Efflorescence

This is the process by which water of crystallization is lost from a hydrated salt to the atmosphere.

Ice-like crystal of sodium carbonate decahydrate

Sodium carbonate decahydrate, Na₂CO₃.10H₂O is an efflorescent salt. This can cause problems in quantitative work if we assume that the original formula (before the loss of water) is accurate. You should be able to see the powdery nature of the salt at the surface where water has been lost.

Sodium carbonate decahydrate after efflorescence