Everyday science, oriented

Temperature tracks how fast molecules move; for a gas, it measures their average energy of motion. Heat is energy on the move, and on its own it moves from the warmer object to the cooler one until the two temperatures match. How much heat changes hands depends on the temperature gap and on heat capacity, and water's is high: a kilogram of water takes 4,184 joules to warm 1 degree C, a kilogram of copper only 385, so water heats up and cools down slowly.

Heat travels three ways. Conduction needs contact, and metals conduct far better than air, so a metal railing draws heat from your hand much faster than air at the same temperature; how cold something feels tracks how fast your skin loses heat. Convection carries heat with a moving fluid, as an air fryer does with circulating hot air. Radiation crosses space as electromagnetic energy, which is why a campfire warms only the side of you facing it.

Once water boils, extra heat goes into making vapor, so a hotter burner cooks boiled food no faster; the water just boils off sooner. Evaporation draws heat from whatever the liquid sits on, which is how sweat and an alcohol wipe cool your skin. A refrigerator makes no cold at all: it uses electricity to move heat out of its box and into the room.

The common mistake treats cold as something wind adds. Say you read a forecast of 0 degrees F with a 15 mph wind: the wind chill is -19 degrees F, because wind pulls heat from bare skin faster. A parked car's radiator, though, only reaches 0 degrees F sooner; wind never cools it below the real air temperature.

Rule: track which way heat flows and how fast, because what feels cold is whatever pulls heat from you fastest.

A water molecule carries both hydrogen atoms on one side of its oxygen, so that side is slightly positive and the oxygen side slightly negative, and neighboring molecules attract. That pull, called cohesion, gathers water into drops and makes its surface act like a stretched elastic sheet that can hold up a carefully placed paper clip.

The same charges let water tug salt apart: its charged ends pull the sodium and chloride ions free and surround each one, which is what dissolving means. That is not melting. Melting is one substance changing state as it gains heat, while dissolving needs a second substance and leaves the salt in the solution. Oil behaves differently: it floats in its own layer unless an emulsifier such as egg yolk keeps the two blended.

Whatever dissolves changes how water behaves. Salt lowers the freezing point, and more salt lowers it further. Water crosses a membrane toward the saltier side, which is how salt on raw meat draws moisture out. Warm water holds less dissolved gas, so a cold soda stays fizzier, and the calcium in hard water leaves scale when heated and turns soap into scum.

The folk explanation, that salt melts ice by warming it, is wrong: pure water freezes at 32 degrees F, and road salt lets ice melt into salty water without getting any warmer. Air pressure sets the other end of the range. At sea level water boils at 212 degrees F, but at 7,500 feet it boils at about 198. Say you cook pasta there: the water is cooler than at sea level, so the pasta needs more time, and a bigger flame does not help.

Rule: when water acts strangely, look at how its molecules pull on each other and on whatever is dissolved in them.

Air has weight. At sea level the air above you presses about 14.7 pounds on each square inch, and you are not crushed because the fluids and air inside your body push outward just as hard. What you notice are differences. Climb and the pressure falls, to about 12.3 pounds per square inch at 5,000 feet, so air in your middle ear pushes the eardrum out until it evens out with a pop. Squeeze a trapped gas into three quarters of its space at a steady temperature and its pressure rises by about a third, which is Boyle's law.

Pressure applied to a sealed liquid passes through undiminished, and a piston's force is that pressure times its area: oil at 1,500 pounds per square inch on a two-square-inch piston pushes with 3,000 pounds. A fluid also pushes up on anything in it with the weight of the fluid it displaces. Ice, about 90 percent as dense as water, floats with roughly a tenth showing, and since it already displaces its own weight, a glass of ice water does not overflow as the cubes melt.

The common mistake is crediting suction, as if your mouth pulls a drink up a straw. Sucking only lowers the pressure inside the straw, and the atmosphere pressing on the drink's surface pushes it up. That push has a ceiling: at sea level it can hold up a water column at most 33.9 feet tall, and only with a perfect vacuum. Say you need a pump at the top of a pipe to raise water from 40 feet down. No suction pump can, however hard it runs, because the water was only ever pushed up from below.

Rule: when a fluid moves, look for the higher pressure pushing it, not a pull from the lower side.

Much household chemistry comes down to which particles grab which. A soap molecule has a head that dissolves in water and a tail that mixes with oil, and in water soap gathers into tiny clusters called micelles whose oily cores hold greasy dirt until rinsing carries it off. Soap and detergent are surfactants, short for surface-active agents: they lower water's surface tension, so soapy water spreads into a film where plain water would sit as a drop. Say you rinse a greasy pan under plain water: oil and water do not mix, so water alone does not take up the grease, but with soap the oily tails take it up and the micelles carry it off in the rinse.

Acidity is measured on the pH scale, 0 to 14 with 7 as neutral, and each whole step is a tenfold change, so pH 4 is a thousand times as acidic as pH 7. Baking soda is a base: when it meets vinegar, the two neutralize each other and release carbon dioxide, the fizz you see.

Oxidation means losing electrons. Iron rusts when its atoms give electrons to dissolved oxygen, which needs a thin film of water on the metal, and dissolved salt makes the rusting more aggressive. A battery splits its chemistry between two electrodes, so electrons travel through your device while ions cross the electrolyte. Fire needs fuel, oxygen and heat at the same time; take away any one and it goes out.

Among materials, metals carry current and heat readily while glass, plastic and dry wood resist current, and glue must wet a surface before it can grip.

A common mistake is thinking soap's main job is killing germs. Mostly it removes them: scrubbing loosens soil and microbes and rinsing washes them away. Killing is disinfecting, a separate job that household bleach does as an oxidizing agent, and the order matters, because dirt left behind makes it harder for a disinfectant to reach germs. Batteries are misread too: a battery does not hold electricity as such, but stores energy as chemical potential and turns it back into current.

Rule: name the particle doing the work, a soap molecule, an electron or an ion, and the household effect usually explains itself.

Food changes because molecules react, and warmth speeds those reactions up. Browning is two reactions, not one. In the Maillard reaction, sugars join the amino acids of proteins with no enzyme involved, and its products give bread, coffee and chocolate much of their color and flavor. Caramelization browns sugar on its own. Gas makes baked goods rise: yeast makes carbon dioxide slowly by fermentation, while baking soda releases it quickly when it meets an acid.

Microbes run on the same clock. At room temperature, the bacteria that cause foodborne illness can double every 20 minutes, and bacteria grow fastest between 40 and 140 degrees F, a range often called the danger zone. A refrigerator at 40 degrees F or below slows that growth, and freezing leaves microbes dormant rather than dead, so cold buys time rather than stopping change. Drying and salt curing work by pulling water out of food. Fermentation turns the idea around, fostering useful microbes whose acids and alcohol hold back the ones that would spoil the food.

A common mistake is trusting your nose to judge food. Spoilage bacteria cause off smells and textures, but pathogens, the kind behind illness, usually leave food looking and smelling normal, so a sniff test cannot tell you food is safe. Browning has its own myth, that it needs high heat. Say you boil one piece of dough and bake another. The boiled one stays pale, because the water keeps its surface wet and no hotter than about 100 degrees C at ordinary air pressure, where browning runs slowly; the baked crust dries out and browns at around 160 degrees C. Slow is not never: the Maillard reaction creeps along even in frozen meat.

Rule: ask how warm and how wet a food is, because those set the pace of both its chemistry and its microbes.

Light travels as a wave yet comes in packets of energy called photons, and each color is a different wavelength. Your eyes see only a narrow slice, from about 380 nanometers, seen as violet, to about 700, seen as red; a nanometer is a billionth of a meter. An object's color is what its material sends back after absorbing the rest: leaves look green because chlorophyll takes in red and blue light and reflects green.

The sky's colors come from scattering, and particle size decides which kind. Nitrogen and oxygen molecules are much smaller than light waves, and particles that small scatter short wavelengths far more than long ones, blue about four times as strongly as red, so blue reaches you from every part of the sky. Violet scatters even more, but your eyes are more sensitive to blue. Near sunset, light crosses much more air, so most of its blue is scattered out and what reaches you is reddened. Cloud droplets, about 0.02 millimeter across, are dozens of times wider than light waves and scatter all colors about equally, so clouds look white.

Two common beliefs are wrong here. Dust and smog are said to brighten sunsets, but haze in the lower air usually mutes them; clean air is the common ingredient of vivid ones. And mixing paint is not mixing light. Say you overlap a blue and a yellow spotlight on a wall: the wavelengths add, and the patch looks white. Mix blue and yellow paint and each pigment swallows part of the spectrum, blue taking the reds and yellows and yellow taking the blues and violets, so only green comes back.

Rule: ask whether light is being added, absorbed or scattered, and whether the particles are smaller or larger than its wavelength.

Light changes direction in two ways. It reflects: measured from a line perpendicular to the surface, a ray leaves at the same angle it arrived, so a mirror's image appears as far behind the glass as the object stands in front. And it refracts: light moves more slowly in water than in air, so a ray crossing the surface at a slant bends. Your eye traces the bent rays straight back, which is why a straw looks broken at the waterline and a lake bottom looks shallower than it is. Lenses put refraction to work: glasses bend light so it focuses on the retina, a concave lens for a nearsighted eye that focuses in front of it.

The sky runs the same optics. Each raindrop bends sunlight going in, reflects it off its curved back and bends it again leaving, bunching much of the light about 42 degrees opposite the sun, and because each color bends a slightly different amount, the colors separate into a bow. You see one only with the sun behind you and rain ahead. A second reflection inside the drops makes a secondary bow at about 50 degrees with its colors reversed. Ice crystals in thin cirrus cloud refract light into halos, and into sun dogs about 22 degrees to either side of the sun. Over hot sand, heated air bends low-angle light from a distant object back upward, so you see an image much like a reflection in a pool, with no water there.

The folk explanation says a mirror flips left and right. Say you raise your left hand to one: its image sits directly across the glass from that hand, and your nose, nearer the glass than the back of your head, has its image nearer too. What the mirror actually reverses is toward and away.

Rule: trace the ray, where it bends, where it bounces, and where your eye assumes it came from.

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are one kind of radiation, differing in wavelength, the distance between crests, and shorter waves carry more energy per photon. Warm bodies give off infrared, so a thermal camera shows emitted heat rather than reflected light. The ultraviolet B band causes sunburn; plain window glass blocks almost all of it but only about a fifth of ultraviolet A. The line between ionizing radiation, which can strip electrons from atoms, and the rest falls inside the ultraviolet, so microwave and radio photons carry too little energy each, and microwaves do not make food radioactive.

A microwave oven's waves bounce off its metal walls, pass through glass, ceramic and paper, and are absorbed by the water, fat and sugar in food, reaching only about 1 to 1.5 inches in. The folk explanation, that a microwave cooks from the inside out, is wrong: the middle of a thick piece heats by conduction from the outer layers. The reflections also overlap into a standing wave with strong and weak spots, which is why a turntable carries food through the pattern.

Sound is a vibration passed from molecule to molecule, so it cannot cross the vacuum of space the way light can. Pitch tracks frequency, and loudness mainly tracks how wide each vibration swings. Decibels run on a logarithmic scale. Say you run a machine that reads 80 decibels and switch on a second, equally loud one: the reading rises to about 83, not 160, because doubling the sound power adds only about 3 decibels. Sound is also slow next to light: thunder takes about 5 seconds to travel a mile, so a 15-second gap from flash to bang puts the lightning about 3 miles away.

Rule: identify the wave and its wavelength or frequency, then ask what it passes through, what absorbs it, and how much energy each piece carries.

Electricity flows in closed circuits. Voltage is the push, current is the flow, and resistance, measured in ohms, opposes it; Ohm's law says voltage equals current times resistance, so 12 volts across 6 ohms drives 2 amps. Nothing flows without a complete path, which is why a bird perched on one power line and touching nothing else is not shocked. A shock comes when your body links a live wire to the ground, and fuses and breakers open the circuit when current runs high enough to overheat the wiring.

Electricity and magnetism are tied together three ways. Current through a coil makes a magnetic field, an electromagnet, which grows stronger as you raise the current, add turns of wire or insert an iron core. A wire carrying current inside a magnetic field is pushed sideways, which a motor uses to turn its rotor. And a magnet moving relative to a coil induces a current, the principle behind the generators that make nearly all US electricity. A changing field in one coil induces a voltage in a nearby one, so a transformer's turns ratio steps alternating current up or down. Radio rides on waves: amplitude modulation (AM) varies a wave's strength and frequency modulation (FM) its frequency, and since most static changes strength, FM resists it better.

A common mistake pictures a compass needle pointing at a magnetic pole. It actually lines up with the horizontal part of Earth's field where you stand, and at most places that differs from true north by an angle called declination, which varies with place and time.

Units cause the other everyday mix-up: watts measure a rate, and kilowatt-hours measure an amount. Say you leave a 40-watt bulb on for five hours. It uses 200 watt-hours, or 0.2 kilowatt-hours, the unit utilities generally meter.

Rule: look for the closed loop and the changing field, because current needs the first and induction needs the second.

Something moving keeps going straight at the same speed unless a net force acts on it, which is why an unbelted passenger keeps going when a car stops suddenly. Newton's second law says acceleration grows with force and shrinks with mass, so the same push starts and stops a loaded cart more slowly than an empty one. His third law says forces come in pairs, equal and opposite, acting on different objects, so the two never cancel each other: a resting book balances because two forces act on the book itself, its weight and the table's push.

Two folk explanations get these laws wrong. The outward pull you feel in a car on a curve is only apparent; moving in a circle needs a force toward the center, and a whirled model plane whose string is cut flies off along a tangent, not straight outward. Heavier objects do not fall faster either: weight grows with mass exactly as resistance to acceleration does, so the mass cancels, and with no air a beach ball and an airliner fall together.

Machines trade force for distance, because they cannot create energy. Work is force times the distance moved, so a ramp lets you push less by pushing farther. Say you lift a load hung from a block and tackle with six rope sections. A 50-pound pull can ideally hold 300 pounds, but you must haul six feet of rope to raise it one foot. Turning works the same way: torque is force times its perpendicular distance from the pivot, so a longer wrench needs less push, and a push aimed straight at a door's hinge turns nothing.

Rule: for any push, ask what it acts on and over what distance or lever arm.

Much of the weather you can see comes from air cooling to its dew point, the temperature at which it would reach 100 percent relative humidity with its moisture unchanged. A windshield that cools below it collects dew, and air cooled further sheds water as fog. Rising air expands and cools, about 5.5 degrees F for every 1,000 feet while unsaturated, until vapor condenses on specks of dust, smoke or sea salt; a cumulus cloud's flat underside marks the height where that begins.

Relative humidity misleads, because the amount of moisture present at saturation depends on temperature. Say you compare a morning at 30 degrees F with a 30-degree dew point and a summer afternoon at 80 degrees F with a 60-degree dew point. The morning shows 100 percent and the afternoon only 50, yet the afternoon feels far more humid; the dew point says more about the moisture actually there.

The sky's cycles come from geometry. Earth's axis tilts about 23.5 degrees, so each hemisphere takes a turn leaning toward the sun, with a high sun angle and long days. The common mistake blames distance, but Earth comes closest to the sun in January, during Northern Hemisphere winter. Moon phases are geometry too: the sun always lights half the moon, and over about 29.5 days you see different amounts of that lit half. Earth's shadow plays no part; it touches the moon only during a lunar eclipse, at full moon. Eclipses are rare because the moon's orbit tilts about 5 degrees from Earth's, so most months the three do not line up. And the moon's uneven pull raises two ocean bulges, one toward it and one away, so most coasts get two high tides a day.

Rule: for weather, compare the air's temperature with its dew point; for the sky, draw the angles before blaming distance or shadow.

Your body runs on sensors and automatic responses, and many everyday sensations are their side effects. The hypothalamus, a brain region that works like a thermostat, makes muscles shiver to add heat in the cold and sends out sweat in the heat, and sweat cools you only as it evaporates. Your ears pop when swallowing or chewing opens the Eustachian tube, the narrow passage from the middle ear to the upper throat, letting pressure even out across the eardrum. After a steady spin, stopping bends the inner ear's motion sensors the other way, so you feel yourself turning while standing still.

Two popular explanations are wrong. Muscle soreness that peaks one to three days after hard exercise cannot be lactic acid, whose levels return to normal within an hour. And fingers wrinkle in warm water through an active, nerve-driven narrowing of blood vessels, not by soaking up water: skin whose nerves are cut or blocked does not wrinkle in the same soak.

Perception is the brain interpreting what the senses report, and an illusion is a percept that differs from the physical stimulus. The rising moon looks larger, yet photos taken with the same zoom show it as wide as the moon overhead. Much of what you call taste is smell: chewing releases odors that travel from the roof of the throat to the nose, so flavor fades when a cold blocks that channel. And the smallest change you can notice grows with the starting intensity. Say you drop an apple into a nearly empty grocery bag, then into a full one: the added weight that is obvious in the light bag may go unfelt in the heavy one.

Rule: when a sensation surprises you, ask which sensor produced it and what your brain added in interpreting it.