(a) Define the boiling point of a liquid.
(b) Describe an experiment to determine the boiling point of small quantity of a liquid.
(c) A piece of copper of mass 300 g at a temperature of 950°C is quickly transferred to a vessel of negligible thermal capacity containing 250 g of water at 25°C. If the final steady temperature of the mixture is 100°C, calculate the mass of the water that will boil away.
[Specific heat capacity of copper = \(4.0 \times 10^{2} Jkg^{-1} K^{-1}\); Specific heat capacity of water = \(4.2 \times 10^{3} Jkg^{-1} K^{-1}\); Specific latent heat of vaporization of steam = \(2.26 \times 10^{6} Jkg^{-1}\)
(d) State four other effects of heat on a substance other than expansion.
Explanation
(a) The boiling point of a liquid is the temperature at which its s.v.p is equal to the external atmospheric pressure.
(b) Experiment to determine the boiling point of a small quantity of a liquid.
Some liquid is introduced into the shorter length of the J - tube above the mercury. The beaker and its contents are heated and stirred continuously. At a stage when the levels of mercury in the two limbs of the J - tube are the same the temperature is read. At this temperature the s.v.p. of the liquid is equal to the atmospheric pressure. This temperature is the boiling point of the liquid.
(c) Heat lost by copper = Heat gained by water
\(mc \theta = m_{1} c_{1} \theta_{1} + ml\)
\(0.3 \times 400 \times (950° - 100°) = 0.25 \times 4200 \times (100° - 25°) + 2.26 \times 10^{6} M\)
\(M = \frac{102000 - 78750}{2.26 \times 10^{6}}\)
\(M = 10.3g\)
Therefore, the mass of water that will boil away is 10.3g.
(d) Four effects of heat other than expansion are:
(1) Change of state (2) Change in temperature (3) Chemical change (4) Change in electrical resistance of conductivity. (5) Melting sublimation.