The kinetic theory of matter assumes that all matter is made up of tiny, ultimate particles of matter called molecules. A molecule is defined as the smallest particle of a compound, or a gaseous element, that can exist and still retain the characteristic properties of that substance. Molecules interact, attracting each other through a force of cohesion. Liquids, solids, and gases are the phases of matter that are explained by the molecular arrangements and forces of attraction between their molecules. A solid has a definite shape and volume because it has molecules that vibrate in a fixed equilibrium position with strong cohesive forces. A liquid has molecules that have cohesive forces strong enough to give it a definite volume but not strong enough to give it a definite shape. The molecules of a liquid can flow, rolling over each other. A gas is composed of molecules that are far apart, with weak cohesive forces. Gas molecules move freely in a constant, random motion.

The temperature of an object is related to the average kinetic energy of the molecules making up the object. A measure of temperature tells how hot or cold an object is on two arbitrary scales, the Fahrenheit scale and the Celsius scale. The absolute scale, or Kelvin scale, has the coldest temperature possible (-273°C) as zero (0 K).

The observable potential and kinetic energy of an object is the externalenergy of that object, while the potential and kinetic energy of the molecules making up the object is the internal energy of the object.Heat refers to the total internal energy and is a transfer of energy that takes place (1) because of a temperature difference between two objects, or (2) because of an energy-form conversion. An energy-form conversion is actually an energy conversion involving work at the molecular level, so all energy transfers involve heating and working.

A quantity of heat can be measured in joules (a unit of work or energy) or calories (a unit of heat). A kilocalorie is 1,000 calories, another unit of heat. A Btu, or British thermal unit, is the English system unit of heat. The mechanical equivalent of heat is 4,184 J = 1 kcal.

The specific heat of a substance is the amount of energy (or heat) needed to increase the temperature of 1 gram of a substance 1 degree Celsius. The specific heat of various substances is not the same because the molecular structure of each substance is different.