Is glass really a very slow liquid? The myth that glass flows comes from old window panes that are thicker at the bottom than the top. In fact, glass is neither a liquid nor an ordinary solid. It is an amorphous solid, a state somewhere between the two. Most solids are crystalline, with particles in a regular pattern. A liquid that freezes before its molecules line up becomes an amorphous solid instead. Candle wax is an amorphous solid made of large hydrocarbon molecules. The particles in a solid vibrate in place, but they do not move around past one another.

In a liquid, the particles are close together but can slide past one another. In a gas, the particles are far apart and move around on their own. Scientists describe the state of a substance as a balance between the kinetic energy of its particles, which keeps them apart and moving, and the forces that pull them together. The average kinetic energy of a gas molecule is directly proportional to its absolute temperature. 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, and the higher the pressure.

Pressure is force divided by the area it pushes on, so the same force pushes harder on a smaller area. A liquid boils when its vapor pressure rises to equal the pressure of the air around it. The normal boiling point of a liquid is its boiling point when the surrounding pressure is 1 atmosphere, or 101.3 kPa. A phase diagram plots pressure against temperature and shows which state a substance is in under each condition. The line separating liquid from gas gives the boiling point at any pressure. High up in the mountains, water boils below 100 degrees Celsius, at about 90 degrees in Leadville, Colorado. The air pressure there is about 68 kPa.

The melting point of a solid and the freezing point of its liquid are the same temperature: where solid and liquid are in equilibrium. 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. While ice is melting in water, the mixture stays at 0 degrees Celsius. A mixture of ice and water in a very good thermos bottle stays as ice and water for hours, because melting and freezing happen at equal rates.

Ice floats because it is less dense than liquid water, but in most other substances the solid is denser than the liquid. Water expands when it freezes, so ice takes up more room than the liquid water it came from. Water's density is highest at about 4 degrees Celsius. So cold water at 4 degrees sinks below water at 0 degrees in a lake. If water were densest at its freezing point, a lake could freeze from the bottom up. A hydrometer measures a liquid's density: it floats lower in less dense liquids and higher in more dense ones. Heavy ice made from heavy water, D2O, is about 10.6 percent denser than normal water. So it sinks in a glass of normal water.

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. When liquid water is heated enough, it turns into a gas called steam. Evaporation happens when heat energy makes water molecules break free of their liquid, and it takes heat away. Cooling towers at power plants use evaporation to cool hot water before it goes back into the environment.

Plasma is an ionized state of matter made of positively charged ions and negatively charged electrons. In plasma, some electrons separate from their atoms, and 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. The aurora borealis is also caused by plasma. The Sun is made of super-hot, electrically charged gas called plasma.

Words to know

• amorphous: no set pattern
• crystalline: regular pattern
• equilibrium: balance point
• hydrometer: density gauge
• plasma: charged gas
• kinetic: moving energy