In this topic you will learn about the differences between solids, liquids, and gases.

Any of the forms matter can exist in is called a state of matter. Most substances exist in one of three states—solid, liquid, or gas. Adding or removing heat can make a substance change from one state to another.

When a change of state occurs, the substance remains the same. When a solid absorbs heat, the molecules vibrate faster and faster. At some point the molecules separate from one another. This causes the solid to become a liquid. When even more heat is absorbed, the molecules move even further apart. This causes the liquid to become a gas.

The temperature at which a solid changes state into a liquid is the melting point. The temperature at which a liquid changes state into a gas is the boiling point. Every substance has its own particular melting point and boiling point. While a substance is melting or boiling, its temperature stays the same. It warms up only before or after the change of state. The temperature at which a liquid changes state into a solid is the freezing point.

Instead of boiling, a liquid can also slowly change to a gas. This process is called evaporation. When a liquid is at room temperature, a few molecules escape into the air. As the liquid is heated, more molecules escape, so the liquid evaporates faster.

When the temperature of a material is increased the particles move faster. In general these particles tend to spread out more. This causes materials to expand—spread out—as they get hotter or contract—shrink—as they get cooler. Gases expand or contract the most with changing temperature, but liquids and solids are also affected. Roads and bridges have expansion joints so they do not crack when heated.

Many of the objects around you are solids. You bathe in and drink liquids. Changes of state allow you to cook food and enjoy cool drinks and ice cream. Gases are what you breathe and what allow you to smell odors at a distance.