Identifying the materials we see ensures that we use them correctly. We do this by observing physical and chemical properties. Physical properties include such things as boiling point, color, density, hardnenss, melting point, odor, taste, and even electrical conductivity. Chemical properties are usually a measure of how a material reacts or fails to react with other substances. Will it burn? Does it dissolve in water? Does it produce bubbles of gas when dropped into acid? These allow us to tell the difference between substances with the use of our senses.