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When you heat ice its molecules gain energy and the molecular bonds weaken. This melts the ice into the water at a temperature of 0 °C. When you further heat that water, at 100 °C the molecular bonds weaken further and finally with enough energy some molecules escape from the liquid phase to the gaseous phase. This is called boiling. The escaped molecules are colourless and odourless and better known as steam (water vapour is the condensed steam).

If you curtail this steam in an air-tight vessel, it will exert pressure against the wall. It will be rampantly growing until it explodes the vessel. But if this pressure (force per area) is craftily applied to some mechanism, steam can be converted to useful mechanical work. We will learn about this mechanism in the next lesson.