[Keywords] Kinetic Model of Matter

Important Facts That You Must Know First!

  1. All matter is made up of tiny particles called molecules and molecules are in continuous random motion.
  2. Generally, the strength of the intermolecular forces determinne the shape and volume of matter whereas the arrangement determine the density and compressibility. There is one exception.
    • Liquid takes the shape of the container because particles can move about/slide past each others relatively freely. However, due to the strong intermolecular forces of attraction, it has a fixed volume at a constant temperature.
  3. PV = nRT (ideal gas law) is helpful in deducing the relationship between the variables when one of them changes. It is a good approximation of the behaviour of normal gas though with many limitations. n represents the number of moles [amount of substance of gas], R is a gas constant [something that you can treat like a constant which will disappear when you do ratio], T represents absolute temperature of gas [in Kelvin], P represents pressure [in Pa] and V represents volume [m3].
  4. The purpose of Brownian motion experiment is to detect the existence of molecules in continuous random motion.
  5. Pressure is force acting per unit area. Force will exist due to collision between two particles. Collision can become more frequently (no change in temperature), and more forcefully/vigorously (increase in temperature).

Commonly seen questions

  • State and explain what happens to the molecules in a block of ice when the temperature of the ice is increased and the ice does not melt.

When the temperature of the ice is increased, the molecules gained thermal energy which is converted into higher average kinetic energy. The molecules vibrate more frequently and vigorously without any change in the arrangement.

  • A syringe contains trapped air. The piston inside the syringe is free to move up and down in the syringe. When the syringe is placed in hot water, the piston is seen to move upwards. Explain (1) why the air inside the syringe exerts a pressure on the piston (2) the movement of the piston.

(1) Air molecules are in continuous random motion. When the air molecules collide with the walls of the syringe, they exert a force on the area of the walls. Since pressure is force per unit area, a pressure is exerted.

NOTE: you need to understand that there must be pressure exerted on the piston or else the piston will be coming down all the way until there is no space for the trapped air.


(2) When the syringe is placed in hot water, the temperature of air inside the syringe increases. This causes an increase in average kinetic energy of the air molecules and thus they move faster. As a result, the air molecules collide more frequently and more forcefully with the walls of the syringe. Force exerted per unit area by air molecules increases and hence the interior pressure increases and exceeds the exterior pressure. A resultant upward force acts on the piston and it moves upwards.

NOTE: we do not use atmospheric pressure to replace exterior pressure because exterior pressure = atmospheric pressure + pressure due to the weight of the piston acting downward.

  • Explain, by writing about molecules, why the pressure in the tyre increases as more air is pumped into the tyre?

As the number of air molecules increases, there are more air molecules per unit volume. This causes the air molecules to collide with the inner walls of the tyre more frequently, resulting in an increase in force per unit area. Hence, the pressure exerted increases.

NOTE: There is an assumption to this question. That is the temperature of the tyre remains constant. If the question states that the temperature is also increased, then you must remember to add “more forcefully” on top of more frequently.

  • Explain why a liquid cools as it evaporates.

When evaporation occurs, the more energetic molecules near the liquid surface have sufficient energy to overcome the attractive forces with other liquid molecules and push back against the atmosphere in order to escape from the liquid. The less energetic molecules are remained and this decreases the average kinetic energy of the liquid molecules. Since temperature of the liquid depends on the average kinetic energy of the molecules, the temperature decreases.