Na₂CO₃. 10H₂O when exposed to air looses water of crystallisation and gives finally 
 

${\mathrm{Na}}_2{\mathrm{CO}}_3 .10{\mathrm{H}}_2\mathrm{O} \stackrel{ }{\to } {\mathrm{Na}}_2{\mathrm{CO}}_3 .{\mathrm{H}}_2\mathrm{O} +\hspace{0.17em}9{\mathrm{H}}_2\mathrm{O}$

The process of losing water of crystallization from hydrated salt when exposed to air is called efflorescence.
Hydration means the addition of water, and dehydration means the removal of water.
Deliquescence means tending to become a liquid.