Question

Identify the true statements regarding evaporation from the following:
(i) Rate of evaporation increases with humidity.
(ii) Evaporation is a surface phenomenon.
(iii) Evaporation results in a decrease in the temperature of the surroundings.
(iv) Rate of evaporation is independent of external pressure.

• A. (i) and (iii)
• B. (ii) and (iv)
• C. (ii) and (iii)
• D. (i) and (iv)

Answer 1

The correct option is C

(ii) and (iii)

Evaporation is a process that occurs on the liquid surface as it changes into the gaseous phase.

Explanation for correct option: (C)

From the statements given above in the question, the only statements which are true for the process of evaporation are statement (ii) and (iii):

(ii) Evaporation is a surface phenomenon:

• Evaporation is a surface phenomenon because the molecules with the highest kinetic energy disperse into the air from the liquid's uppermost layer, making it a surface occurrence.
• When water is exposed to sunlight for an extended period of time, the temperature of the water rises and the hydrogen bonds of ${\mathrm{H}}_2\mathrm{O}$ on the surface begin to break, causing the dissociated molecules to rise.
• As a result, evaporation occurs from the outside but not from the inside.

(iii) Evaporation results in the decrease in the temperature of the surroundings:

• The molecules with the highest kinetic energy are able to escape from the liquid's surface during evaporation.
• As a result, the surviving molecules have a lower average kinetic energy, resulting in a lower liquid temperature.

Explanation for incorrect options:

(A) Statement (i) and (iii) are given in this option, statement (iii) is true for the process of evaporation but statement (i) is not.

Statement (i) says, rate of evaporation increases with humidity, which is false, because:

• Evaporation is also influenced by humidity, or the amount of water vapour in the air. The lower the relative humidity, the dryer the air is and the faster it evaporates.
• The higher the humidity, the closer the air is to saturation, and the less evaporation is possible. When relative humidity is high, it slows evaporation; when it hits 100%, it reduces evaporation to zero (no evaporation at all).
• The amount of moisture in the air compared to what the air can "hold" at that temperature is known as relative humidity. If we state that air humidity is high, we mean that the amount of water vapour in the air is also high.
• As a result, the air's water vapour intake capacity has diminished.
• Evaporation, on the other hand, is the process through which more water vapour is released into the atmosphere.
• As a result, as humidity rises, the rate of evaporation reduces.

(B) Statement (ii) and (iv) are given in this option, statement (ii) is true for the process of evaporation but statement (iv) is not.

Statement (iv) says, rate of evaporation is independent of external pressure, which is false because:

• Evaporation is a surface process in which solution molecules migrate to the atmosphere from the surface.
• When atmospheric pressure rises, molecules on the surface are unable to travel to the atmosphere due to external pressure, resulting in a decrease in the rate of evaporation.
• Therefore, the rate of evaporation is dependent on the external pressure.

(D) Statement (i) and (iv) are given in this option and both of these statements are false for the process of evaporation.

Conclusion: Hence, option (C) is the correct answer. Statement (ii) and (iii) are true for the process of evaporation.