Question

define efflorescence give an example

Answer 1

efflorescence means to loss the water of crystallization
for example gypsum converted into plaster of Paris

Answer 2

Efflorescence, in chemistry, is the loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.

 in chemistry it reffers to loosing of molecules of water attached to a compound,but accord. to x ,when Na2CO3.10H2O loses nine molecules of water of crytilization is known as efflorosence 
Na2CO3.10H2O----Na2CO3.H2O + 9H2O