Question

9(d). A quantity of pepper soup of mass 800g poured into a plastic container with a tight fitting lid has a temperature of 30°C. The container is placed in a microwave oven, rated 1200W, and operated for 3 minutes. (i) Calculate the final temperature attained by the soup (assuming no heat losses); (ii) Explain why containers with tight-fitting lids are not suitable for use in microwave cooking; (iii) When the soup is brought out and allowed to cool, a dent is observed on the container. Explain. [specific heat capacity of soup=4000 J kg⁻¹K⁻¹]

Accepted Answer

(i) Heat supplied = Power × time = 1200×(3×60) = 1200×180 = 216,000J
Heat = mcΔT ⟹ 216,000 = 0.8×4000×ΔT ⟹ ΔT = 216,000/3200 = 67.5°C
Final temperature = 30+67.5 = 97.5°C

(ii) The heat generated causes the liquid and trapped air/vapour to expand, building up pressure inside the sealed container. Since the tight lid does not allow this pressure to escape, it can cause the container to burst.

(iii) As the soup and trapped vapour inside the container cool, they contract, lowering the internal pressure below the surrounding atmospheric pressure. The greater external atmospheric pressure then pushes inward on the container walls, causing the observed dent.

WAEC Physics 2014 Theory — Question 18

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