(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^\circC. The air is cooled until the temperature falls to 24^\circC. 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}]
Verified Correct Answer: .
Physical Principles & Numerical Solution
(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.
Source: Myschool Past Questions Archive (Record #53162)