Heat Transfer

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A molecule is the smallest particle that can exist freely in nature and retains the properties of the substance. Its size is nearly 10^-10 m, and it cannot be seen even by a microscope.

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Heat increases the internal energy of a substance. The molecules gain kinetic energy, so their motion becomes faster. When a substance is cooled, molecular motion becomes slower and kinetic energy dec…

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Heat is the internal energy of a substance. It is measured in joule (J).

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During change of state, the absorbed heat is used to change the forces of attraction between molecules. The average kinetic energy remains unchanged, so the temperature stays constant. This heat is ca…

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Evaporation is the change of liquid into vapour from the surface at all temperatures. Boiling is the change of liquid into vapour at a fixed temperature throughout the whole mass of the liquid.

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The rate of evaporation increases with wind. Moving air removes vapour near the surface, so more molecules can escape from the liquid surface. Evaporation is also faster when temperature and surface a…

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Water boils at 100°C.

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Pressure and boiling point are directly related. In a pressure cooker, water boils at nearly 125°C because the pressure is higher than normal. Higher pressure raises the boiling point.