Matter, or everything around us is composed of small particles called atoms, ions, and molecules. These particles are in constant motion and, therefore, possess kinetic energy. The average kinetic energy pf a group of particles determines the group's temperature. The temperature of a sample of matter is a measure of the average kinetic energy of the particles that make up the sample. If we add energy in the form of heat to an object, the kinetic energy of its particles increases. The greater the average kinetic energy of the particles, the higher the temperature of the material.

A thermometer is used to measure temperature. When the bulb is heated, the liquid inside expands and rises in the tube. When the bulb is cooled, the liquid contracts and the height of the liquid column decreases. The height of the liquid column can thus be used to measure temperature. The temperature can be read directly from the scale on the tube.

The SI unit of temperature is the kelvin (K). The kelvin has a direct connection with more familiar unit, the Celsius degree (C^o).

The Celsius temperature scale is based on the fact that the freezing and boiling temperatures of pure water under normal atmospheric pressure are constant. The difference between the boiling and freezing points of water is divided into 100 equal intervals. Each interval is called a Celsius degree. The point at which water freezes is labeled zero degrees Celsius (0^oC). The point at which water boils is labeled 100^oC. The size of a Celsius degree is exactly equal to that of a kelvin. Thus, there are also 100 kelvins between the freezing point and boiling point of water. Note that Celsius temperatures are expressed in ^oC, whereas temperature intervals and changes are expressed in C^o.