a) Define heat capacity and state its unit.
(b) List two effects of heat on a substance.
(c) Explain how a tightly fitted glass stopper could be removed from a reagent bottle.
(d) A quantity of pepper soup of mass 800 g poured into a plastic container with a tight-fitting lid has a temperature of 30°C. The container is then placed in a microwave oven, rated 1200 W 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. [Take specific heat capacity of the soup = 4000 Jkg\(^{-1}\) K\(^{-1}\)]
Explanation
(a) Heat capacity is the quantity of heat required to change the temperature of a body by 1K. The unit is JK\(^{-1}\)
(b) Effects of heat on a substance are:
(i) Increase in temperature.
(ii) Change of state.
(iii) Change in colour .
(iv) Change in electrical resistance, etc.
(c) To remove a tightly fitted glass cork from a reagent bottle, warm the bottle neck using warm cloth to make the glass bottle to expand creating space for easy removal of the cork.
(d)(i) Pt = MC\(\Delta \theta\)
\(\Delta \theta = \frac{pt}{Mc} = \frac{1200 \times 3 \times 60}{0.8 \times 4000}\) =67.5°
= final temp \(\theta\) = 67.5 + 30° = 97.5°C
(ii) Containers with tight-fitted lids are not suitable because with increase in temperature the steam molecules from the container are confined in the container, thereby increasing the rate of collission of the steam with the walls of the container, hence in pressure inside the container. Thus the high pressure can burst the tightly-fitted container and the hot content released.
(iii) The observed dent on the container is due to the condensation of the steam in the container which-will lead to a decrease in pressure in the greater container making the atmospheric pressure greater than the pressure in the container.