Each substance has properties that allow it to be distinguished from others. These properties can be classified into two types: physical and chemical.
Some important physical properties are: density, melting point, boiling point, electrical and thermal conductivity, hardness, color. These properties depend on the state of aggregation in which the substance is (solid, liquid or gas). The density of solid water and gas are very different.
The chemical properties are related to the reactivity of each substance. A chemical property of sodium is its reaction with water. It is a redox process in which protons in water are converted to hydrogen and sodium to sodium cation. Magnesium oxidizes in the presence of oxygen to give magnesium oxide, it is a chemical property of magnesium.