The crystals of washing soda undergo efflorescence because it is hydrated salt containing 10 molecules of water of crystallization. When the crystals of washing soda exposed to air loose water of crystallization and form white powder of sodium carbonate monohydrates.
Na 2CO3 .10 H2 O ------air---- --> Na2 CO2. H2 O + 9H2O
The loss of water of crystallization a substance when exposed to air is called efflorescence.
