When dilute hydrochloric acid is added to sodium chromate solution the colour changes to:

orange

The correct answer is option 4. orange. 

Let us break down the process and the color change in detail when dilute hydrochloric acid (\(HCl\)) is added to a sodium chromate (\(Na_2CrO_4\)) solution:

Sodium Chromate (\(Na_2CrO_4\))

Sodium chromate is a salt that dissociates in water to produce chromate ions (\(CrO_4^{2-}\)). The chromate ion has a yellow color in aqueous solution.

Hydrochloric Acid (\(HCl\)):

Hydrochloric acid is a strong acid that dissociates in water to produce hydrogen ions (\(H^+\)) and chloride ions (\(Cl^-\)).

Chemical Reaction:

When hydrochloric acid is added to a sodium chromate solution, the following chemical reaction occurs:

\(2 \text{CrO}_4^{2-} + 2 \text{H}^+ \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}\)

In this reaction:

Chromate ions (\(CrO_4^{2-}\) are converted into dichromate ions (\(Cr_2O_7^{2-}\)). The chromate ions are yellow, while the dichromate ions are orange.

Detailed Steps of the Color Change:

Initial State:

Sodium Chromate Solution: Contains \(CrO_4^{2-}\) ions, which impart a yellow color to the solution.

Addition of Hydrochloric Acid:

Acidic Environment: The hydrogen ions (\(H^+\)) from the hydrochloric acid react with the chromate ions. The acidic conditions shift the equilibrium towards the formation of dichromate ions.

Reaction and Color Change:

The reaction transforms yellow chromate ions (\(CrO_4^{2-}\)) into orange dichromate ions (\(Cr_2O_7^{2-}\)). The solution changes color from yellow to orange as a result of this conversion.

Summary:

In a neutral or basic environment, sodium chromate gives a yellow solution due to the presence of \(CrO_4^{2-}\) ions. When hydrochloric acid is added, the acidic conditions lead to the formation of \(Cr_2O_7^{2-}\) ions, which results in an orange-colored solution.