When crystals of Na₂CO₃·10H₂O (sodium carbonate decahydrate) are exposed to air, they:

Sodium carbonate decahydrate (Na₂CO₃·10H₂O) is a hydrated salt containing ten molecules of water of crystallization. When exposed to air, these crystals tend to lose water gradually because the surrounding atmosphere usually has lower humidity than the salt itself. This phenomenon is called efflorescence.

During efflorescence, the water of crystallization evaporates from the crystals, but the salt remains solid. The compound does not dissolve or turn into liquid. This behavior distinguishes efflorescent salts from deliquescent salts, which absorb moisture and form a solution.

The reaction can be summarized as:

Na₂CO₃·10H₂O (s) → Na₂CO₃·xH₂O (s) + (10 − x) H₂O (g)

where x < 10 represents the remaining water molecules. Sodium carbonate decahydrate gradually loses water to become sodium carbonate heptahydrate (Na₂CO₃·7H₂O) or eventually anhydrous Na₂CO₃, depending on exposure and humidity.

This property is important in inorganic chemistry and practical applications. Understanding efflorescence helps in storing hydrated salts correctly and predicting their behavior in chemical reactions.

In summary, when Na₂CO₃·10H₂O is exposed to air, it loses water molecules but stays solid, illustrating a key property of efflorescent salts. This behavior is a classic example used in chemistry education to explain the differences between efflorescent and deliquescent compounds.