Since, the chamber is thermally insulated ∆Q=0 ∴ Elastic potential energy of spring = work done by gas or work done by gas =
1
2
kx02 This work is done in the expense of internal energy of the gas. Therefore, internal energy of the gas is decreased by
1
2
kx02 Internal energy of an ideal gas depends on its temperature only. Internal energy of the gas is decreasing. Therefore, temperature of the gas will decrease.