(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.
