What happens when crystals of washing soda are kept open in the air for long time?
When crystals of washing soda (Na2SO4.10H2O) are kept open in the air for long time then the water of crystallization will lose and the crystals will turn to white. This process of loosing water of crystallization when kept in air is called efflorescence.
Na2SO4.10H2O → Na2SO4 + 10H2O