Questions

The amount of water vapor ranges from a trace amount up to 4% of the mass of air. Water vapor exists as an invisible gas in the air.

The amount of water vapor in the air is called absolute humidity. The amount of water vapor in the air as compared with the amount of water that the air could hold is called relative humidity.

The maximum amount of water vapor that can be in the air depends on the air temperature.

Black  surfaces are good emitters of heat ( They are also good absorbers of heat). If the black surface is rough too, the emitting will be better. So (b) is the correct answer. Shining surfaces reflect back radiation. So if the black surface is also shining the effectiveness of emitting heat is lessened.

Molecular compounds are those with covalent bonds between the elements and those elements are typically non metals.

Here in the given options, Ar is an element. NaI has an ionic bond. Hence, N2O4 has a covalent bond between the two non metals and is a molecular compound.

Oxygen is the final electron acceptor of cellular respiration.

Salivary glands of man are 3 pairs = 6. They are one pair of parotid glands, one pair of sub-maxillary or sub-mandibular glands and one pair of sub-lingual glands.