Discussion
When a fixed mass of air-water vapour mixture is heated at constant pressure,
Answer: B
On heating at constant pressure, relative humidity decreases, but specific humidity remains unchanged.
Specific humidity is the mass of water vapour per unit mass of dry air. Heating a fixed mass of mixture adds no water and removes none, so it stays the same, and at constant total pressure the vapour partial pressure stays the same too.
Relative humidity is the vapour partial pressure divided by the saturation pressure at the mixture's temperature. The saturation pressure rises steeply with temperature, so the relative humidity falls. That is why heated air feels drier. Any option in which specific humidity changes is wrong, since no moisture is added or condensed.
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