Question

When a gas is expanded at room temperature:
A.The pressure decreases
B.The kinetic energy of the molecules remains the same
C.The kinetic energy of the molecule decreases
D.The number of molecules of the gas decreases

Answer 1

Hint: To answer this question, you must recall the Ideal gas equation. The ideal gas equation gives us a relation between the volume, pressure, number of moles and temperature of a gas. Since, we have to deal with the expansion of gases in the given question, we must know that temperature and pressure, both govern the change in the volume of a fixed amount of gas.

Complete answer:
We are given the question that the expansion of the gas is taking place at a constant temperature, i.e., room temperature.
On expansion, the volume of the gas increases. Using Boyle's law, we can say that pressure decreases. Therefore, option A is correct.
We know that the expression of average kinetic energy of a gas is given as $K.E. = \dfrac{3}{2}nRT$.
We can see that the kinetic energy for a fixed amount of gases varies only with temperature. Since, there is no change in the temperature of the system, hence there is no change in the kinetic energy of the gas molecules.
Therefore, option B is correct and option C is incorrect.
In option D we are given that the number of molecules of gas decreases. When the temperature is constant, the pressure of the gas decreases with an increase in the volume. There is no change in the amount of the gas and the number of molecules remains the same.
Therefore, option D is not correct.

Thus, the correct options are A and B.
Note:
We know from Boyle's law, that at constant temperature, the pressure exerted by a gas on the walls of a container is inversely proportional to the volume occupied by the gas.
$P\alpha \dfrac{1}{V}$ or $PV = {\text{constant}}$