JEE Main · 2026 · 23 Jan 2026 · Shift 1 · Chemical Thermodynamics

The water absorbs energy from the microwave radiation, so heat flows into the system and $q$ is positive. As it reaches the boiling point and vaporises, the large volume increase of the gas does expansion work against the surroundings, so work is done by the system and $w$ is negative. The internal energy climbs as the temperature rises from $5^{\circ }\text{C}$ to $100^{\circ }\text{C}$ and the endothermic phase change stores further energy, so $\Delta U$ is positive. Hence $q=+\text{ve}$, $w=-\text{ve}$ and $\Delta U=+\text{ve}$.