All material must exist in one of the three forms of matter—a solid, a liquid, or a gas. These are different physical states of being and each form has implications for the substance in question.

When we consider chemical substances most can exist in any of the three states. Which state of matter is encountered depends upon the physical conditions that they are being studied under. If the conditions are not specified then standard temperature and pressure is assumed. As a result of this it can be said that sodium chloride (table salt) is a solid. What in fact should be said is that at a pressure of one atmosphere (1.013 x 10⁵Nm ²) and a temperature of 273.15 K (32°F or 0°C) sodium chloride is a solid.

The three states of matter have different ways of responding to changes in temperature and...