The decahydrate form of sodium carbonate i.e. washing soda on standing in air effloresces and crumbles to powder. The number of water molecule(s) present in the compound formed is:

Hydrated crystals of sodium carbonate exposed to air it losses it is 9 molecules of water of crystallization leaving only powder sodium carbonate with one molecule of water
The process of losing water of crystallization is called efflorescence
$\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3.10{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\left(\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\right)\stackrel{\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{c}\mathrm{e}\mathrm{s}-9{\mathrm{H}}_2\mathrm{O}}{\to }\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\left(\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\right)$