On heating below 373K, it loses 9 molecules of water of crystallisation to form monohydrate (Na2CO3.H2O). On heating above 373K, the monohydrate changes to an anhydrous white powder called soda ash but does not decompose further.
Na2CO3 is a salt of strong base (NaOH) and weak acid (H2CO3). When Na2CO3 is dissolved in water, it undergoes hydrolysis to produce strong base (NaOH) and weak acid (H2CO3). Hence its aqueous solution is alkaline in nature.