What matter is

An astronaut on the Moon weighs one sixth of what she weighs on Earth, but her mass does not change at all.

Chemists define matter in one short sentence: anything that takes up space and has mass. A brick is matter, and so is the water in a glass. Air is matter too, even though you cannot see it. Blow into a balloon and it swells, because the air you push in needs room. Light, sound and heat are not matter. They carry energy, but they have no mass of their own and do not fill a container.

Mass is not the same as weight

Mass measures how much matter an object contains. You can find it with a balance by comparing the object with standard masses, or by seeing how much force it takes to speed the object up: a car needs far more push than a bicycle. Weight is the pull of gravity on that mass. It is proportional to the mass, but it depends on where you are. The Moon's gravity is about one sixth of Earth's, so an astronaut there weighs about one sixth as much. Her mass, the amount of her, stays exactly the same. In orbit she may feel weightless, but she is never massless.

Three everyday states

On Earth, matter is usually found in one of three states. A solid is rigid and keeps its own shape; a spoon stays a spoon whether it lies in a drawer or a cup. A liquid flows and takes the shape of its container, but gravity gives it a flat upper surface. In orbit, where the water and the spacecraft fall freely together, nothing pulls it flat, and a blob of water pulls itself into a sphere. A gas fills its whole container, taking both its shape and its volume. Squeeze a solid or a liquid and its volume hardly changes; squeeze a gas and it shrinks a lot.

There is a fourth state that is rare on Earth but common in the universe. A plasma is a gas in which many of the particles carry an electric charge. Stars are made of it, and you also see it in lightning, in plasma cutting torches and in some older television screens. Some things look as if they belong to more than one state. Sand pours like a liquid, but each grain is a solid. A cloud drifts like a gas, but it is air mixed with tiny drops of liquid water or bits of ice.

Matter is not lost when it changes

When ice melts or water boils, the substance does not appear or disappear. The total amount of matter stays the same through a change of state and through a chemical reaction. This is the law of conservation of matter. It is hard to see in daily life because some of the products usually escape as gases, but in a sealed container the balance shows no change. A kettle seems to lose water as it boils; the water is still there, spread through the kitchen as vapour.

The model in this course explains all of this with particles: atoms or molecules that are always moving. In a solid they are held in fixed places and can only vibrate. In a liquid they still touch but slide past one another. In a gas they are far apart and fly in straight lines until they hit something. Temperature tells you how fast they move. The kinetic-molecular theory states it exactly: the average kinetic energy of the particles is proportional to the temperature measured in kelvins.