JEE Main 2026 — Jan 23, Shift 1 — Chemistry solution

JEE Main2026Jan 23, Shift 1Chemistry

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?

A
$q = + ve, w = 0, Δ U = - ve$
B
$q = + ve, w = - ve, Δ U = + ve$
C
$q = - ve, w = - ve, Δ U = - ve$
D
$q = + ve, w = - ve, Δ U = - ve$

Solution

Heat: The microwave delivers energy to the water. Heat flows into the system, so $q = + ve$ .

Work: Boiling water expands as it converts to vapour, doing work against the surroundings. By the chemistry sign convention $w$ done on the system is negative, so $w = - ve$ .

Internal energy: The water heats up from 5°C to 100°C and some of it vaporises to gas (which has greater internal energy than liquid at the same temperature). Net $Δ U = + ve$ .

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