When molecules are heated, they gain heat energy in addition to the kinetic energy they already have. If the molecules in a solid gain enough energy, they can break free of each other and become liquid. This is called melting. If they gain even more heat energy, the liquid becomes a gas.
Water freezes, or becomes solid, at temperature of 0oC (32oF) or below. If the temperature outside drops to this level, the water on the surface of ponds and lakes will freeze, although the water below may hold enough heat to remain liquid.
When solid water (ice) is heated, it melts to become liquid. Generally speaking, we think of water as being liquid at a “room temperature” of 200C (68oF), or, in other words, under normal conditions, Copper, however, is a solid under such conditions, because it needs a temperature of 1083oC (1981oF) to melt into a liquid.
When water is heated and boils, it turns into a gas. We can see this when a kettle boils. In fact, it is not the billowing steam that is the gas – that is the water turning back in tiny droplets of liquid as it comes into contact with cool air. The real steam is invisible. It can be “seen” in the gap between the spout of the kettle and the visible vapour.
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