When an ideal gas expands in vaccum, there is neither absorption nor evolution of heat. Why?

Explanation

In an ideal gas, there are no intermolecular forces of attraction and therefore, no force opposes the expansion in vaccum.Hence, no energy is required to overcome these forces.Morever, when a gas expands against vaccum, work done is zero because `P_(ext) = 0`.Therefore, internal energy of the system does not change, i.e.there is no absorption or evaluation of heat.