chem ce 1997 paper 1 #8a(iv) 個題ge filter paper(KI)有D咩野用??

In the experiment, chlorine bleach reacts with dilute hydrochloric acid to give chlorine gas. The experiment also shows that chlorine gas can react with KI solution (actually the I- ions in the solution).
Cl2(g) + 2I-(aq) → 2Cl-(aq) + I2(aq)
or Cl2(g) + 2KI(aq) → 2KCl(aq) + I2(aq)

But what container is used to hold the KI solution ?
If a common container (e.g. small test tube or spatula) is used, it is difficult for the chlorine gas to make contact with KI solution. A simple method is used a piece of filter paper to be moistened by KI solution. In this case, the filter paper acts as a "container" to hold KI solution. The colourless I-(aq) ion soaked in the filter paper changes to brown I2(aq) solution, but the filter paper does not change.

Similarly, when a drop of dilute acid is added to a piece of blue litmus paper, the litmus paper changes to red. The paper also acts as a "container" of litmus. Actually, the blue litmus changes to red, but the paper does not change at all.