Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Liquid, one of the three physical states in which matter can exist. The other two states are solid and gaseous. The molecules of liquids are arranged less tightly than those of solids but more closely than those of gases. Liquids and gases take the shape of their container, unlike solids, which keep their own shape. Liquids and solids maintain a definite volume, or size, while gases will expand to fill a container. For example, a liter of liquid will not expand to fill a two-liter container, but a liter of gas will.

Most substances can exist in the liquid state at the right temperature and pressure. Water, for example, exists as a liquid at room temperature (20° C/68° F) and normal atmospheric pressure (the pressure of the atmosphere at sea level). Helium, on the other hand, is a gas under these conditions. It must be cooled to a very low temperature or compressed to a very high pressure to become a liquid. Iron is a solid at room temperature and normal pressure and must be heated to 1535° C (2795° F) to become a liquid. Only three chemical elements—bromine, gallium, and mercury–exist as liquids at room temperature and normal pressure. All other elements exist as either solids or gases under these conditions. Many compounds (combinations of elements) exist as liquids. Alcohol, gas, oil, and water are examples of compounds that are liquids at room temperature and normal pressure. Many familiar liquids, such as juice, milk, and soda, are water based, meaning they contain substances mixed with or dissolved in water.