When sodium carbonate crystals lose their water of crystallisation on exposure to air, this behaviour is known by which term?

Introduction / Context:
This question deals with the characteristic behaviour of hydrated salts such as sodium carbonate decahydrate when exposed to air. Knowing the terms efflorescence, deliquescence and hygroscopy is important in physical chemistry, especially in topics about hydration and interaction of solids with atmospheric moisture.

Given Data / Assumptions:

• Sodium carbonate crystals, such as washing soda, are hydrated and contain water of crystallisation.
• On exposure to air, they lose some or all of this water.
• We are asked to name this property correctly.
• Options include hygroscopy, deliquescence, efflorescence, sublimation and hydrolysis.

Concept / Approach:
Efflorescence is the process by which a crystalline hydrated salt loses water of crystallisation when exposed to air, often due to the vapour pressure of the water in the crystal being higher than that of the surrounding atmosphere. Deliquescence is the opposite behaviour, where a substance absorbs so much moisture from the air that it dissolves in the absorbed water. Hygroscopic substances simply absorb moisture from the air without necessarily dissolving or losing water of crystallisation. Sublimation refers to solid to vapour transition, and hydrolysis refers to chemical reaction with water, so neither of these match the described behaviour.

Step-by-Step Solution:
1) Sodium carbonate crystals, Na2CO3·10H2O, contain water molecules in their crystal lattice.2) When left exposed to dry air, these crystals gradually lose part or all of their water of crystallisation.3) As a result, the crystals may become powdery and less transparent.4) This loss of water of crystallisation to the atmosphere is termed efflorescence.5) Therefore, the property described in the question is efflorescence, not hygroscopy or deliquescence.

Verification / Alternative check:
Common textbook examples of efflorescent substances include sodium carbonate decahydrate and certain hydrated sodium sulphates. These salts visibly lose water and appear crusty when left in open air. Deliquescent substances, in contrast, include calcium chloride and magnesium chloride, which absorb moisture and become wet or liquefied. Sodium carbonate does not typically become liquid; instead, it dries out, confirming that efflorescence is the correct classification for its behaviour.

Why Other Options Are Wrong:
Hygroscopy: Hygroscopic substances mainly absorb moisture; they do not lose water of crystallisation to the surroundings as described here.
Deliquescence: Deliquescent solids absorb so much water from air that they eventually dissolve in the absorbed water, which is opposite to the behaviour of sodium carbonate in the question.
Sublimation: This refers to a solid changing directly to vapour, which is characteristic of substances like naphthalene, not hydrated salts losing water of crystallisation.
Hydrolysis: This involves chemical reaction with water to form new products; simply losing water molecules from the crystal lattice is not hydrolysis.

Common Pitfalls:
Students often mix up the terms efflorescence and deliquescence because both involve interaction with atmospheric moisture. The key difference is the direction of water movement. In efflorescence, water leaves the crystal and moves to the air, whereas in deliquescence water from the air enters the solid, sometimes leading to a solution. Remembering a few standard examples like sodium carbonate for efflorescence and calcium chloride for deliquescence helps avoid confusion in exam questions.

Final Answer:
The loss of water of crystallisation from sodium carbonate crystals on exposure to air is called efflorescence.