocean basins
- c.where the crust is thickest and oldest
- d.along the boundaries between tectonic plates✓
Plates grind past, pull away from and push under one another at their edges, and the sudden slip that releases the accumulated strain is an earthquake, which is why seismic maps trace out the plate boundaries. Continental interiors are comparatively quiet. Great ocean depth marks trenches, which are themselves plate boundaries, but depth is not the cause. And thick old crust is among the most stable ground there is.
A chain of volcanoes forms above a subduction zone because—
- a.the ocean floor is being lifted upward
- b.the descending plate melts material at depth✓
- c.two plates are pulling apart at the surface
- d.erosion has exposed old volcanic rock
As one plate sinks beneath another it carries water down, lowering the melting point of the rock above it, and the magma that forms rises to build volcanoes in an arc. Plates pulling apart create ridges and rift valleys, a different setting. Uplift of the sea floor produces mountains without necessarily producing magma. And erosion can expose volcanic rock but cannot create an active chain.
In the water cycle, the release of water vapor from the leaves of plants is called—
- a.transpiration✓
- b.condensation
- c.infiltration
- d.precipitation
Transpiration is the movement of water up through a plant and out through its leaves, and over a forest it returns an enormous volume to the air. Condensation is vapor turning back into liquid, which forms clouds. Precipitation is that water falling as rain or snow. And infiltration is water soaking down into the ground.
The passage of a cold front over a region is typically followed by—
- a.cooler air and often brief storms✓
- b.clear skies with no change in air
- c.a long period of steady warming
- d.a rise in humidity that lasts weeks
A cold front is the leading edge of advancing cold air that shoves warm moist air upward, which commonly triggers showers or thunderstorms along the boundary and leaves cooler, drier air behind. Steady warming describes a warm front instead. Some change always accompanies a front, which is what makes it a front. And humidity generally falls after the front passes rather than climbing for weeks.
Carbon dioxide is called a greenhouse gas because it—
- a.traps heat radiating from the Earth✓
- b.blocks sunlight from reaching the ground
- c.reflects heat back out into space
- d.destroys ozone in the upper atmosphere
Carbon dioxide lets visible sunlight through but absorbs the longer-wave infrared the warmed surface radiates, and re-emitting some of it downward keeps the lower atmosphere warmer than it would otherwise be. It does not block incoming sunlight, which is how the surface gets warm in the first place. Ozone destruction is caused by other compounds entirely. And reflecting heat away would cool the planet, the opposite of the effect.
Earth has seasons mainly because—
- a.its axis is tilted as it orbits the Sun✓
- b.its distance from the Sun changes greatly
- c.it spins more slowly in winter months
- d.the Sun's output rises and falls yearly
The tilt means each hemisphere leans toward the Sun for part of the year and away for the rest, changing the angle of the sunlight and the length of the day. Earth's distance from the Sun does vary slightly, but far too little to drive the seasons, and it would give both hemispheres the same season at once. The Sun's output is nearly steady. And the rotation rate does not change with the calendar.
The phases of the Moon are caused by—
- a.clouds in the Earth's atmosphere
- b.the rotation of the Moon on its own axis each month
- c.the changing positions of the Sun, Earth and Moon✓
- d.the Earth's shadow falling across the face of the Moon
Half the Moon is always lit; what changes is how much of that lit half faces us, and that depends on where the Moon is in its orbit relative to the Sun and Earth. Earth's shadow on the Moon is a lunar eclipse, which happens occasionally rather than monthly. Clouds hide the Moon without changing its phase. And the Moon's rotation keeps one face toward Earth and does not produce the cycle.
Ocean tides on Earth are caused mainly by—
- a.prevailing winds over the ocean
- b.the Moon's gravitational pull✓
- c.the Earth's magnetic field
- d.differences in ocean salt content
The Moon's gravity pulls hardest on the water nearest it and least on the far side, raising bulges that sweep around as the Earth rotates; the Sun contributes a smaller share. Winds pile up water in storm surges but do not produce the twice-daily rhythm. The magnetic field does not move water. And salinity differences drive slow deep currents, not tides.
The Sun is best classified as—
- a.a star✓
- b.a galaxy
- c.a comet
- d.a planet
The Sun is a star, a ball of gas generating energy by fusing hydrogen in its core, and it is ordinary except for being close. A planet orbits a star and shines only by reflected light. A comet is a small icy body on a long orbit. And a galaxy contains billions of stars, so the Sun is one member of ours rather than a galaxy itself.
A light-year is a unit of—
- a.brightness
- b.time
- c.distance✓
- d.mass
A light-year is how far light travels in one year, which makes it a measure of distance despite the word year in its name. Time is measured in seconds and years themselves. Brightness is reported as magnitude or luminosity. And mass is given in kilograms or in solar masses.
Which layer of the Earth lies directly beneath the crust?
- a.The atmosphere
- b.The inner core
- c.The outer core
- d.The mantle✓
The mantle is the thick layer of hot rock between the thin crust and the core, and its slow motion is what drags the plates around. The outer core lies beneath the mantle and is liquid metal. The inner core is deeper still and solid. And the atmosphere sits above the crust rather than below it.
A deep canyon with a river running along its floor was most likely shaped by—
- a.a single earthquake event
- b.erosion by running water✓
- c.the eruption of a volcano
- d.the weight of a glacier melting
Running water cuts downward over long spans of time, carrying away rock and deepening its own channel, which is the standard explanation for a river canyon. An earthquake can open a fissure but does not carve a valley with a river in it. Volcanic eruptions build landforms up. And glaciers do carve valleys, but they leave broad U-shaped troughs rather than a narrow canyon cut by a stream.
Coal, oil and natural gas are called fossil fuels because they formed from—
- a.minerals crystallizing out of seawater
- b.volcanic ash settling on the sea floor
- c.rainwater collecting in deep rock layers
- d.the remains of organisms buried long ago✓
Fossil fuels are the altered remains of plants and microscopic marine life buried under sediment and cooked by heat and pressure across millions of years, which is why they are not replaced on any human timescale. Minerals precipitating from seawater form evaporite deposits such as salt. Volcanic ash forms tuff. And water in deep rock is groundwater, not fuel.