# Thermodynamics Questions and Answers – Properties of Atmospheric Air

This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Atmospheric Air”.

1. Dry air consists of
a) oxygen, nitrogen
b) carbon dioxide
c) hydrogen, argon
d) all of the mentioned

Explanation: Dry air is a mixture of all these gases.

2. Complete dry air exists in nature.
a) true
b) false

Explanation: Complete dry air does not exist in nature.

3. Which of the following is true?(here pa=partial pressure of dry air, pw=partial pressure of water vapour, p=atmospheric pressure)
a) p=pw
b) p=pa
c) p=pw + pa
d) all of the mentioned

Explanation: This comes by Dalton’s law of partial pressures.

4. In a mixture of dry air and water vapour,
a) mole fraction of dry air = pa/p
b) mole fraction of water vapour = pw/p
c) both of the mentioned
d) none of the mentioned

Explanation: This comes from the concepts of Dalton’s law of partial pressures.

5. When pw is very small,
a) saturation temperature of water vapour at pw is less than atmospheric temperature
b) water vapour in air exists in superheated state
c) air is said to be in unsaturated state
d) all of the mentioned

Explanation: Mostly partial pressure of water vapour is very small.

6. Relative humidity is defined as
a) (saturation pressure of pure water) / pw
b) pw / (saturation pressure of pure water)
c) (saturation pressure of pure water) / p
d) p / (saturation pressure of pure water)

Explanation: Here pw is the partial pressure of water vapour and ps is the saturation pressure of pure water at same temperature of mixture.

7. For saturated air, relative humidity is 0%.
a) true
b) false

Explanation: For saturated air, relative humidity is 100%.

8. If water is injected into a container with has unsaturated air,
a) water will evaporate
b) moisture content of air will decrease
c) pw will decrease
d) all of the mentioned

Explanation: The moisture content of air will increase and pw will increase.

9. Humidity ratio is given by the ratio of
a) (mass of dry air per unit mass of water vapour)^2
b) 1/(mass of dry air * mass of water vapour)
c) water vapour mass per unit mass of dry air
d) mass of dry air per unit mass of water vapour

Explanation: Humidity ratio is also called specific humidity.

10. The degree of saturation is the ratio of
a) (saturated specific humidity / actual specific humidity)^2
b) 1/(saturated specific humidity * actual specific humidity)
c) saturated specific humidity / actual specific humidity
d) actual specific humidity / saturated specific humidity

Explanation: Here both saturated specific humidity and specific humidity are at same temperature.

11. The degree of saturation varies between -1 and 0.
a) true
b) false

Explanation: The degree of saturation varies between 0 and 1.

12. Which of the following statement is true?
a) dew point temperature is the temperature at which water vapour starts condensing
b) dry bulb temperature is recorded by thermometer with dry bulb
c) wet bulb temperature is recorded by thermometer when bulb is covered with a cotton wick which is saturated with water
d) all of the mentioned

Explanation: These are the definitions of dew point temperature, dry bulb temperature and wet bulb temperature.

13. The wet bulb temperature is the ____ temperature recorded by moistened bulb.
a) lowest
b) highest
c) atmospheric
d) none of the mentioned

Explanation: This is a property of wet bulb temperature.

14. At any dbt, the ____ the difference of wbt reading below below dbt, ____ is the amount of water vapour held in mixture.
a) smaller, smaller
b) greater, greater
c) greater, smaller
d) smaller, greater

Explanation: This is an important property of dbt and wbt.

15. When unsaturated air flows over a sheet of water in an insulated chamber
a) specific humidity of air decreases
b) the water evaporates
c) both air and water are cooled during evaporation
d) all of the mentioned

Explanation: The specific humidity of air increases during this process.

Sanfoundry Global Education & Learning Series – Thermodynamics.
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