Short Notes:

Of all the substances on Earth, only water can exist as all states of matter under natural conditions, that is, as a gas, a liquid, or a solid. But no matter which state water is in, each of its molecules consists of one atom of oxygen and two atoms of hydrogen: only its molecular behaviour changes. When water freezes, it becomes a solid. The water molecules line up with one another to from rigid structures giving ice a low density. The molecules cannot move freely because the inter-molecular forces holding them together are greater than the kinetic energy available to propel them into action. When the temperature rises to 0 °C, the heat loosens the forces holding the molecules together, and the molecules slip freely past one another, flowing as a liquid.

Although the molecules retain a weak inter-molecular attachment, the kinetic energy supplied by the heat keeps the molecules in constant motion. If the temperature increases further, the movement of the molecules becomes even livelier.

Finally, at 100 °C, the kinetic energy completely overcomes the energy of the inter-molecular forces, and the molecules break apart, escaping into the air. The liquid has become a gas. Water vapour, as with any gas, has the lowest density of any state of matter. If water is contained in a tight plastic bag, it will expand and break the bag when it turns into vapour. This happens because the volume of a gas is greater than the volume of the solid or liquid form of the same substance.

In ice, the intermolecular forces hold water molecules together in a rigid pattern. These molecules retain their structure at temperatures below 0 °C. When ice begins to melt, at 0 °C, the increase in kinetic energy overcomes the hold of the intermolecular forces, and the molecules loosesn. The solid turns into a liquid.

In the liquid state, water molecules cling loosely together, gliding freely around each other. The weakness of the intermolecular forces lets liquids flow.

When water boils, the force that holds the molecules together is less than the kinetic energy of the molecules. The intermolecular forces are pulled apart, and the liquid becomes a gas.

Water vapour molecules are free of their intermolecular attachments. Individual molecules dart through the air, colliding with one another and flying off in different directions. The molecules are spaced widely apart and occupy a greater volume than they do in their liquid or frozen state.