General Science: 40 Most Important Scientific Concepts

Question 1. Describe the process of photosynthesis. Include the balanced chemical reaction.
Photosynthesis: Your explanation should mention chlorophyll in plants, sunlight, and the conversion of carbon dioxide and water into carbohydrates and oxygen. The balanced chemical reaction is 6 CO2 + 6 H2O → C6H12O6 + 6 O2.

Question 2. How do the phases of the moon form? 
Lunar phases: Your explanation should mention sunlight illuminating one-half of the moon’s surface, but that depending on the relative position of the earth, sun, and moon, the amount of sunlight seen from earth may vary from 0 (new moon, in which the sun illuminates the side opposite from earth) to 100% (full moon, in which the sun illuminates the side facing earth). 

Question 3. What causes earth’s seasons? 
Earth’s seasons are caused by the tilt of earth’s axis (not by the elliptical nature of earth’s orbit). Due to this tilt, in the summer the northern hemisphere receives more direct sunlight (while at the same time the southern hemisphere receives less direct sunlight, which explains why it is winter in the southern hemisphere when it is summer in the northern hemisphere, and vice-versa).

Question 4. Describe the water cycle.
Water cycle: Your description should mention the oceans, water vapor in the atmosphere, and ground water. It should also mention evaporation/condensation, precipitation/runoff/seepage, and the role of plants (e.g. transpiration).

Question 5. The earth exerts a gravitational force on the moon. Why doesn’t this cause the moon to crash into the earth?
Earth’s pull of the moon: If the moon started from rest, earth’s pull would cause it to crash into the earth. Fortunately, the moon isn’t starting from rest. It has tangential speed, and it has enough of it to maintain its orbit. In this case, the earth’s pull serves as a centripetal force, causing the direction of the moon’s velocity to change (but not significantly changing the moon’s speed). It would be satisfactory to draw the earth, a tall mountain, projectile orbits, and a satellite orbit, as Galileo did in a famous diagram a few hundred years ago.

Question 6. What is the significance of the equation E = mc2?
E = mc2: This equation expresses an equivalence between mass and energy. You should highlight at least one common example, such as its significance in nuclear reactions, the extreme change in rest-mass that occurs in pair annihilation, or the role it plays in the sun providing energy to earth.

Question 7. Describe the metamorphosis of a butterfly.
Metamorphosis: Your description should mention the caterpillar and chrysalis, and should also mention the egg, pupa, larva, and adult.

Question 8. What are the different blood types? What is the Rh factor? Which types of blood transfusions are possible?
Blood types/transfusions: A, B, AB, and O. The first three carry type A and/or B antigens. Another type of antigen is designated by the Rh factor: Rh+ carries this, Rh– doesn’t. A successful transfusion does not give a person a new type of antigen. Type A may receive type A or type O, type B may receive type B or type O, type O may only receive type O, and type AB may receive any. However, there is the added restriction of Rh: Rh– may only receive Rh– (but Rh+ may receive Rh+ or Rh–). (Note that plasma transfusions are different from blood transfusions.)

Question 9. Describe how convection works in the process of boiling water in a pot on a stove.
Convection: Your description should begin with the hot bottom of the pot heating the water at the bottom. It should also mention that this heated water at the bottom expands and rises, and that the cooler water at the top is more dense and sinks, and that the volume of the water heats through these convection currents.

Question 10. Describe the role that certain gases play in the greenhouse effect. (This question is asking for a scientific explanation of what it is, not your opinion, belief, or conclusions about it.)
Greenhouse effect: Your description should mention that certain gases, like CO2, CH4, H2O (vapor), and O3, in earth’s atmosphere influence earth’s surface temperature by absorbing heat (analogous to the way that glass traps heat in a greenhouse). You should also state that if the concentrations of these gases increase dramatically, it may increase the average temperature at earth’s surface. You might also provide examples of reasons that these concentrations might be changing, such as through the combustion of fuels. (No deductions for stating that this is a complex problem and may not yet be fully understood. But your answer should sound impartial.)

Question 11. Describe how to use a star map (on paper, not an app on a phone).
Star map: A star map shows the night sky at various times of the year (often by month or by season) for a particular hemisphere (northern or southern). A star map includes compass directions. Hold the star map so that its compass direction and the direction you are actually facing match up (for example, both pointing north). Try to identify constellations. Once you find these, you can search for specific stars. (Phone apps these days greatly simplify this process.)

Question 12. Explain why the sky is blue. On a related note, why does the sun appear red at sunrise or sunset, but yellow at noon?
Why the sky is blue: (What we really mean is, when you look above the surface of the earth, why does your eye perceive blue light? The sky itself doesn’t have color, but the light entering our eye has color.) The main point in your explanation should mention that shorter wavelengths of light tend to scatter more on average. (Bonus if you know that the intensity of scattered light varies as the inverse of the fourth power of the wavelength.)

Your diagram should show at least two different rays of sunlight traveling through earth’s atmosphere: One should travel in a straight line, while the other should scatter one or more times. When looking toward the sun, you see light that hasn’t scattered, so that light on average is a longer wavelength. At noon, the sun appears yellow because sunlight has traveled a shorter distance through the atmosphere (those last three words are critical; it’s not the overall distance that matters, just the part through the atmosphere) so that less scattering has occurred. At sunrise or sunset, the sun appears red because sunlight has traveled a longer distance through the atmosphere so that more scattering (of all the shorter wavelengths) has occurred (primarily red wavelengths reach your eye in this direction). When you look anywhere in the sky other than the sun, you see blue light because you’re looking at an average of scattered light (so these are mostly shorter wavelengths like blue, violet, and green, all averaged together).

Question 13. How does a magnet work? What happens if you cut a magnet in half? What does the term “magnetic monopole” mean? What is known about “magnetic monopoles”?
Magnet: Your answer should mention the alignment of microscopic magnetic fields (in the form of atomic magnetic dipoles, but if you ascribe this to magnetic fields produced by moving charged particles, we won’t ask you to deduct for that), at least partially (as opposed to the random alignment in nonmagnetic materials). If you cut a magnet in half, you get two smaller magnets, and each new magnet has one pair of north and south poles. The idea is that you can’t isolate one pole simply by cutting a magnet in pieces. Rather, north and south poles always come in pairs. An isolated pole would be called a magnetic monopole (as of yet undiscovered).

Question 14. How does a Punnett square work? Fill one out as an example.
Your description should include writing the genotypes of the parents, drawing a table with enough squares for the different gametes possible, labeling the genotypes for the parents, and pairing letters to form genotypes for the gametes. Your description should also mention dominant and recessive alleles, and determining probabilities. Our example below is for parents who are both Bb, where B = brown eyes and b = blue eyes. Our example has a genotype ratio of 1 BB : 2 Bb : 1 bb and a phenotype ratio of 3 brown : 1 blue (with probabilities of 75% and 25%).

Question 15. Describe the eruption of a volcano, including its cause.
Volcano: Your description should mention a build-up of pressure in the earth and the rising of magma, and might also mention tectonic plates, viscosity, and how magma forms. Your description should describe features like the magma chamber and lava flow.

Question 16. Describe osmosis and osmotic pressure in chemistry. Give one example from biology.
Osmosis: One way to illustrate osmotic pressure is to consider a tube with a semi-permeable membrane separating two solutions. You should mention that the solvent will have a net movement towards the solution where the solute is more concentrated. Your description should mention that osmotic pressure is the pressure that must be applied to a pure solvent in order to prevent it from passing into a solution through osmosis. You should also mention one example from biology, such as osmosis in red blood cells or plants absorbing water from the soil.

Question 17. Given that steel is more dense than water, explain how a boat made of steel can float.
Steel boat: The main idea is that the shape of the boat includes a large volume of air. A steel ball would sink because it is more dense than water. However, air is much less dense than water. Thus, if there is enough air inside the boat that its overall density is less dense than water, the boat will float. You might also discuss buoyancy and Archimedes’ principle.

Question 18. Describe how a rainbow forms. Where is the sun in relation to the observer?
Rainbow: Your description should state that the observer stands with his or her back to the sun and that rainbow formation involves the dispersion of sunlight in raindrops.

Your diagram should show the refraction and reflection of the red and violet rays (the two extremes). The red ray emerges from the raindrop at a steeper angle than the violet ray (violet light travels slower than red light in water, so that the violet ray changes direction more). An observer doesn’t see red and violet rays leaving the same raindrop. Rather, the observer sees red rays coming from a higher angle (because they exit the raindrops at a steeper angle). A secondary, weaker rainbow forms when light enters the bottom of the raindrop and reflects twice inside. The colors of the secondary are reversed compared to the primary.

Question 19. Explain why solar eclipses are rare. That is, why don’t we have a solar eclipse every time there is a new moon?
Solar eclipses: You should mention that the plane of the moon’s orbit around the earth is tilted about 5° compared to the plane of the earth’s orbit around the sun. As a result, the moon only crosses the plane of earth’s orbit twice every 27.2 days (a draconic month). Compare this to a new moon forming once every 29.5 days (a synodic month). For a solar eclipse to occur, the moon must be close to the earth’s orbit during a new moon (otherwise, the earth, moon, and sun won’t lie in a straight line and the moon won’t be blocking sunlight from reaching earth). Furthermore, even when a solar eclipse does occur, the shadow formed by the moon on earth’s surface is only a couple of hundred kilometers wide (note that this shadow moves as the earth rotates and the moon revolves), so only a small portion of the earth will experience a given solar eclipse.

Question 20. Draw and label a prokaryote and a eukaryotic cell.
Prokaryote and eukaryotic cell: Indicate whether your eukaryotic cell is a plant or animal cell. If it is a plant cell, it should feature a cell wall and chloroplasts but not centrioles, but if it is an animal cell, it should feature centrioles but not chloroplasts or a cell wall. A prokaryote should include ribosomes, DNA, a cell wall, and a cell membrane. The eukaryotic cell should also contain other features, such as a nucleus, vacuoles, mitochondria, an endoplasmic reticulum, and Golgi apparatus. 

Question 21. Discuss how valence electrons are shared by atoms in the covalent bond CH4.
Covalent bond in CH4: The main idea is that hydrogen has 1 valence electron and its outer shell can hold 2, while carbon has 4 valence electrons and its outer shell can hold 8. Therefore, each hydrogen atom in CH4 shares one pair of electrons with the carbon atom (its own electron and one of carbon’s valence electrons). By sharing electrons this way, carbon’s valence shell is effectively filled with 8 valence electrons (4 pairs make 8) and each hydrogen’s valence shell is effectively filled with 2.

Question 22. Discuss how electrons are transferred by atoms to form the ionic bond MgCl2.
Ionic bond in MgCl2: The main idea is that magnesium is a metal with 2 valence electrons, while chlorine is a nonmetal with 7 valence electrons and its outer shell can hold 8. The magnesium atoms each need to lose 2 electrons to have filled outer shells, so they form Mg2+. The chlorine atoms each need to gain 1 electron to have filled outer shells, so they form Cl– (the 1 is implied). A single Mg2+ ion pairs with two Cl– ions (because magnesium loses 2 electrons and each chlorine gains 1).

Question 23. Describe the life of a main sequence star. What are the three possible fates of a main sequence star? What determines the star’s fate?
Life of a main sequence star: You should mention the various stages, including the nebula, protostar, main sequence star, and red giant. The fate of a star depends on its mass. If the core after supernova has less than about 1.4 solar masses it becomes a white dwarf, if it is between about 1.4 and 3 solar masses it becomes a neutron star, and if it exceeds about 3 solar masses it becomes a black hole.

Question 24. Describe the rock cycle.
Rock cycle: Your description should include the five forms (magma, igneous, sediment, sedimentary, metamorphic) and the processes (crystallization, weathering/ transport, lithification, metamorphism, and melting – at a minimum).

Question 25. Describe the cell cycle for a typical eukaryotic cell.
Eukaryotic cell cycle: Your description should include interphase, prophase, metaphase, anaphase, telophase, and cytokinesis, and also describe what each is.

Question 26. Describe the physics of a spinning ice skater who is changing her angular speed.
Spinning ice skater: The main idea is that she has a smaller moment of inertia when she brings her arms and legs inward, which increases her angular speed such that her angular momentum remains constant. In contrast, she has a larger moment of inertia when she spreads her arms, torso, and one leg outward, which decreases her angular speed in order to conserve her angular momentum.

Question 27. What are specific ways that aerobic exercise helps to fight heart disease?
Aerobic exercise and heart disease: You should mention that it makes the heart stronger, increases blood flow, helps keep the blood vessels open, and lowers blood pressure. You might also mention that it prevents clogged arteries and may also help with stress.

Question 28. What causes tides?
Tides: The main idea is that the moon exerts a greater force on water that is on the side of the earth that faces the moon and exerts a weaker force on water that is on the side of the earth opposite to the moon (since gravitational force varies with distance), which creates a squeezing effect on earth’s oceans (which cover its surface). The parts of the ocean nearest or furthest from the moon experience high tides, and the part of the ocean in between experiences low tides. As the earth rotates on its axis once every 24 hours, different parts of the earth experience low and high tides. In one 24-hour period, any location experiences 2 high and 2 low tides.
Spring and neap tides each occur twice per month, and have to do with the combined effects of the sun and moon. When the earth, sun, and moon are aligned, the tidal effects are maximum (spring tides). When they form a right angle, the tidal effects are at a minimum (neap tides).

Question 29. Describe the close relationship between the respiratory and circulatory systems.
Respiratory/circulatory systems: Your description should describe the exchange of gases that occurs between the alveoli and the capillaries, especially the diffusion of oxygen. It should also mention the importance of hemoglobin.

Question 30. What did Galileo observe with his telescope that supports the heliocentric theory and rules out the geocentric theory of the solar system? Explain.
Galileo and the heliocentric theory: Galileo observed a range of phases of Venus that aren’t possible in the geocentric model. In the heliocentric model, Venus can be on the other side of the sun compared to earth and can experience a full phase, but in the geocentric model Venus and earth are always on the same side of the sun, so Venus can’t experience a full phase in the geocentric model. (In the geocentric model, Venus and Mercury are constrained to lie within a cone, as illustrated below, in order to agree with the observation that Mercury and Venus don’t stray far from earth’s horizon at night. In the heliocentric model, no such cone applies.)

Question 31. Describe the Doppler effect. Include an example.
Doppler effect: Whereas a stationary source produces concentric wave fronts, a moving source produces off-center wave fronts, as illustrated below. If an observer is in front of the source, the wave fronts are closer together (smaller wavelength, but higher frequency). Behind the source, the wave fronts are farther apart (longer wavelength, but lower frequency).

Question 32. Describe how carbon-14 dating works. Which reaction is central to this?
Carbon-14 dating: The main idea is that carbon-14 decays to nitrogen-14 via radioactive beta decay. The reaction is:
The half-life of carbon-14 is 5715 years. When an organism dies, the ratio of carbon-14 to carbon-12 decreases, which allows scientists to estimate the age of organic remains.

Question 33. Explain how a basic mercury thermometer works.
Mercury thermometer: Your explanation should mention the thermal expansion of mercury, for which the expansion is proportional to the temperature change, and how it is calibrated based on standard measures, such as the freezing and boiling points of water.

Question 34. Of all the elements, what makes carbon so important for life?
Carbon: You should mention that carbon has 4 valence electrons and that its outer shell can hold 8, which allows it to form a very wide variety of bonds with many other types of atoms, typically with each carbon atom forming 4 bonds. This helps carbon form strong bonds, and it can even bond with itself. It can form double or triple bonds, and can form chains, rings, and large molecules. (Why is carbon more important for life than silicon, which also has 4 valence electrons? One factor is that carbon bonds tend to be stronger.) An element like carbon that can form very large molecules is essential for making proteins and other substances critical for life.

Question 35. What is the importance of nitrogen in the soil?
Nitrogen: You should mention the importance of nitrogen for plants (growth, chlorophyll, metabolism), and how the food produced by plants is important for animals.

Question 36. In what sense is Newton’s first law of motion a special case of Newton’s second law of motion?
Newton’s laws: The second law states that the net force on an object equals its mass times its acceleration. For the special case that the net force equals zero, the second law simplifies to the first law. That is, if the net force is zero, the acceleration is zero, meaning that the velocity is constant.

Question 37. Explain how the van der Waals equation modifies the ideal gas law.
Van der Waals: Gases with high enough density do not obey the ideal gas law as well. The van der Waals equation corrects for this in two ways: It subtracts the volume of the molecules from the volume of the gas, and corrects the pressure by accounting for attractive intermolecular forces.

Question 38. Describe Archimedes’ principle and how it relates to floating versus sinking.
Archimedes’ principle: You should state that the buoyant force equals the weight of the displaced fluid. The buoyant force is FB = mfg = dfgVf (where f stands for the fluid). The weight of the object is mog = dogVo (where o stands for the object). If the object is wholly submerged in the fluid, Vo = Vf. The net force equals FB – mfg = dfgVf – dogVo = (df – do)gVo, which according to Newton’s second law equals mass times acceleration. The object accelerates downward (it sinks) if it is more dense than the fluid and accelerates upward (it rises) if it is less dense than the fluid.

Question 39. Which weather conditions can cause a tornado? How?
Tornado: Your description should mention a warm and humid lower atmosphere and cooler upper atmosphere (common in the US in states north of the Gulf of Mexico in spring and early summer), severe thunderstorms, and wind shear.

Question 40. Describe how a heat engine works. How does the cycle of a heat engine relate to the second law of thermodynamics?
Heat engine: Your description should mention that the natural flow of heat from a thermal reservoir at high temperature to a thermal reservoir at low temperature is adapted in order to perform mechanical work. You should also mention that a heat engine can’t be 100% efficient according to the second law of thermodynamics.