[Keywords] Thermal Properties of Matter

Important Facts That You Must Know First!

  1. Only for pure substances that the changes of state of matter occur at a fixed temperature.
  2. Boiling and evaporation are both processes of vaporisation. However, you need to know their differences.
  3. Instead of using break the inter-molecular bond, you should use “enough energy to overcome the forces of attraction between them”. “Them” refers to the molecules.
  4. How do you define condensation? Thermal energy is given out during condensation as intermolecular bonds are formed.
  5. When a substance has lower specific heat capacity, the substance is relatively more sensitive because for the same amount of heat, the increase in temperature will be more significant. This is one of the reasons why mercury is used instead of water in the thermometer.
  6. Five factors affecting the rate of evaporation:
    1. Temperature of the liquid: a warmer liquid means that a greater number of molecules at the surface layer have higher average internal kinetic energy, making them energetic enough to overcome the attractive forces by neighbouring water molecules and push against the atmospheric pressure.
    2. Exposed surface are of the liquid: evaporation only takes place at the surface of the liquid. A larger surface area means more molecules can escape from the surface at any one time.
    3. Humidity of the surrounding air: when humidity of the air increases, there are more water vapour present in the air. The escaping molecules from the liquid surface will easily hit these water vapour molecules and bounce back into the liquid.
    4. Movement of air above the surface of the liquid (e.g wind): the moving air removes the molecules of the liquid as soon as they escape from the surface.
    5. Air pressure: higher pressure means there is a lot more air molecules above the surface of the liquid blocking the escaping molecules. This reduces the rate of evaporation.

Commonly Seen Questions

  1. Explain why there is no temperature change when ice is melting and when water is boiling.
    • When ice is melting, thermal energy absorbed is used to overcome the attractive forces between the molecules/ break the inter-molecular bonds, resulting in an increased internal potential energy. None of the thermal energy supplied is converted to internal kinetic energy, and so the average kinetic energy of the substance/molecules remains constant and hence there is no change in temperature.
    • During boiling, the thermal energy supplied is used by the escaping water molecules to overcome the attractive forces of other water molecules and the force of atmospheric pressure. Internal potential energy increases but none of the thermal energy supplied is converted into internal kinetic energy (average kinetic of the molecules), there is no change in temperature during boiling.
  2. Explain why during condensation the temperature of water remain constant?
    • During condensation, molecules come closer together and strong inter-molecular bonds are formed and thermal energy is released and lost to the surroundings. The average kinetic energy of the molecules remains constant/the same. Thus, the temperature does not change.
  3. Explain why evaporation causes cooling (using kinetic model of matter/kinetic theory).
    • The molecules in a liquid move randomly at different speeds and therefore have different kinetic energies. At the liquid surface, some liquid molecules have sufficient energy to overcome the attractive forces of their neighboring molecules and the force of atmospheric pressure (or push back against the atmospheric pressure). They escape into the atmosphere leaving behind the less energetic ones. This lower the average speed and hence average kinetic energy of the liquid molecules. As a result, the temperature of the liquid decreases (cooling).
    • NOTE: Out of all the energetic molecules trying to escape, not all can escape into the atmosphere. Some may collide with each other or with the air molecules and bounce back into the liquid. As such, the process is slow.
  4. Explain why the value of specific latent heat of vaporisation of water is greater than its specific latent heat of fusion.
    • Hint: fusion (overcome the strong inter-molecular forces); vaporisation (overcome the inter-molecular forces and push back against the surrounding air molecules (atmospheric pressure)).
  5. Why does dew form on the surface of a cold soft-drink can?
    • Water vapor in the warm air is chilled when it makes contact with the cold can. This causes them to move slower and collide at lower speed. There is a higher chance of them sticking together, forming dew or condensation.
  6. Explain how a misting fan can be used to cool the air in a room.
    • The small water droplets will evaporate gradually and remove heat from the air in the room in the process.
    • NOTE: This method can be a double edged sword depending on the set up of the room and the context of the question. By increasing the humidity in the air, it will also lower the rate of evaporation. Furthermore, water is a poor conductor of heat but air is better at doing that job. Thus, you need to read the question carefully in order to decide from which angle that you should tackle the question.
  7. Why is there no temperature change during freezing?
    • During freezing, strong bonds are formed and thermal energy released is lost to the surroundings. Internal potential energy decreases while internal kinetic energy remains constant/ the same. Hence, there is no change in temperature.