Every substance is made up of very small particles called molecules. A molecule is the smallest particle of matter that exists independently. It exhibits all the properties of that substance. For example, a molecule of sugar exhibits all the properties of sugar. Molecules of different substances differ in size. Some are only few billionths of a centimetre in size while some molecules are a thousand times larger. Did you know that one cubic centimetre of air contains about 2.5 × 1019 (25,000,000,000,000,000,000) molecules? Even though a tiny piece of matter contains a very large number of molecules, there are vast empty spaces in between them. Molecules of matter are constantly in random motion. As the temperature increases, the motion of the molecules also increases. The hotter the substance, the faster the motion of the molecules. Molecules of gases move very fast in comparison to the molecules of liquids and solids. Even the molecules in ice are in motion. Now you must be surprised that even though molecules of all substances are in motion, why don’t the substances appear shaky? The reason is that there exists a force of attraction between the molecules, which binds them together. If this force did not exist, the molecules of matter would have scattered in all directions. When the molecules of a solid are heated, their speed increases and the force of attraction between them decreases If the heating continues, the speed of the molecules will increase and the solid will change into liquid and, finally, into gas. The molecules of solids are very closely packed; the molecules in liquids are less closely packed while those of gases are quite widely spreaded.