Final answer:
The source of energy for work done by a gas during isothermal expansion is heat absorbed from its surroundings, as the temperature remains constant and the internal energy change is zero.
Step-by-step explanation:
When a gas expands isothermally, it performs work as it increases in volume. The source of energy required for this work is heat that the gas absorbs from its surroundings.
During isothermal expansion, the temperature remains constant, and according to the first law of thermodynamics, the heat absorbed by the gas is utilized to do the work of expanding against external pressure. This process is described by the relationship Δ Eint = Q - W, where Δ Eint is the change in internal energy (which is zero for an ideal gas during isothermal processes), Q is the heat absorbed, and W is the work done.