Properties of gases: gas pressure: gases expand to fill their container and assume the shape of their containers, they diffuse into one another and mix in all proportions. Four properties determine the physical behaviour of a gas: The amount of the gas (in moles: the temperature of the gas (in. K: the volume of the gas (in l, pressure of gas (in atm or kpa) One good argument is that the molecules of the gas within the balloon are in constant motion and thus colliding with each other as well as the walls of the container, the balloon, keeping the balloon in shape. However it is difficult to measure the total force exerted by a gas. So in chemistry, we speak in terms of pressure. In translation, it is the force divided by the area over which the force is distributed. It is difficult to measure the pressure of gas directly.