Question
A cup of water at 5^ C (system) is placed in a microwave oven and the oven is turned on for one minute during which the water begins to boil. Which of the following option is true ?
JEE Main
Chemistry
Thermodynamics (C)
2026
JEE Main 2026 (23 January Shift 1)
JEE Main Chemistry Question (2026) — Solution
Options
- A. q =+ ve , w =0, U =- ve
- B. q =+ ve , w =- ve , U =- ve
- C. q =- ve , w =- ve , U =- ve
- D. q =+ ve , w =- ve , U =+ ve
Answer
D. q =+ ve , w =- ve , U =+ ve
Step-by-step solution
For water heating in a microwave: Heat (q): The microwave radiates electromagnetic energy that is absorbed by water molecules, so q = +ve Work (w): As water heats and boils, it expands against atmospheric pressure. The system does work on the surroundings, so w = -ve Internal Energy (ΔU): From the first law, ΔU = q + w. Since heat absorbed (+ve) is much larger in magnitude than work done against atmosphere (-ve), the net change is ΔU = +ve (confirmed by temperature increase from 5°C to boiling) Therefore: q = +ve, w = -ve, ΔU = +ve
Related: Chemistry — Thermodynamics (C) · All PYQ Banks