Matter is anything that has mass and takes up space. Solids, liquids and gases are the three states of matter commonly found on Earth. All three are made of tiny particles such as atoms and molecules. Yet matter can act like more than one state at once. A cloud is a mixture of air, a gas, and tiny drops of water. The drops are liquid or solid. Sand can be poured like a liquid, even though each grain of it is a solid. In zero gravity, a liquid forms a ball shape instead of settling flat.

Scientists describe the state of a substance as a balance between two things. On one side is the kinetic energy of its particles, which keeps them apart and moving. On the other side are the forces that pull them together. In a solid, the tiny particles are packed tightly together, often in a regular pattern. They vibrate in place. 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. Heating a gas speeds up its molecules. Gas pressure comes from molecules colliding with the walls of their container.

Temperature stays constant while a substance changes state, even if heat keeps being added. In a pot on the stove, once water gets to 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. Melting is a solid changing into a liquid. Freezing is a liquid changing into a solid. Gases can be turned into liquids by cooling them. Liquids can be turned into solids by cooling them more. Solids can expand and contract a little when the temperature changes.

Atmospheric pressure is caused by the weight of the column of air molecules above an object. At sea level, it is about the same as a full-grown African elephant standing on a doormat. 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. At normal air pressure, water boils at 100 degrees Celsius. High up in the mountains, it boils lower, at about 90 degrees in Leadville, Colorado, elevation 10,200 feet.

Density is the mass of a substance in a chosen amount, or volume, of it. Water's density is about 1 gram per milliliter. Upon freezing, it decreases by about 9 percent. So ice is about 90 percent as dense as liquid water. Ice has its water molecules lined up in a regular lattice that is more spread out than the random arrangement of liquid water. 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. Ocean water is denser than pure water because it has salt dissolved in it. About 10 percent of an iceberg shows above the water, and about 90 percent is below.

Evaporation happens when heat energy makes water molecules break free of their liquid. It takes heat away: when water on your skin evaporates, you feel cold. Water vapor condenses on cold surfaces, and as it turns back into liquid it releases heat into the air. 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. This is because water vapor condenses on them. Frost forms by deposition, when water vapor turns directly into ice.

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. Plasma makes up about 99 percent of the visible matter in the universe. Much of it is 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

• kinetic: moving energy
• pressure: pressing weight
• density: mass in volume
• lattice: regular pattern
• plasma: charged gas
• evaporation: liquid to gas