Matter is anything that has mass and takes up space. In a gas, the particles are far apart and move around on their own. A gas spreads out to fill the whole container it is in. A gas is easy to squeeze into a smaller space. The average kinetic energy of a gas molecule is directly proportional to its absolute temperature. Air, the gas around us, is matter. Gases take up space, which is why a balloon inflates when filled. Heating a gas speeds up its molecules. Gas pressure comes from molecules colliding with the walls of their container. The more gas molecules there are in a container, the more collisions there are with the walls. So the pressure is higher.

In a liquid, the particles are close together but can slide past one another. A liquid has a definite volume that stays the same whatever the size or shape of its container. Liquids have short-range order only: their molecules pack tightly but move rapidly past one another. As temperature rises, more molecules in a liquid have enough kinetic energy to escape into the gas phase. So its vapor pressure goes up. In a closed container, liquid and vapor reach a dynamic equilibrium: molecules keep moving both ways at equal rates, so the amounts stop changing. A liquid boils when its vapor pressure rises to equal the pressure of the air around it.

Scientists track phase changes with heating curves. On a heating curve, temperature climbs, then levels off in flat plateaus wherever the substance melts or boils. During a phase change, added heat goes into weakening the attractions between particles instead of making them move faster. Temperature stays constant while a substance changes state, even if heat keeps being added. In a pot on the stove, once water reaches its boiling point, more heat does not raise its temperature. The boiling just becomes more vigorous. The melting point of a solid and the freezing point of its liquid are the same temperature: where solid and liquid are in equilibrium. Gases can be turned into liquids by cooling them, and liquids into solids by cooling them more.

The triple point is the one temperature and pressure where the solid, liquid and gas phases of a substance exist together in equilibrium. Below water's triple-point pressure, water cannot be a liquid at any temperature. Carbon dioxide cannot be a liquid at normal air pressure, so dry ice never melts into liquid in the open air. Until 2019, the kelvin temperature unit was defined as 1/273.16 of the temperature of the triple point of water. Water's melting point falls slightly as pressure rises. This is unusual, because for most substances it rises. Above its critical temperature, a gas cannot be turned into a liquid no matter how much pressure is applied.

Most solids are crystalline, with particles in a regular pattern. A liquid that freezes before its molecules line up becomes an amorphous solid. Candle wax is an amorphous solid made of large hydrocarbon molecules. Rapidly cooling molten silicon dioxide makes an amorphous solid called fused silica. Glass is neither a liquid nor an ordinary solid. It is an amorphous solid, a state somewhere between the two. In a glass the atoms are not as ordered, which is what amorphous means. The myth that glass flows comes from old window panes that are thicker at the bottom than the top. For practical purposes, such as holding a drink, glass is like a solid, although a disorganized one.

Plasma is an ionized state of matter made of positively charged ions and negatively charged electrons. In plasma, some electrons separate from their atoms. These free electrons make plasma different from other states of matter. Unlike a gas, plasma can conduct electricity. Plasma makes up about 99 percent of the visible matter in the universe, much of it in space, such as in nebulae and stars. It is found in lightning strikes, certain television screens and stars, as well as in lab instruments. Ball lightning is one type of lightning that is plasma, and the aurora borealis is also caused by plasma. In a lab, scientists make plasma by blasting atoms with high-voltage electricity, lasers or electromagnetic fields.

Oobleck is a mixture of cornstarch and water that is a non-Newtonian fluid: the harder you stir it, the thicker it gets. It is named after a gooey substance that fell from the sky in a Dr. Seuss story. Hit with a hammer, oobleck hardens enough to shatter. A person can run across a pool of oobleck because the pressure of a foot hitting the surface makes the liquid thicken under it. Its thickening under force is called shear thickening, and it happens in mixtures of tiny solid particles in a fluid. Shear-thickening fluids are used in drilling mud and in one type of body armor that is flexible until a bullet's impact turns it solid.

On a global scale, the amount of water evaporating is about the same as the amount falling as precipitation. Over land, rain and snow usually exceed evaporation. Over the oceans, evaporation is more common than precipitation. About 90 percent of the moisture in the atmosphere came from oceans, seas, lakes and rivers. Evaporation takes heat away. When water on your skin evaporates, you feel cold. Cooling towers at power plants use evaporation to cool hot water before it goes back into the environment. Dew is made of tiny water drops that form by condensation on cool surfaces such as grass and spider webs. Eyeglass lenses fog up when you walk from a cold room into warm, humid air, because water vapor condenses on them.

Words to know

• kinetic: moving energy
• equilibrium: balance point
• amorphous: no set pattern
• crystalline: regular pattern
• plasma: charged gas
• condensation: gas to liquid