(a) Define boiling point of a liquid.
(b) Describe how water in a round bottom flask could be made to boil without heating it. [diagram not necessary]
(c) State three applications of expansion of metals.
(d) A room with floor measurements 7m x 10 m contains air of mass 250 kg at a temperature of 34°C. The air is cooled until the temperature falls to 24°C. 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? Give a reason for your answer. [ Specific heat capacity of air = 1010 Jkg(^{-1})K(^{-1}); density of air = 1.25 kg m(^{-3}]
Explanation
(a) Boiling point of a liquid is the temperature at which saturated vapour pressure of a liquid equals the external atmospheric pressure.
(b) Fill a round bottom flask partially with water, then pump out the air gradually with a vacuum pump. At a state when the saturation vapour pressure equal atmospheric pressure, the water is observed to be boiling.
(c) Application of i expansion of metals are in:
– Riveting two or more metal plates.
– Fixing of metal rims on metal wheels.
– Fusing of platinum wire through walls of glass vessels.
– Automatic fire alarms and
– Thermostat.
(d)(i) Volume = (frac{Mass}{Density} = frac{250}{1.25})
= 200(m^2)
But volume = Ibh
h = (frac{v}{Ib} = frac{200}{7 times 10})
= 2.86m
(ii) Q = mc(theta)
V = 250 x 1010(34 – 24)
= 252500 or 2.525 x 10(^6)
(iii) The actual energy value will be higher in value than the calculated value because -Part of heat is extracted. from the walls in the room and other materials. he room also is not airtight, there are heat leakages in and out of the room.