Why do we regard the gaseous state of water as vapours while that of ammonia a gas?

Question: Why do we regard the gaseous state of water as vapours while that of ammonia a gas?

Water and ammonia are both compounds that can exist in a gaseous state at room temperature and atmospheric pressure. However, we commonly refer to the gaseous state of water as "water vapor" while the gaseous state of ammonia is simply called "ammonia gas". The reason for this difference lies in the properties of these two compounds.

Water has a significantly higher boiling point than ammonia. The boiling point of water at standard atmospheric pressure is 100°C, while the boiling point of ammonia at standard atmospheric pressure is -33°C. This means that at room temperature, water molecules tend to have weaker intermolecular forces of attraction, and a significant portion of them can escape into the gas phase to form water vapor. This process is commonly referred to as "evaporation". The water vapor can mix with air, and it remains as a gas as long as the temperature and pressure remain constant.

On the other hand, ammonia molecules have a stronger tendency to stick together due to their intermolecular forces of attraction, which are stronger than those between water molecules. This makes it more difficult for ammonia molecules to escape into the gas phase and form a gas. Instead, they tend to form a gas more readily when they are heated or when the pressure is reduced, which allows the intermolecular forces to be overcome.

Therefore, we refer to the gaseous state of water as "water vapor" because it is formed by evaporation, whereas the gaseous state of ammonia is simply called "ammonia gas" because it requires heating or a reduction in pressure to form a gas.

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