Science trivia, oriented

Every atom has a nucleus of protons and neutrons, with electrons around it. The number of protons, the atomic number, decides the element: six protons make carbon and eight make oxygen. Atoms of one element with different numbers of neutrons are isotopes, such as carbon-12 and carbon-14, and the unstable ones decay with a half-life, the time it takes their activity to fall by half.

The periodic table lists the elements by atomic number, in rows called periods and columns called groups. Elements in a group behave alike: group 1, below hydrogen, holds the soft, reactive alkali metals, group 17 the halogens, and group 18 the very unreactive noble gases. The transition metals, including iron, copper, silver, and gold, fill the middle. Each element has a symbol of one or two letters with only the first letter capitalized, so Co is the element cobalt, while a second capital letter would mean two separate elements.

Elements combine into compounds in fixed ratios: water is always two hydrogen atoms bonded to one oxygen atom, and table salt is sodium chloride, sodium and chlorine one to one. A solution is a uniform mixture, with a solute dissolved in a solvent. The pH scale measures acidity from 0 to 14, with 7 neutral, and each step is a tenfold change. Oxidation is the loss of electrons and reduction the gain, and the two always happen together.

Rule: when an answer hinges on an element, start from its atomic number and its column, because much of its behavior follows from where it sits on the table.

Science measures in SI units. The seven base units are the meter, kilogram, second, ampere, kelvin, mole, and candela, and each is now defined through a fixed constant of nature, such as the speed of light for the meter. Prefixes scale them by powers of ten: kilo is a thousand, milli a thousandth, giga a billion, and nano a billionth.

Newton's laws describe how forces change motion. Without a net force, a resting object stays put and a moving one carries on at the same speed and direction; acceleration is proportional to the net force and inversely proportional to mass; and every force has an equal and opposite partner. Weight is a force, the pull of gravity on a mass, so an astronaut on the Moon keeps the same mass but weighs about a sixth as much. Momentum is conserved in every collision, while kinetic energy is conserved only in elastic ones.

Energy comes as kinetic energy of motion and potential energy of position, and the first law of thermodynamics says the total is conserved. The second law says heat flows on its own from hot to cold, and entropy measures the energy no longer available for work. Heat travels by conduction through materials, by convection in moving fluids, and by radiation, which needs no medium at all. At normal pressure water freezes at 0 °C, or 32 °F, and boils at 100 °C, or 212 °F, and absolute zero, about −273 °C, is the bottom of every absolute scale.

Rule: separate a quantity from its unit and from the law that governs it, because many trap answers swap mass for weight or heat for temperature.

Electric charge comes in two kinds: electrons are negative and protons positive, and unlike charges attract while like charges repel. A conductor such as copper lets charge flow freely. Ohm's law ties a circuit together: voltage equals current times resistance, so a higher resistance means less current for the same voltage. Moving a conductor through a magnetic field, or changing the field around it, induces a voltage; with no relative motion there is none.

Light behaves both as a wave and as a stream of photons, packets of energy with no mass. Visible colors run from violet, the shortest wavelength at about 380 nanometers, to red, the longest at about 700. Light reflects off smooth surfaces, refracts as it changes speed passing from one material into another, and diffracts around obstacles, and in a vacuum it travels at exactly 299,792,458 meters per second. Sound, by contrast, is a pressure wave carried through a medium such as air, and a moving source changes the pitch you hear as it passes, the Doppler effect.

Einstein's E = mc² says mass and energy are equivalent, which is why a tiny amount of mass releases enormous energy in nuclear fusion. Nothing with mass can reach the speed of light, because the energy needed grows without limit. Antimatter particles are partners of ordinary ones with the opposite charge.

Rule: for a wave question, find the wavelength first, since color and pitch both follow from it.

In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, about eleven times Earth's diameter, and all four giant planets have rings, not Saturn alone. The classic trap is temperature: Venus, not Mercury, is the hottest planet, because its thick atmosphere holds the heat in. Earth has one moon, Mars two small ones, and Mercury and Venus none.

Earth's seasons come from the tilt of its axis, 23.4°, not from its distance to the Sun; Earth is in fact closest to the Sun in January. A solar eclipse happens when the Moon gets in the way of the Sun's light and casts its shadow on Earth, and a lunar eclipse when Earth's shadow falls on the Moon. The daytime sky is blue because the air scatters blue light more than other colors. A space rock that burns up in the atmosphere is a meteor, and one that reaches the ground is a meteorite.

Stars shine by fusing hydrogen into helium in their cores. A star like the Sun ends by shedding its outer layers as a planetary nebula and leaving a white dwarf; a massive star explodes as a supernova, leaving a neutron star or a black hole, from which not even light escapes. Our galaxy, the Milky Way, is a spiral more than 100,000 light-years across, and the Big Bang theory holds that the universe began hot and dense and has been expanding ever since.

Rule: when a question offers an obvious answer, check the classic trap first: Venus is hotter than Mercury, and seasons come from tilt, not distance.

Earth is layered like an egg: a thin, rigid crust, a mantle about 2,900 km thick, and a core whose liquid outer part spins to create the magnetic field. The outer layer is broken into plates, and most earthquakes and volcanoes occur where plates meet. A seismograph records the shaking, and magnitudes are now given on the moment magnitude scale, which replaced the Richter scale.

Rocks form in three ways. Igneous rock solidifies from molten rock, called magma underground and lava once it erupts. Sedimentary rock builds up from layers of sediment that are buried and cemented. Metamorphic rock is older rock changed by heat and pressure. Minerals are ranked by hardness on the Mohs scale, from talc at 1 to diamond at 10, and fossils preserve evidence of ancient life.

The atmosphere is mostly nitrogen and oxygen, and its lowest layer, the troposphere, is where weather happens; above it, the stratosphere holds the ozone that blocks much of the Sun's ultraviolet light. Water evaporates, condenses into clouds, and falls as precipitation. Hurricanes are rated from 1 to 5 on the Saffir-Simpson scale, and tornadoes on the Fujita scale. One global ocean covers about 71 percent of the surface and holds about 97 percent of Earth's water, and its tides come mainly from the Moon's gravity, with the Sun adding about half as much.

Rule: place every Earth fact by layer, from the core out to the atmosphere, and the right answer is usually the one at the right level.

DNA, deoxyribonucleic acid, carries an organism's genetic instructions in a double helix whose four bases pair adenine with thymine and cytosine with guanine. A gene is a stretch of DNA, and humans have about 20,000 genes that code for proteins. Each person inherits one allele of a gene from each parent; a dominant allele shows in the trait, while a recessive one shows only when two copies are present. Human body cells carry 46 chromosomes in 23 pairs, and the X and Y chromosomes determine sex.

Inside the cell, ribosomes build proteins from chains of the twenty amino acids, and mitochondria produce most of the cell's energy. Mitosis copies one cell into two identical ones, while meiosis makes egg and sperm cells, each with a single set of chromosomes.

The human body divides its work among systems. The heart's four chambers pump blood, the kidneys filter it, and the pancreas releases insulin to control blood sugar. Hormones act as chemical messengers, the brain and spinal cord form the central nervous system, the skin is the largest organ, and the adult skeleton has 206 bones.

Across living things, producers such as green plants make their own food from sunlight using chlorophyll, consumers eat other organisms, and decomposers break down the dead. Vertebrates have backbones; mammals feed their young milk, while reptiles and amphibians are cold-blooded and rely on their surroundings for heat.

Rule: tie each fact to its level, from molecule to cell to organ to organism, because many wrong options borrow the right word from the wrong level.