The correct option is 1 and 3 only.
Explanation
Condensation is the process by which water vapour in the air transforms into liquid water. This phase change occurs when the air becomes saturated with moisture. Saturation can be achieved either by cooling the air to its dew point or by increasing the moisture content in the air.
Statement-wise Analysis
- Statement 1 is Correct: When the temperature of an air mass is reduced to its dew point, its capacity to hold water vapour decreases. If the volume remains constant and the temperature drops to the dew point, the air becomes saturated (100% relative humidity), leading to condensation. This is the primary mechanism behind the formation of dew, frost, and fog.
- Statement 2 is Incorrect: While an increase in the volume of air (adiabatic expansion) leads to cooling and potential condensation (as seen in cloud formation), it is not the only condition under which condensation occurs. Condensation also takes place when air is cooled by contact with a cold surface (conduction/radiation) without an increase in volume, or when moisture is added to the air. The use of the word "only" makes this statement invalid.
- Statement 3 is Correct: Adding moisture to the air through evaporation increases the specific humidity. If enough moisture is added while the temperature remains stable, the air will eventually reach its saturation point, triggering condensation.
Key Takeaway
Condensation is caused by the cooling of air to its dew point or by the addition of water vapour. It depends on the interplay between air temperature, volume, and moisture content, not solely on volume expansion.