A steel cylinder with a moving piston on top is filled with helium (He) gas. The force that the piston exerts on the gas is constant, but the volume inside the cylinder doubles, pushing the piston up. Which is the cause for the change?

The temperature increased.

The pressure decreased.

The helium atoms increased in size.

The density of the helium atoms decreased.

I think the answer might be the last one but I'm not sure.

The temperature increased.

Explanation:

Let consider the Charles Law.

According to this law, The volume of given amount of a gas is directly proportional to its temperature at constant number of moles and pressure.

It means when the number of moles and pressure of gas is constant, the volume of gas is directly proportional to its temperature. So when temperature increases volume also goes to increase.

Mathematical expression:

V₁/T₁ = V₂/T₂

V₁ = Initial volume

T₁ = Initial temperature

V₂ = Final volume  

T₂ = Final temperature