Hydrochloric acid (HCl) reacts with methyl orange to cause a color change. Methyl orange is a pH indicator that changes color from red to yellow as the pH of the solution decreases. When HCl is added to methyl orange, the pH of the solution decreases, causing the methyl orange to turn yellow.

The chemical reaction between HCl and methyl orange can be represented as follows:

$$HCl + H_2O \rightarrow H_3O^+ + Cl^-$$

$$H_3O^+ + Methyl\ orange \rightarrow H_2O + Methyl\ orange^+$$

In this reaction, the H3O+ ions from the HCl react with the methyl orange molecules to form water and methyl orange+ ions. The methyl orange+ ions are yellow, which is why the solution turns yellow when HCl is added.