Qúéßtîóñ : A quantity of pepper soup of mass 800g poured into a plastic container with a tight fitting lid has a temperature of 30oC. The container is then placed in a microwave oven, rated 1200 W and operated for3 minutes.
(i) Calculate the final temperature attained by the soup. (Assuming no heat losses)
Àñßwër : Pt = mc
1200 x 3 x 60 = x 4000 x
= 67.5o
Final temp = 67 . 5o + 30o
= 97.5oC
(ii) Explain why containers with tight-fitting lids are not suitable for use in microwave cooking.
Àñßwër : The molecules of the steam from the content are confined within the container. As the temperature increases the rate of collision of the molecules with walls of the container increases, thus the pressure inside the container increases. If it is tightly fitted, the pressure can make the container burst thus releasing its hot content.
(iii) When the soup is brought out and allowed to cool, a dent was observed on the container. Explain.[ Take specific heat capacity of the soup = 4000 Jkg-1 K-1 ]
Àñßwër: The container will dent because the condensation of the steam in the container will lead to a decrease in pressure in the container making the atmospheric pressure greater than the pressure in the container.
Explanation:
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