A solid is a sample of matter that retains its shape and density when not confined. The adjective solid describes the state, or condition, of matter having this property.
The atom s or molecule s of matter in the solid state are generally compressed as tightly as the repulsive forces among them will allow. Some solids, called crystalline solids, tend to fracture along defined surfaces that have a characteristic shape depending on the arrangement of, and the forces among, the atoms or molecules in the sample. Other solids, known as amorphous solids, lack any apparent crystalline structure.
Examples of solids are common table salt, table sugar, water ice, frozen carbon dioxide (dry ice), glass, rock, most metals, and wood.
When a solid is heated, the atoms or molecules gain kinetic energy . If the temperature becomes sufficiently high, this kinetic energy overcomes the forces that hold the atoms or molecules in place. Then the solid may become a liquid or a gas, or it may react with chemicals in the environment. Water ice is an example of a solid that becomes liquid when it is heated gradually. Dry ice sublimates directly into the gaseous phase. Wood combines with oxygen in the atmosphere, undergoing combustion.