When sodium is heated in air, it burns with a golden yellow flame to form sodium peroxide \(2 Na _{( s )}+ O _{2( g )} \rightarrow Na _{2} O _{2( s )}\)
If it is exposed to air (without heating) it forms sodium oxide \(\left( Na _{2} O \right)\). The moisture present in the air combines with some of the oxide to form the hydroxide and finally, after a time, the carbon dioxide of the air combines with the sodium hydroxide to form sodium carbonate which crystallizes to form the decahydrate.
\(4 Na _{(s)}+ O _{2( s )} \rightarrow 2 Na _{2} O _{(s)}\)
\(Na _{2} O _{( s )}+ H _{2} O _{(n)} \rightarrow 2 NaOH _{( aq )}\)
\(2 NaOH _{( aq )}+ CO _{2(g)} \rightarrow Na _{2} CO _{3(s)}+ H _{2} O\)
\(Na _{2} CO _{3( s )}+10 H _{2} O _{(0)} \rightarrow Na _{2} CO _{3}, 10 H _{2} O _{( s )}\)