Classifying Matter by State

Matter is anything that has mass and occupies space. Everything around us is matter, including our bodies, the air we breathe, and the ground we walk on.

Matter exists in three main forms — solid, liquid, and gas. Scientists have also identified a fourth fundamental state of matter — plasma. 

Solids have a definite shape and volume. For example, take ice cubes in a glass. The ice cubes have a rigid shape, do not flow, and cannot be compressed. This rigidity is maintained because the particles in the ice are arranged close to each other, with strong interparticle forces holding them in place. These particles vibrate only slightly at their otherwise fixed positions, giving them fixed shape and volume. 

Liquids, on the other hand, have a fixed volume, but no definite shape. If the same amount of water is placed into another container, its shape will vary according to the container, unlike the ice cube. Like solids, liquids too cannot be compressed. Their particles are still close together but the interparticle forces in a liquid are weaker than in solids, meaning the particles are freer to move and slide past over each other. This gives liquids their fluidity.

Gases have no fixed shape or volume. Gases take the shape and volume of their container. For example, imagine air from a cylinder inflating a balloon. In a gas, the particles are very loosely held together, with little to almost no interparticle forces. As a result, they have freedom of shape and volume, can move rapidly, and are less dense than both solids and liquids.

Gases are highly compressible, due to the large empty spaces between their particles, meaning they can be confined inside a tiny balloon, or expanded to occupy a large room.

Plasma is the fourth state of matter — produced when gas gets heated to very high temperatures. It is comprised of electrically charged particles. Plasmas make up the interior of stars, auroras surrounding the poles, and fluorescent light bulbs.

The different types of matter are also capable of transitioning from one state to another, typically triggered by changes in temperature or pressure or both. For example, upon heating, solid ice melts to become liquid water. With further heating, water changes into a vapor. With cooling, these state changes can be reversed.