Hydrochloric acid (HCl) is a colorless solution when pure. However, if certain compounds are dissolved in it, it can turn red. One such example is the reaction between HCl and ferric chloride (FeCl3). When FeCl3 is added to HCl, it undergoes a chemical reaction to form a complex ion called [FeCl4]-. This complex ion is responsible for the red color.

The reaction can be represented as follows:

FeCl3 + HCl → [FeCl4]- + H+

The red color of the solution is due to the absorption of light by the [FeCl4]- complex ion. This complex ion absorbs light in the blue-violet region of the spectrum, and reflects the red light, which gives the solution its red color.

The reaction between HCl and FeCl3 is a reversible reaction. This means that if the concentration of HCl is increased, the reaction will shift to the left, and the red color will disappear. Similarly, if the concentration of FeCl3 is increased, the reaction will shift to the right, and the red color will become more intense.