100 questions
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.