(a)(i) State two advantages of alcohol over mercury as a thermometric liquid. (ii) When the bulb of a thermometer is placed in a beaker of hot water, the level of the mercury first falls and then rises gradually. Explain this observation. (b) List two factors, other than temperature, that affect the rate of evaporation of a liquid. (b) A block of lead of mass 100 kg in a crucible and at a temperature of 40 °C was placed in an electric furnace rated 10 kW. If the melting point of lead is 320 °C, calculate the: (i) quantity of heat required to heat the lead to its melting point; (ii) additional heat energy required to melt the lead; (iii) time taken to supply this additional energy. (Specific heat capacity of lead = 120 Jkg\(^{-1}\) K\(^{-1}\) (Specific Idtent heat of fusion lead = 2.5 x 10\(^{4} JK^{-1}\)) (d) State two precautions necessary in an experiment to determine the specific latent heat of vaporization of water.
Explanation
a)(i) Advantages of alcohol over mercury as a thermometric liquid are: - Alcohol is cheaper -Alcohol is easily available -Alcohol has higher expansivity - Alcohol has lower freezing point. (ii) The glass bulb of the thermometer first got heated and expands, therefore the mercury level in it falls. Then the mercury expands and increases more than that of the glass and then rises. (b) Factors that affect the rate of evaporation of a liquid, temperature exclusively are: - Pressure, area of liquid surface exposed, humidity, nature of the liquid, wind and dryness of air, etc. (c)(i) Q = mc \(\Delta \theta = 100 \times 120 \times (320 - 40)\) ii) Q = Ml = 100 x 0.25 x 10\(^5)\) = 2500000J = 2.5MJ ii) Pt = ml t = \(\frac{ml}{p} = \frac{2500000}{10000} = 250s\) (d) Precautions to be considered are: - Calorimeter should be well lagged. - Use only dry steam. - Stir the mixture gently and continually. - Shield the calorimeter from heat sources.
