(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.
(a) Boiling point
The boiling point of a liquid is the constant temperature at which the liquid changes to vapour throughout its bulk, when its saturated vapour pressure equals the external (atmospheric) pressure.
(b) Experiment to determine the boiling point of a small quantity of liquid
- Put the small quantity of liquid in a narrow ignition tube.
- Attach a thermometer to the tube with a rubber band so the bulb is level with the liquid.
- Immerse the tube and thermometer in a beaker of water (a heating bath) clamped on a stand.
- Heat the bath steadily while stirring, and observe the thermometer.
- The temperature at which the liquid boils steadily (bubbles rise continuously and temperature stays constant) is read and recorded as the boiling point.
(c) Mass of water that boils away
Heat lost by copper cooling from 950°C to 100°C:
\[ Q_{Cu} = m_{Cu} c_{Cu}\,\Delta\theta = 0.300 \times 4.0\times10^{2} \times (950-100) = 0.300\times400\times850 = 102000\,\text{J} \]
Heat used to raise the water from 25°C to 100°C:
\[ Q_{w} = 0.250 \times 4.2\times10^{3} \times (100-25) = 0.250\times4200\times75 = 78750\,\text{J} \]
Heat left for vaporization:
\[ Q_{v} = 102000 - 78750 = 23250\,\text{J} \]
Mass boiled away:
\[ m = \frac{Q_{v}}{L} = \frac{23250}{2.26\times10^{6}} = 1.03\times10^{-2}\,\text{kg} \approx 10.3\,\text{g} \]
(d) Four other effects of heat (besides expansion)
- Change of state (melting, boiling, evaporation).
- Rise in temperature of the substance.
- Chemical change (e.g. decomposition or burning).
- Change in electrical resistance (or thermionic/thermoelectric effects).
