Physics Paper 2, May/June. 2010
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength
Question 12

1. (i) State two advantages of alcohol over mercury as a thermometric liquid.
(ii) When the bulb of a thermometer is placed in a beaker of hot water, the level of mercury first falls and then rises gradually.  Explain this observation.
2.  List two factors, other than temperature, that affect the rate of evaporation of a liquid.
3. A block of lead of mass 100kg in a crucible and at a temperature of 40oC was placed in an electric furnace rated 10kW.  If the melting point of lead is 320 oC, calculate the:
(i)  quantity of heat required to heat the lead to its melting point;
(iii) time taken to supply this additional energy.
(Specific heat capacity of lead = 120JKg-1 K-1
(Specific latent heat of fusion of lead = 2.5 x 104 JK-1)
4.  State two precautions necessary in an experiment to determine the specific latent heat of vaporization of water.
Observation

In part (a)(i), performance was fair however, some candidates took the advantages to mean just the properties.  In (a)(ii), candidates are expected to know that the glass of the bulb of the thermometer first expands and  that the linear expansivity of mercury is more than that of glass.  Most candidates failed to explain the observation correctly.  Performance was low.

In part (b), high performance was recorded.  However, few candidates wrote ‘area of liquid’   instead of ‘area of liquid exposed’.
In (c), this numerical part was correctly done by most candidates except in few cases where
wrong units was quoted for the final answer.  Also in (d), performance was fair.

1. (i)   Advantages of alcohol over mercury as a thermometric liquid

-      lower freezing point
-      cheaper

-      easily expansively

• -     the glass of the bulb of the thermometer first expands; hence the mercury level

fails
-     the expansion of the mercury increases more than that of the glass
-     and the liquid rises

•     Factors other than temperature affecting the rate of evaporation of a liquid

-     wind
-     humidity/dryness of air
-     nature of liquid/viscosity/density
-     area of liquid exposed
-     pressure.

• (i)   Q  = mC Dθ

=  100 x 120 x (320 – 40)
=  3360000 J

(ii)   Q  =  ml
= 100 x 0.25 x 105
=  2500000 J

(iii)   Pt  =  ml
10,000 t = 2500000
t  =  250s

•       Precautions

-    calorimeter should be (well) lagged
-    calorimeter should be shielded from heat source
-     only dry steam should be used
-     mixture should be gently and continually stirred.