Physics (Essay) Paper 2 WASSCE (PC), 2022

Question 9

 

(a)  (i)  The s.v.p. of water vapour on acertain day at air temperature 28 0C is 12.2 mmHg.  Calculate the relative humidity of a day when the s.v.p. of water vapour at dew point is 15.9 mmHg.

       (ii)  State two effects of expansion of water when it freezes to ice.

(b)   A quantity of liquid of mass 600 g placed in a plastic container with a tight fitting lid has a temperature of 30 0C. The container is placed in a microwave oven rated 1250 W.

(i)   if the microwave is operated for 4 minutes, calculate the final temperature attained by the liquid. (assuming no heat losses [Specific heat capacity of the liquid  = 4100 j kg-1 K-1]

(ii)  If the liquid is brought out and allowed to cool, a dent is observed on the container. Explain.

(c) Explain why containers with tight-fitting lids are not suitable for use in microwave cooking.

(d) (i)  Define fixed points on a temperature scale.
(ii)   State one effect of heat on a substance.

Observation

 

(ii)  The container is dented because the condensation of the steam will lead to a decrease in pressure making the atmospheric/external pressure greater than the pressure in the container.

     

(c)        Explanation of observation

  • The molecules of the steam are confined. As the temperature increases, the rate of
  • collision of the molecules with walls of the container increases, the pressure
  • inside the container increases, causing it to burst.

 

(d)  (i)  Fixed point on a temperature scale

Accurate and standard temperatures used as reference point on the temperature scale.                                                                                   

(ii)  Effects of heat on a substance:

  • Change:
  • in temperature
  • in (electrical) resistance
  • of state
  • in colour  
  • in dimension