(a) State two factors that affect the rate of evaporation of a liquid.
(b) Explain the term latent heal.
(c) Explain each of the following phenomena:
(i) On a dry day, water in a clay pot is cooler than water in a rubber container:
(ii) Cooking of food is faster in a pressure cooker than in an ordinary pot.
(d) A 40 V electric heater is used to supply a current of 12 A for 1400 seconds to a body mass of 1.5 kg at its melting point. The body melts and its temperature rises by 60°C in an extra 1.2 minutes, Calculate the:
(i) Latent heat of fusion of the body
(ii) Specific heat capacity of the body.
(e) State two differences between evaporation and boiling.
Explanation
(a) Factors affecting the rate
i. Surface area of the liquid (exposed surface area)
ii. Temperature of the liquid
iii. Wind/ drought.
iv. Humidity/ amount of water vapour
v. Nature of the liquid
vi. Air pressure
(b) Explanation of latent heat;
The energy needed to change the state of a substance without a change in temperature.
(c)(i) Water in a clay pot cooler than in a rubber container;
A clay pot has pores which allows evaporation to occur while a plastic container has no pores.
(ii) Pressure cooker cooks faster than ordinary pot;
Pressure cooker raises the pressure within it, which raises the temperature to/above the boiling point thus cooks faster.
(d)(i) Calculation of latent heat of fusion of the body
Ivt = mL
mL = 12 x 40 x 1400
= 6.72 x 10\^{5}\)j
(ii) Calculation of specific heat capacity of the body
C = \(\frac{Ivt}{m \bigtriangleup \theta}\)
= \(\frac{12 \times 40 \times 72}{1.5 \times 60}\)
= 384 jkg\(^{-1}\) K\(^{-1}\)
(e) difference between evaporation and boiling
| Evaporation | Boiling |
| It takes place at the surface of the liquid | It occurs throughout the entire volume of the liquid |
| It takes place across all temperatures | It takes place at specific temperatures |
| It produces a cooling effect | It produces no cooling effect |
| It is wind aided | It is not wind aided |