1.

When thermal energy is supplied to melt a solid, the temperature remained constant during the melting point. Explain what happened to the heat energy supplied.

• During melting, thermal energy absorbed is used to break the intermolecular bonds between the particles, resulting in increase in potential energy of the substance.

• Thermal energy is not used to cause an increase in kinetic energy of the particles, hence temperature remains constant.

2.

*Using the kinetic model of gases, explain why evaporation of a liquid causes a fall in temperature.

• In a sample of water, the molecules move at different speeds, and therefore different kinetic energies.

• At the water surface, molecules with sufficient energy to overcome the attractive forces of other molecules escape into the atmosphere.

• When energetic molecules escapes from the surface of a liquid, its leaves behind the less energetic molecules.

• As a result, the average kinetic energy of the remaining molecules decreases and therefore the temperature of liquid falls.

3.

Which will cause a more severe injury? Scalding by boiling water at 100 degree Celsius or steam at 100 degree Celsius?

• Scalding by steam at 100 degree Celsius.

• Although both are of the same temperature, the latent heat of vaporization of steam is greater than specific heat capacity of water.

• A larger amount of latent heat of vaporization is released as steam condenses on the skin, as compare to thermal energy released as water contacts the skin.