Physics Paper 2, MAy/June. 2013
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main

Weakness/Remedies
Strength

Question 12

1. Define boiling point of a liquid
2.  Describe how water in a round bottom flask could be made to boil without heating it.

(Diagram not necessary)

1.  State three applications of expansion of metals
2.  A room with floor measurements 7m x 10m contains air of mass 250 kg at a temperature of 34oC.  The air is cooled until the temperature falls to 24oC. Calculate the:  (i)  height of the room;

(ii)  quantity of energy extracted to cool the room;
(iii)  Which is higher; the calculated value or the actual energy needed to cool
the room?
[Specific heat capacity of air = 1010Jkg-1K-1; density of air = 1.25kg m-3]

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Observation

Part (a) Many candidates attempted this question but majority of them failed.  They used the word point instead of temperature in their definition e.g. boiling point of a liquid is the point at which the s.v.p of the liquid equates the external atmospheric pressure.  Performance was fair.
Part (b) Most candidates failed to link boiling to external atmospheric pressure.  Infact most of the description by the candidates stated that it was not possible to boil a liquid without heating it.  They did not even realize that the round bottom flask must not be completely filled with the water.  Performance was poor.

Part (c)  This part was satisfactorily tackled by most responding candidates.  However, few candidates stated the effects of expansion instead of applications of expansion of metals.
Part (d)  This part was satisfactorily tackled by most candidates. However, many candidates could not give correct reasons for the answer to d(iii)
(a)  Boiling point of a liquid is the temperature at which saturated vapour pressure of a liquid                              equals the ( external ) atmospheric pressure

(b)   A round bottom flask not fully/ partially filled with water is connected to a vacuum
pump Air is gradually pumped out of the flask until s.v.p = atmospheric
pressure At this stage the water is observed to be boiling

(c)      Application of expansion of metal

e.g -  Thermostat
-  Riveting two or more metal plates
-   Automatic fire alarms
-   Fixing of metal rims on metal wheels
-   fusing of platinum wire through walls of glass vessels
NOTE:  Do not accept bimetallic strips

(d)       (i)      v  =  m/D    OR  D  =  m/v
=

=    200 m3
v  =  x bx h          OR
h  =

=

=  2.86m

(ii)     Q  =  mc
=   250 x 1010 x (34 – 24)
=  2525000  = 2.525 x 106 J
(iii)     The actual energy value will be higher in value than the calculated value

Reason
e.g   -      Part of heat is extracted from other materials/walls in the room

•    Room is not air tight; hence heat leakages in and out of the room need to be

Compensated for.