Iron on reaction with HCl forms FeCl3 as the hydrogen liberated during the process prevent the oxidation of ferrous chloride to ferric chloride.
ferric chloride solution appears as a colourless to light brown aqueous solution that has a faint hydrochloric acid (HCl) odor.

Introduction:
When making an aqueous solution of ferric chloride laboratory reagent, it is common to add some HCl solution in addition to FeCl3. This is done for various reasons related to the solubility, stability, and reactivity of ferric chloride.

Reasons for adding HCl solution:

1. Solubility: FeCl3 is sparingly soluble in water, especially at lower temperatures. The addition of HCl helps in increasing the solubility of FeCl3 by forming a complex ion. The reaction between FeCl3 and HCl leads to the formation of FeCl4- complex ion, which is more soluble in water than FeCl3 alone.

2. Stability: FeCl3 can hydrolyze in water, forming insoluble Fe(OH)3 precipitate. This hydrolysis reaction is more significant at higher pH levels. By adding HCl, the solution becomes more acidic, which helps in preventing the hydrolysis of FeCl3 and maintaining its stability.

3. Redox reactions: HCl can act as a reducing agent in certain redox reactions. In the presence of HCl, FeCl3 can be reduced to FeCl2, which may be desirable in certain experimental procedures.

4. Controlling pH: In some cases, the addition of HCl is necessary to adjust the pH of the solution. The pH can affect the reactivity and behavior of the ferric chloride solution in various reactions.

Procedure:
To prepare an aqueous solution of ferric chloride, you can follow these steps:

1. Measure the desired amount of FeCl3 solid reagent using a balance.
2. Dissolve the FeCl3 in a small amount of distilled water in a beaker.
3. Slowly add concentrated HCl solution drop by drop while stirring the solution.
4. Continue adding HCl until the desired concentration or pH is achieved.
5. Transfer the solution to a volumetric flask and dilute it to the desired volume with distilled water.
6. Label the flask with the concentration and date of preparation.

Conclusion:
The addition of HCl solution while making an aqueous solution of ferric chloride laboratory reagent is necessary for increasing solubility, maintaining stability, controlling pH, and facilitating certain redox reactions. It helps in obtaining a stable and reactive ferric chloride solution, suitable for various laboratory applications.