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13.7 — Science: Physics and Chemistry
Units, laws, elements and the discoveries that get asked. Every answer carries the reason, because a formula with no reason attached is a formula you will misremember.
Units and constants
What are the seven SI base units? Metre, kilogram, second, ampere, kelvin, mole and candela — length, mass, time, current, temperature, amount of substance and luminous intensity.
What is the SI unit of force, energy, power and pressure? Newton, joule, watt and pascal. One watt is one joule per second; one pascal is one newton per square metre.
Unit of frequency, resistance, charge, capacitance and magnetic field? Hertz, ohm, coulomb, farad and tesla.
What is the speed of light, and why is it exact? 299,792,458 m/s in vacuum — exact by definition, because the metre is now defined from it.
What is absolute zero? −273.15 °C, or 0 K. Nothing can have less thermal motion than none, and it has never been reached.
What is Avogadro's number? About 6.022 × 10²³, the number of particles in a mole. It is why chemists can weigh out a countable number of atoms.
What is the acceleration due to gravity on Earth? About 9.8 m/s². It is slightly lower at the equator, because of the bulge and the rotation.
What is Planck's constant, and why does it matter? About 6.626 × 10⁻³⁴ joule-seconds, the quantum of action. It now defines the kilogram.
Which quantities are scalar and which vector? Speed, mass, distance, energy and temperature are scalar; velocity, force, displacement, acceleration and momentum are vector — they carry direction.
The laws
State Newton's three laws. A body stays at rest or in uniform motion unless a force acts; force equals mass times acceleration; every action has an equal and opposite reaction.
What is Newton's law of gravitation? Every mass attracts every other with a force proportional to the product of the masses and inversely proportional to the square of the distance.
What are the three laws of thermodynamics, in one line each? Energy is conserved; entropy of an isolated system never decreases; you cannot reach absolute zero in finite steps. The joke version: you cannot win, you cannot break even, you cannot leave the game.
What is Archimedes' principle? A body in fluid is buoyed up by a force equal to the weight of fluid it displaces. It is why steel ships float and why a bath overflows.
What is Pascal's law? Pressure applied to an enclosed fluid transmits equally in all directions. Every hydraulic brake and jack works on it.
What is Bernoulli's principle? Faster-moving fluid exerts lower pressure. It contributes to aircraft lift, but lift is mainly from the wing deflecting air downward.
What is Ohm's law? Voltage equals current times resistance.
What is Hooke's law? Extension is proportional to the applied force, within the elastic limit. Beyond that limit a spring does not return.
What are Kepler's three laws? Orbits are ellipses with the sun at one focus; a planet sweeps equal areas in equal times; the square of the orbital period is proportional to the cube of the semi-major axis.
What is Faraday's law of induction? A changing magnetic field induces a voltage in a conductor. That single fact is every generator and transformer on Earth.
What is Lenz's law? The induced current opposes the change that produced it — which is conservation of energy showing up as a direction.
What is Snell's law? It relates the angles of incidence and refraction when light crosses between media. It is why a straw looks bent in water.
What is the Doppler effect? The change in observed frequency when source and observer move relative to each other. It explains the pitch drop of a passing siren and the redshift of distant galaxies.
What is the photoelectric effect, and who explained it? Electrons ejected from a metal by light, explained by Einstein in 1905 using light quanta — and that paper, not relativity, won him the Nobel.
What is the uncertainty principle? Heisenberg's result that position and momentum cannot both be known precisely. It is a property of nature, not a limitation of instruments.
What are the four fundamental forces? Gravity, electromagnetism, the strong nuclear force and the weak nuclear force. Gravity is by far the weakest and the only one with unlimited range that never cancels.
Light, sound and heat
What is the visible spectrum, in order? Violet, indigo, blue, green, yellow, orange, red — VIBGYOR — from shortest to longest wavelength, roughly 400 to 700 nanometres.
Why is the sky blue and the sunset red? Air scatters short wavelengths far more strongly. At sunset the light travels through much more air, so the blue has been scattered away.
What is the speed of sound in air? About 343 m/s at 20 °C — roughly a kilometre in three seconds, which is how you time a lightning strike.
Does sound travel faster in water or air, and why? Water, about four times faster, because the molecules are closer together and transmit the disturbance more quickly. Faster still in steel.
What is the human audible range? About 20 Hz to 20 kHz, narrowing at the top with age. Below is infrasound, above is ultrasound.
What is the decibel scale? Logarithmic — a 10 dB rise is ten times the power and sounds roughly twice as loud. Sustained exposure above 85 dB damages hearing permanently.
How does a microwave oven heat food? Radio waves at 2.45 GHz twist water molecules billions of times a second; the collisions are heat. It cannot brown food, because wet food cannot exceed 100 °C.
What are the three modes of heat transfer? Conduction through contact, convection through fluid movement, radiation through electromagnetic waves. Only radiation crosses a vacuum, which is how the sun reaches you.
What is latent heat? The energy absorbed or released during a change of state at constant temperature. It is why sweat cools you and why steam burns are worse than boiling-water burns.
What is a black body? An idealised object that absorbs all radiation falling on it. Trying to predict its emission spectrum is what forced Planck to invent the quantum.
The atom and the nucleus
Who proposed the nuclear atom, and how? Rutherford, in 1911, after alpha particles fired at gold foil occasionally bounced back — which he compared to a shell rebounding from tissue paper.
Who discovered the electron, the proton and the neutron? J. J. Thomson, Rutherford and James Chadwick.
What is an isotope? Atoms of the same element with different numbers of neutrons. Carbon-14 dating uses one.
What is half-life? The time for half of a radioactive sample to decay. Carbon-14's is about 5,730 years, which sets the practical limit of radiocarbon dating at around 50,000 years.
What are alpha, beta and gamma radiation? A helium nucleus, an electron or positron, and a high-energy photon. Alpha is stopped by paper, beta by aluminium, gamma needs lead or concrete.
What is the difference between fission and fusion? Fission splits heavy nuclei; fusion joins light ones. Fusion powers stars and releases more energy per kilogram; fission powers reactors and bombs.
Who explained nuclear fission, and who got the Nobel? Lise Meitner and Otto Frisch explained it; Otto Hahn received the 1944 chemistry prize alone.
What is antimatter? Particles with opposite charge to their ordinary counterparts. Matter and antimatter annihilate on contact, converting entirely to energy.
What is the Higgs boson, and when was it confirmed? The particle associated with the field that gives other particles mass, confirmed at CERN in 2012.
What is the Standard Model? The theory describing all known particles and three of the four forces. It leaves out gravity, and it assumed neutrinos are massless, which turned out to be wrong.
Chemistry: the table
How many elements are known, and which are man-made? 118 confirmed. Everything above uranium, element 92, is made rather than found in nature.
Which element is most abundant in the universe, in Earth's crust, and in the human body? Hydrogen, oxygen and oxygen by mass — though by number of atoms the body is mostly hydrogen.
Which is the lightest and heaviest naturally occurring element? Hydrogen and uranium.
Which element is liquid at room temperature? Mercury among metals, bromine among non-metals. Gallium melts in your hand at about 30 °C.
Which metal is the best conductor of electricity? Silver, then copper, then gold. Copper wins commercially because silver is expensive and gold worse than both.
Which is the most abundant gas in the atmosphere? Nitrogen, at about 78 per cent, with oxygen at 21 and argon at nearly 1.
Which is the most reactive metal, and the most reactive non-metal? Caesium or francium among metals; fluorine among non-metals.
What are the noble gases, and why are they inert? Helium, neon, argon, krypton, xenon and radon. Their outer electron shells are full, so they have no chemical business with anybody.
Which element has the symbol that looks least like its name? Several: Fe iron from ferrum, Pb lead from plumbum, Au gold from aurum, Ag silver from argentum, Sn tin from stannum, Hg mercury from hydrargyrum, K potassium from kalium, Na sodium from natrium, W tungsten from wolfram, Sb antimony from stibium.
Who created the periodic table, and what made him right? Mendeleev, 1869 — and he left gaps and predicted the missing elements' properties, which is what turned a filing system into a theory.
What actually orders the table? Atomic number, the number of protons, established by Henry Moseley in 1913 — not atomic weight, which is why Mendeleev had to break his own rule in places.
Which element is named after India, and which after a country? None after India directly; but germanium, francium, polonium, americium, nihonium and ruthenium are all named after places, and gallium after France.
Which elements are named after scientists? Curium, einsteinium, fermium, mendelevium, bohrium, rutherfordium, meitnerium, seaborgium — Seaborg being the only one named while alive at the time.
Chemistry: reactions and compounds
What is the pH scale, and what does it measure? Hydrogen ion concentration, logarithmically. Each step is a factor of ten; 7 is neutral, below is acidic, above is alkaline.
What are the strong acids and strong bases to know? Hydrochloric, sulphuric and nitric acid; sodium and potassium hydroxide. Sulphuric acid is often called the king of chemicals because industrial output tracks it.
What is an oxidation and a reduction? Loss of electrons and gain of electrons. The mnemonic is OIL RIG — oxidation is loss, reduction is gain.
What is a catalyst? A substance that speeds a reaction without being consumed, by offering a lower-energy path. Enzymes are biological catalysts.
What is the difference between a mixture and a compound? A mixture keeps its components' properties and can be separated physically; a compound is a new substance with new properties, separable only by chemical means.
What is an alloy, and what are brass, bronze and steel? A metal mixed with another element. Brass is copper and zinc, bronze is copper and tin, steel is iron and carbon; stainless steel adds chromium.
What is heavy water, and what is it used for? Water with deuterium in place of hydrogen, used as a moderator in some nuclear reactors.
What are the formulas to know? Water H₂O, carbon dioxide CO₂, methane CH₄, ammonia NH₃, common salt NaCl, baking soda NaHCO₃, washing soda Na₂CO₃, plaster of Paris CaSO₄·½H₂O, quicklime CaO, slaked lime Ca(OH)₂, chalk and marble CaCO₃, laughing gas N₂O, vinegar CH₃COOH, sugar C₁₂H₂₂O₁₁, ethanol C₂H₅OH.
What is the noble gas used in balloons, in signs, and in bulbs? Helium, neon and argon.
What is dry ice? Solid carbon dioxide, which sublimes directly to gas at −78 °C without becoming liquid.
Which chemical is called the philosopher's wool, and which the king of chemicals? Zinc oxide and sulphuric acid.
What is aqua regia? A mixture of nitric and hydrochloric acid, one of the few things that dissolves gold. The name means royal water.
What is the difference between soap and detergent? Both have a water-loving head and a grease-loving tail; soap is made from natural fats and fails in hard water, detergents are synthetic and do not.
What causes acid rain, and what causes the greenhouse effect? Sulphur and nitrogen oxides from burning fossil fuels; carbon dioxide, methane, water vapour and nitrous oxide trapping outgoing infrared.
What destroys the ozone layer, and what fixed it? Chlorofluorocarbons; the Montreal Protocol of 1987, the most successful environmental treaty signed.
What is the difference between LPG and CNG? Liquefied petroleum gas is mostly propane and butane, stored as a liquid under moderate pressure; compressed natural gas is mostly methane, stored as a gas at high pressure.
Why is a diamond hard and graphite soft, when both are carbon? Diamond bonds each atom to four others in a rigid three-dimensional lattice; graphite bonds three in flat sheets that slide over each other.
What is the rarest naturally occurring element on Earth? Astatine — the total quantity in the crust at any moment is estimated in grams.
What comes next
The next page is biology and medicine, where the useful facts are the ones about your own body.