P) This was well answered by most candidates however, few candidates omitted the
phrase “ in a closed system or “isolated system” in their statement.
(b) The three effects of heat on a substance fell an easy prey to most candidates.
Performance was satisfactory.
- Explanation of how mercury level changes when dipped in ice-cold water proved difficult to most respondent candidates as they could not provide satisfactory explanation.
- The numerical section was poorly tackled by most respondent candidates. The
formula m = v eluded most of the candidates.
Also applying the formula Q = mc appropriately was another problem due to their poor understanding of the concepts of specific heat capacity and latent heat.
The expected answers are:
(a) In an isolated / closed system, the total amount of energy is (always) constant
(although energy may be transformed from one form to another)
(b) The effect of heat on a substance
e.g Change
- in temperature
- in size (expansion/contraction)
- of phase/state
- in resistance
- in colour
(c) The bulb comes in contact with the cold water before the mercury, hence the glass cools first, contracts and squeezes the mercury up the stem until (thermal) equilibrium is reached. The mercury being a better conductor of heat cools faster than the glass; hence the fall in mercury level
(d)(i) m = x V
= 1000 x 200
1,000,000
= 0.2 kg
Q = MwCwӨw + Mwlf + Mi Ci Өi
= 0.2 x 4200 x (20-0) + 0.2 x 3.36 x 105 + 0.2 x 2100 x [0-(-10)
= 16800 + 67200 + 4200
= 88200J
(ii) Q = Pt or P = Q
t
t =
= 88200
120
= 735s or 12.25 minutes
(iii) Why the calculated time is less than the actual time
e.g (No consideration is given to)
- Heat absorbed by the aluminum container
- Heat lost / absorbed from the surroundings
- Effect of impurities on freezing point of liquid.
