Reaction of Iodine with Sodium Thiosulphate
When iodine reacts with sodium thiosulphate, a series of chemical reactions take place, resulting in the formation of different products. Let's break down the reaction step by step:

1. Formation of Iodine Solution
First, iodine is added to water to form a solution of iodine. This solution appears as a brown color due to the presence of iodine molecules.

2. Interaction with Sodium Thiosulphate
When sodium thiosulphate solution is added to the iodine solution, a reaction occurs where iodine is reduced to iodide ions. This reaction can be represented by the following equation:
I2 + 2S2O3^2- → 2I^- + S4O6^2-

3. Formation of Iodide Ions
As a result of the reaction, iodine is converted into iodide ions, which causes the brown color of the iodine solution to fade. The solution turns colorless as iodine is converted into iodide ions.

4. Endpoint Detection
The reaction between iodine and sodium thiosulphate is often used in titrations to determine the concentration of a solution. The endpoint of the reaction is detected using a starch indicator, which forms a blue-black complex with any remaining iodine in the solution.

5. Overall Reaction
The overall reaction between iodine and sodium thiosulphate can be summarized as the reduction of iodine by thiosulphate ions to form iodide ions and tetrathionate ions. This reaction is often used in analytical chemistry for titrations and quantitative analysis.
In conclusion, the reaction between iodine and sodium thiosulphate results in the reduction of iodine to iodide ions, leading to a color change in the solution. This reaction is important in analytical chemistry for determining the concentration of solutions.