JAMB - Chemistry (2024 - No. 113)

If a salt weighs 2g and upon exposure to the atmosphere weighs 1.5g, it simply means it has lost some moisture content from it.  This process is called efflorescence.

Efflorescence is the process by which a hydrated salt loses water spontaneously when the aqueous vapour pressure of the hydrate is greater than the partial pressure of water vapour in the air. This causes the salt to lose all or part of its water of hydration, resulting in a powdery appearance on its surface.