Water — practice questions

1. State the formula: The energy ($Q$) required for evaporation is calculated using the formula $Q = m \times L_v$, where $m$ is the mass and $L_v$ is the specific latent heat of vaporisation.
2. Convert units: The mass is given in grams, but the latent heat is in kilojoules per kilogram. We must convert the mass to kilograms.
   • $m = 20 \text{ g} = \frac{20}{1000} \text{ kg} = 0.020 \text{ kg}$
3. Substitute values into the formula:
   • $L_v = 2260 \text{ kJ kg}^{-1}$
   • $Q = 0.020 \text{ kg} \times 2260 \text{ kJ kg}^{-1}$
4. Calculate the result:
   • $Q = 45.2 \text{ kJ}$

Answer: The evaporation of 20 g of sweat removes 45.2 kJ of heat energy from the body.

1. High specific heat capacity: Water requires a large amount of energy to change its temperature. This means that the water within a mammal's body, which makes up a significant proportion of its mass, can absorb or release considerable heat energy without a drastic change in the mammal's internal body temperature. This helps to maintain a stable internal environment (homeostasis) for enzyme activity, even when external temperatures fluctuate.
2. High latent heat of vaporisation: A large amount of energy is absorbed when water evaporates. Terrestrial mammals can dissipate excess heat by sweating (evaporating water from the skin surface). As sweat evaporates, it draws a significant amount of heat from the body, providing an efficient cooling mechanism with minimal water loss relative to the cooling effect achieved.