 The three states of matter. All matter exists in three forms. Solid, liquid, and gas. Each form is called a state. Gas. Air is composed primarily of nitrogen and oxygen gases. Solid. Calcium carbonate is a common solid on the earth's surface. Liquid. Water is the most common liquid on earth. Compressibility. It is extremely difficult to change the volume of solids and liquids. Even extreme pressures cause very little change in the volume of these states of matter. The volume of gases, on the other hand, can be changed easily. All that is required is a change in the pressure or temperature of the gas. Air can be forced into the rocket using a pump. When the pipe is released, the air escapes. Density. The densities of solids and liquids are measured in grams per milliliter. For example, water has a density of one gram per milliliter, while gold has a density of nineteen point three grams per milliliter. In contrast, gases have densities measured in grams per liter. That is, gases are roughly one thousand times less dense than solids and liquids. For example, oxygen has a density of one point three grams per liter at room temperature and pressure. Solids and liquids are more dense than gases, so they will fall through gases. Mixtures. Some liquids, such as oil and vinegar, do not mix together, but remain as two separate layers. Other liquids, such as water and alcohol, do mix together. Unlike liquids, gases always mix. Non-mixing liquids. When the oil and water are combined, the two liquids separate. Mixing gas. When the divider is removed, the two gases mix. Shape. Solids have distinct shapes and volumes. Liquids take on the shape of their containers, but maintain their volumes. Gases fill their containers completely. They have neither a distinct shape nor a distinct volume. Gas pressure. Because gases fill containers uniformly, they exert pressure uniformly in all directions. This explains why a balloon full of air is firm in all directions. Let's do a quick review. True or false? A gas can be compressed into a smaller volume. True. Gases can be compressed. The density of a sample of matter is 1.5 grams per milliliter. Is the matter a gas or is it a liquid or solid? It is a liquid or a solid. Solids and liquids have densities in the grams per milliliter range. True or false? Gases do not have fixed shapes. True. Gases take the shape of the container they are in. You have completed this activity, the three states of matter.