A Physics Digest—Part 2: Properties of Matter

Matter is what physics is all about—in contrast to mind and spirit (which are the province of other specialties). All matter is composed of identical, infinitesimal building blocks: quarks and leptons. They come together to form a little bit bigger building blocks we call electrons, protons, and neutrons—which then form atoms. Everything in this wide universe is made up of only about a hundred kinds of atoms, arranged in endless possibilities. It’s all in how it’s assembled.

Of those 100 kinds of atoms, only about a dozen are used in the common stuff we see every day (most commonly carbon). When the universe was very young it consisted almost exclusively of hydrogen atoms—which are the simplest: one proton, one electron. Gradually, over time, other kinds of atoms got manufactured in the explosive belly of stars. We are stardust!

Atoms are ageless—once created, they’re pretty much forever. They move through us continually. Oxygen atoms you inhale today were once inhaled by Jesus (or maybe a gnat on his neck). Atoms are mostly nothing—each one is the tiniest speck of a nucleus, surrounded by vast space. We’re mostly nothing!

Matter comes in three basic flavors: solids, liquids, and gases. When atoms combine into molecules and get firmly locked into a structure, we call it a solid. When the molecules are ordered in a precise fashion, we call it a crystal—like the sodium chloride molecules in salt. An electrical attraction between atoms and molecules in a solid binds them tightly together. Solids possess density (how much mass is squeezed into a given volume)—thus the iron atoms in steel are heavier and packed together more tightly than the carbon atoms in wood.

Some solids also possess elasticity: the ability to distort under force and then bounce back, once the force is removed. But inelastic solids, like a lump of clay, get bent and stay bent.

In liquids the molecules are not fixed—there’s less electrical attraction between them—so they slide over each other and can assume the shape of the container they’re in. Analogous to the weight of a solid is pressure in a liquid: the force that a liquid exerts on the surface that contains it. Liquids also possess density; thus less dense oil will float on denser water. Useful properties of a liquid are hydraulics (a property of pressure) and capillarity (which allows trees to suck up water).

The third flavor of matter is gas—in which the molecules are even freer from each other. A gas will expand to fill its container, as its molecules spread out. But gas does have weight (also expressed as pressure), so we experience atmospheric pressure, when all that air piles up above us. Gas can also float less dense objects, such as air floating a helium-filled balloon. Gases can exert forces: hold your hand out of a moving car’s window and you’ll feel it.

The force of gas also holds up an airplane—which is far denser than air. How? Air flowing over the curved top of a wing must travel farther (and thus faster) than that across the flat bottom of the wing. Bernoulli showed us that the faster air moves, the lower is its pressure; so more pressure (force) is applied to the bottom of the wing, lifting it—along with the solid airplane attached to it.