Engineering Hydrology Questions and Answers – Evaporation Process

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Evaporation Process”.

1. Evaporation is a process characterized by which of the following phase transitions of matter?
a) Solid to liquid
b) Liquid to solid
c) Liquid to gas
d) Gas to liquid

Explanation: Evaporation is a process characterized by the change of matter from liquid phase to gaseous phase. The other phase transitions given are melting (solid to liquid), freezing (liquid to solid) and condensation (gas to liquid).

2. Which of the following is true regarding evaporation?
a) Surface phenomenon and cooling process
b) Bulk phenomenon and cooling process
c) Surface phenomenon and heating process
d) Bulk phenomenon and heating process

Explanation: Evaporation is a surface phenomenon as it takes place over the exposed surface of a liquid, as opposed to a process like boiling (bulk phenomenon). The molecules of the liquid require heat energy to evaporate, which is provided by the liquid. As the evaporated molecules take the heat with them, it leaves a cooling effect on the liquid surface.

3. Which of the following is true regarding the range particle velocity values and its average speed just prior to which the process of evaporation?
a) Both range of velocities and average speed increases
b) Range of velocities reduces and average speed increases
c) Range of velocities remains same and average speed increases
d) Both range of velocities and average speed remain same

Explanation: The molecules of a liquid are in constant motion with a wide range of velocities. Evaporation takes place through the transfer of heat energy. When heat is added to the liquid, the velocity range and average speed of molecules both increases causing the crossing over from one state to another.

4. Which of the following relations is true for the rate of evaporation (E) as per Dalton’s Law? Ps and Pa represent the saturation and actual vapour pressure.
a) E∝(Ps-Pa)
b) E∝$$\frac{1}{(P_s-P_a)}$$
c) E∝(Ps-Pa)n, n=constant
d) E∝Ps and E∝Pa

Explanation: As per Dalton’s law of evaporation, The evaporation rate is directly proportional to the difference between the saturation vapour pressure and the actual vapour pressure of the surrounding air.

5. According to Dalton’s law of evaporation, the process of evaporation ceases when which of the following conditions is met?
a) Saturation vapour pressure = actual vapour pressure
b) Saturation vapour pressure > actual vapour pressure
c) Saturation vapour pressure < actual vapour pressure
d) Saturation vapour pressure = atmospheric pressure

Explanation: Evaporation takes place as long as the vapour pressure is less than the saturated vapour pressure. Once the relative humidity reaches 100%, the air no more has the ability to hold the vapours and this leads to condensation.

6. The rate of evaporation of water increases with which of the following changes?
a) Increase in water temperature and decrease in air temperature
b) Decrease in water temperature and increase in air temperature
c) Increase in both air and water temperature
d) Decrease in both air and water temperature

Explanation: Other parameters remaining constant, the rate of evaporation increases with an increase in the water temperature as well as the surrounding air temperature.

7. As the air temperature increases, what is the nature of increase in the evaporation rate of the liquid?
a) Linear
b) Exponential
c) Logistic
d) No fixed relation

Explanation: As the air temperature increases, the saturation vapour pressure of the air also increases exponentially. As the saturation vapour pressure increases, the evaporation rate also increases. However, there is no exact pattern between the increase in air temperature and increase in rate of evaporation.

8. In a particular year, three months having the same monthly mean temperature values, can exhibit different extents of evaporation for the same water body.
a) True
b) False

Explanation: Since, there is no exact correlation between the temperature and evaporation rate, it is possible to have different degrees of evaporation for a lake during different months having similar temperature readings.

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