Every substance has both physical characteristics, such as color and luster, as well as chemical properties, such as flammability and reactivity. Physical characteristic or physical properties are those that are easily observed without changing the substance chemically. Chemical properties represent a substance's ability to undergo chemical changes and are observable only when these changes take place.
Common physical properties that are used to identify substances include luster, color, malleability, ductility and conductivity. Luster refers to the shininess of a substance. Metals tend to have luster, while non-metals tend to be dull. Malleability refers to the ability of a substance to be pounded into a shape, while ductility refers to its ability to be drawn into a wire. Conductivity is a physical characteristic representing a substance's ability to conduct electricity. Physical properties are divided into two groups: extensive and intensive properties. Extensive properties such as size and weight change when the amount of substance changes, while intensive properties, such as density and luster, are independent of the amount of substance.
Chemical characteristics of a substance include whether it reacts with certain other chemicals. For example, a property of iron is that it reacts with oxygen to form iron oxide, or rust. A chemical property of baking soda is its reaction with citric acid to form water, carbon dioxide and sodium citrate.