Science Quiz — 50 Questions, Including How We Know

What do red blood cells mainly carry?

Answer Oxygen

Hemoglobin picks up oxygen in the lungs and releases it in the tissues. Mature red cells in humans have thrown out their nucleus to make room for more of it, which is also why they cannot repair themselves and last only a few months.

Photosynthesis takes in carbon dioxide and water and produces sugar and what else?

Answer Oxygen

Nearly all the oxygen in the air was put there by photosynthesis. Before that process spread, the atmosphere had almost none, and the change was catastrophic for the organisms already living without it.

Where is most of a cell's DNA kept in plants and animals?

Answer In the nucleus

The nucleus holds almost all of it, but not quite all — mitochondria carry their own small loop of DNA, inherited only from the mother, and that quirk is what lets geneticists trace maternal lines.

What does a species name like Panthera leo consist of?

Answer Genus and species

The two-part naming system dates to Linnaeus in the eighteenth century. The genus is capitalized and the species is not, and the pair is italicized — conventions that let a name mean the same animal in every language.

What are mitochondria mainly for?

Answer Releasing usable energy from food molecules

Mitochondria are thought to descend from free-living bacteria that were absorbed by another cell and stayed. Their own DNA, their double membrane and the way they divide independently are the evidence for that history.

Fungi are more closely related to which of these than to the others?

Answer Animals

Fungi sit closer to animals than to plants on the tree of life. They cannot photosynthesize, they store carbohydrate as glycogen the way animals do, and their cell walls use chitin, the same material as an insect shell.

What does natural selection act on?

Answer Inherited variation that affects survival and reproduction

Selection cannot act on a trait unless it is inherited and unless it makes a difference to survival or reproduction. That is why a skill you acquire in your lifetime does not get passed on by this route.

Antibiotics work against which of these?

Answer Bacteria

Antibiotics target structures bacteria have and our cells do not, such as their cell wall. Viruses have neither, which is why antibiotics do nothing for a cold and why taking them anyway helps resistance spread.

What is a food web made of?

Answer The feeding relationships linking organisms in an ecosystem

A food chain is a single line; a web is the real thing, with most species eating and being eaten by several others. That redundancy is part of why removing one species can have effects nobody predicted.

Why do most cells need to stay small?

Answer Because surface area grows more slowly than volume, limiting exchange

Double a cell's width and its surface grows fourfold while its volume grows eightfold. Since everything must enter and leave through the surface, growing bigger makes a cell worse at supplying its own interior.

What is at the center of an atom?

Answer A nucleus of protons and neutrons

The nucleus holds nearly all the mass in a space vanishingly small compared with the atom itself. If an atom were the size of a stadium, the nucleus would be about the size of a pea at the center.

What decides which element an atom is?

Answer Its number of protons

Change the proton count and you have a different element; change the neutrons and you have an isotope; change the electrons and you have an ion. Only the first changes what the substance is.

What is an isotope?

Answer An atom of the same element with a different number of neutrons

Isotopes of an element behave almost identically in chemistry because that is set by the electrons. Their differences show up in mass and in stability, which is what makes radiocarbon dating possible.

On the pH scale, what does a value below 7 indicate?

Answer An acidic solution

The scale is logarithmic, so pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6. That is why small-looking pH shifts in the ocean or in soil matter so much.

What happens to matter in a chemical reaction?

Answer Atoms are rearranged but not created or destroyed

Atoms are rearranged, never created or destroyed, which is why a chemical equation must balance. Burning a log does not destroy its matter — most of it leaves as gas you cannot see.

Why does ice float on water?

Answer Because water expands when it freezes, making ice less dense

Almost every substance is denser as a solid. Water is the strange one because its molecules lock into an open lattice when they freeze, and that oddity is why lakes freeze from the top and life survives beneath.

What is a solution?

Answer A mixture in which one substance is evenly dispersed in another

In a true solution the dissolved substance is spread out at the molecular level and will not settle out or be caught by a filter. That is what separates it from a suspension like muddy water.

What are the columns of the periodic table useful for?

Answer Grouping elements that behave chemically alike

Elements in the same column have the same number of outer electrons, and the outer electrons are what react. That is why the arrangement predicted the properties of elements that had not been discovered yet.

What is happening when a liquid evaporates below its boiling point?

Answer The fastest molecules at the surface escape

Molecules in a liquid have a spread of speeds, and the fastest ones at the surface can break free at any temperature. Because the quick ones leave, what remains is cooler — which is how sweating works.

What does a catalyst do?

Answer It speeds a reaction up without being consumed

A catalyst lowers the energy barrier a reaction has to climb, so more collisions succeed. It is not used up, which is why a tiny amount can process an enormous quantity of reactant.

Two objects of different mass are dropped in a vacuum. What happens?

Answer They fall at the same rate

A heavier object is pulled harder but is also harder to accelerate, and the two effects cancel exactly. Astronauts on the Moon demonstrated it with a hammer and a feather.

What does an object's weight depend on that its mass does not?

Answer The strength of gravity where it is

Mass is how much matter there is and does not change with location. Weight is the force gravity exerts on that mass, which is why the same person weighs about a sixth as much on the Moon.

What does the law of conservation of energy say?

Answer Energy changes form but the total stays the same

Energy cannot be created or destroyed, only converted. Machines lose energy in the sense that it turns into heat and spreads out, not in the sense that it vanishes.

Why do you hear thunder after you see lightning?

Answer Because light travels far faster than sound

Light arrives essentially instantly, sound at about 340 meters per second. Counting the seconds between flash and rumble and dividing by three gives the distance in kilometers.

What makes a rainbow's colors separate?

Answer Different wavelengths of light bend by different amounts in water droplets

Each wavelength bends by a slightly different amount entering and leaving a droplet, which fans white light into a band. The bow always appears opposite the Sun, and its position depends on where you stand — no two people see quite the same one.

What is friction?

Answer A force resisting motion between surfaces in contact

Friction comes from surfaces gripping at the microscopic scale, not from how rough they look. It is also what lets you walk at all — on a frictionless floor your foot would simply slide backward.

Sound cannot travel through which of these?

Answer A vacuum

Sound is a pressure wave and needs particles to pass it along. Solids usually carry it fastest because their particles are closest together, which is why you can hear a train through a rail before you hear it through the air.

What does an electric current consist of in a metal wire?

Answer Moving electrons

Individual electrons drift surprisingly slowly, a few centimeters a minute. The signal moves near light speed because the push is passed along the whole wire at once, like water in a full pipe.

Why does a metal spoon feel colder than a wooden one at the same temperature?

Answer Metal conducts heat away from your hand faster

Both are at room temperature; the difference is in how fast each pulls heat out of your skin. Your hand does not sense temperature directly, only the rate at which it is losing or gaining heat.

What is temperature a measure of?

Answer The average kinetic energy of the particles

Temperature measures the average energy of motion of particles, not the total. A spark from a sparkler is far hotter than a bathtub yet carries almost no heat, because there is so little of it.

What causes the seasons?

Answer The tilt of the Earth's axis relative to its orbit

The axis stays tilted in the same direction while the Earth orbits, so each hemisphere leans toward the Sun for half the year. This is why the two hemispheres have opposite seasons on the same date.

What drives the movement of tectonic plates?

Answer Slow motion in the hot mantle beneath them

Heat from the interior sets up slow circulation in the mantle, and the rigid plates ride along on top. The pace is a few centimeters a year, but over millions of years that redraws every coastline.

What is the water cycle mainly powered by?

Answer Energy from the Sun

Sunlight evaporates water, and gravity brings it back down. All of the planet's water has been through this loop many times — none of it is new, and none is lost.

How is sedimentary rock formed?

Answer By layers of particles being deposited and compressed

Sediment settles in layers, and the layers below get squeezed and cemented by the weight above. Because the lower layers were laid down first, the sequence reads like a calendar from the bottom up.

What causes ocean tides?

Answer The gravitational pull of the Moon and Sun

The Moon does most of it and the Sun adds to it. When both pull along the same line the range is largest, which is why the biggest tides come around the new and full Moon.

What does the ozone layer do?

Answer Absorbs much of the Sun's ultraviolet light

Ozone absorbs ultraviolet light that would otherwise damage DNA. The hole found over Antarctica in the 1980s led to a treaty phasing out the chemicals responsible, and the layer has been slowly recovering since.

Which gas makes up most of the Earth's atmosphere?

Answer Nitrogen

Nitrogen is about 78 percent of the air and oxygen about 21 percent. Nitrogen is nearly inert, which is fortunate — an atmosphere that was mostly oxygen would make almost any fire unstoppable.

What does an earthquake's magnitude describe?

Answer The energy released at the source

Magnitude describes the energy released where the quake began, so one earthquake has one magnitude. How hard the shaking felt where you were standing is a separate measure called intensity, and it varies from place to place.

How do scientists work out the age of very old rock layers?

Answer By measuring the decay of radioactive elements in them

Radioactive elements decay at a fixed rate, so the ratio of parent to daughter atoms acts as a clock. Different elements suit different timescales, which is why carbon dating is useless on anything older than about fifty thousand years.

Why is the sky blue on a clear day?

Answer Because air scatters short wavelengths more than long ones

Shorter wavelengths bounce off air molecules far more than longer ones, so blue light arrives at your eye from all over the sky. Violet scatters even more, but our eyes are much less sensitive to it.

What is a control group for?

Answer Providing a comparison that differs only in the thing being tested

The control is what makes a comparison mean anything. Without it you cannot tell whether people improved because of the treatment, or because they would have improved anyway.

Two things rise together over time. What does that establish?

Answer That they are correlated, which is not the same as one causing the other

Correlation can come from coincidence, from a third factor driving both, or from causation running the other way. Ice cream sales and drowning deaths rise together, and neither causes the other — summer causes both.

What does the word theory mean in science?

Answer An explanation supported by a large body of tested evidence

In everyday speech a theory is a guess; in science it is the opposite end of the scale. A theory is a framework that has survived repeated testing and explains a wide range of observations.

Why are studies with human participants often blinded?

Answer Because knowing which group you are in can change the outcome

Expectation changes results, in participants and in the researchers measuring them. Double-blinding hides the assignment from both sides, which is why it is the standard for testing treatments.

What is the SI base unit of mass?

Answer The kilogram

The kilogram was the last unit defined by a physical object, a metal cylinder kept in France. In 2019 it was redefined in terms of a constant of nature, so it no longer depends on an artifact that could drift.

A measurement is written as 2.50 meters rather than 2.5. What does that convey?

Answer That the precision extends to the hundredths place

The trailing zero is not decoration; it says the measurement was good to that place. Writing 2.5 claims less precision than 2.50, and writing 2.500 claims more than you may be able to support.

What makes a claim scientific rather than not?

Answer That it could in principle be shown wrong by evidence

A claim that no possible observation could contradict is not scientific, however true it might be. The requirement is not that a claim be wrong, but that there be some result that would show it wrong.

Why does peer review exist?

Answer To have other specialists check the method and reasoning before publication

Peer review is a filter, not a guarantee. Reviewers check method and reasoning but rarely repeat the work, which is why published results still need replication before they are treated as settled.

What is a hypothesis?

Answer A testable proposed explanation

A hypothesis has to be specific enough that some result would count against it. Wording it so that any outcome can be claimed as support is the most common way for the idea to become untestable.

Why does replication matter?

Answer A result that only appears once may be chance or a flaw in one study

Any single study can produce a result by chance or through a flaw nobody noticed. When many fields tried repeating well-known findings, a large share did not hold up — which is why one striking study is a starting point, not a conclusion.