 The three basic states of matter are solid, liquid, and gas. The main differences between these states of matter are the closeness of the molecules and the energy of the molecules. In solids, the molecules are packed tightly together and remain in a fixed position relative to each other. In the solid state, the molecules have the least amount of energy and move very little. In liquids, the molecules are close together but are not in a fixed position. In the liquid state, the molecules have more energy than in a solid, which allows the molecules to move around each other. In gases, the molecules are very far apart and move randomly. In the gas state, the molecules have the greatest amount of energy which keeps the molecules in constant motion. For changes of state to occur, the molecules will need to gain or lose energy, which affects the amount of movement and the closeness of the molecules to each other, thus determining the state. When heat is removed from a liquid, the molecules lose energy, which decreases the amount of movement and the molecules pack tightly together, forming a solid. This is called freezing. In the reverse process, called melting, heat is added to a solid, causing the molecules to gain energy, which increases the amount of movement and molecules move a little further apart, thus forming a liquid. When heat is added to a liquid, the molecules gain even more energy and the molecules move even faster and farther apart, thus forming a gas. This process is called vaporization. During the process of condensation, heat is removed from a gas to form a liquid. When energy is removed from a gas, the molecules do not move as fast and move closer together, forming the liquid. Under certain conditions, substances can change between the states of solid and gas without going through the liquid state. Sublimation occurs when a solid gains enough energy to be converted directly into a gas. If you have ever seen dry ice turn into a smoky looking fog, you have observed the sublimation of solid carbon dioxide into carbon dioxide gas. In the reverse process, called deposition, a gas loses enough energy to be converted directly to a solid. This occurs when frost forms on grass on a cold morning. The warm water vapor in the air comes in contact with the very cold grass, which turns the water vapor into solid ice crystals. Changes of state involve the loss or gain of energy, in turn changing the amount of movement and closeness of the molecules.