Question

How can anhydrous salts regain their colour?

Answer 1

Hydrated salts: Word Hydrous is referred to a substance that contains water molecules as a constituent.

• These hydrous compounds are called hydrates.
• Here, water means the water of crystallization.
• Here water can get trapped inside the crystal lattice of substances during crystallization.

For example,

$$\begin{gathered} Copper\left(II\right)sulfate\left(CuS{O}_4.7H_{2}O\right) \\ Hydratedsalt\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\left(Blue\right) \end{gathered}$$

Anhydrous salts: Word anhydrous is referred to a substance that does not contain water as a constituent.

• It means the absence of water in a compound.
• These compounds are called anhydrous compounds or anhydrates.

For example,

$$\begin{gathered} Copper\mathit{}sulfate\mathit{}\mathit{}\mathit{}\left(CuS{O}_4\right) \\ \mathrm{Anh}ydratedsalt\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\left(white\right) \end{gathered}$$

How can anhydrous salts regain their color?

• The hydrated $\mathrm{Copper}\left(\mathrm{II}\right)\mathrm{sulfate}{\mathrm{CuSO}}_4.7{\mathrm{H}}_2\mathrm{O}$ has blue color due to water crystallization present in it.
• When this is heated it turns into anhydrous $\mathrm{Copper}\mathrm{sulfate}\left({\mathrm{CuSO}}_4\right)$ and loses its water of crystallization as well as its blue color.
• The color can be regained by adding a little amount of water back again.

So, the color of an anhydrous salt can be regained by adding water to crystallization and making it hydrated again.