Question

Select the correct statements.
I. Increase in relative humidity decreases the density of air.
II. Increase in relative humidity decreases the rate of evaporation of water.
III.Increase in relative humidity decreases the velocity of sound.
IV.Increase in relative humidity increases the velocity of sound.
A.I,II,IV
B.I,II,III
C.I,II
D.I,IV

Answer 1

Hint: We know that humidity is the content of water vapour present in the air. It is a primary element of the climate. We should be well versed with the relation between water vapour and humidity. More the water vapour in the air, higher will be the humidity in the air and it will feel wetter. The concept of humidity in the air is generally discussed under the concept of relative humidity.

Complete step by step solution:
Humidity tells us the amount of moisture present in the air. The humidity present in the air is said to be the relative humidity. Relative humidity is calculated as the ratio of the current absolute humidity to the highest possible absolute humidity.
\[\operatorname{Re} lativeHumidity = \dfrac{{CurrentAbsoluteHumidity}}{{\max imumPossibleAbsoluteHumidity}}\]
We can take an example to have a better understanding of this concept. So let us suppose that we take a reading of 100% relative humidity, this means that the air is totally saturated with water vapour and cannot hold any more moisture which may result in the possibility of rain.
It is measured with the help of a hygrometer. The water in the surrounding evaporates which results in moist air and humidity thus, when relative humidity increases the amount of water vapour in the air increases and so the amount of other gases like oxygen and nitrogen decreases per unit volume and so density of air increases as the mass decreases. Dry air is denser than humid air.
So, statement (I) is true.
Humidity in itself shows the amount of moisture present in the air thus, at a given temperature the air can hold a particular amount of water vapour and thus the increase of humidity will decrease the evaporation rate as air cannot hold more water than its capacity as it reaches saturation.
So, we know that the statement (II) is true.

As humidity increases, the air molecules that are water molecules also increases, density of air decreases and therefore sound waves travel faster than usual thus, the velocity of sound increases.
So, statement (III) is false and (IV) is true.

Thus we conclude that the correct answer for the given question is option A as statements (I), (II) and (IV) are true.

Note: Absolute humidity is the ratio of mass of water vapour to the mass of dry air a volume of air at a given temperature. It is expressed as grams of moisture per cubic meter of air\[\left( {g{m^{ - 3}}} \right)\]. Human skin is very sensitive to humidity and presence of moisture in the air. Hygrometer that measures the difference between the two thermometers (which further helps in calculation of relative humidity) is known as psychrometer.