Halogen/halide displacement reactions

When chlorine, Cl2 is added to an aqueous solution of potassium iodide, KI, the following reaction occurs:

chlorine   +   potassium iodide   —>   iodine   +   potassium chloride

Cl2   +   2 KI    —>    I2   +   2 KCl

The reaction mixture is seen to turn orange/brown because of the iodine formed. Iodine molecules are only slightly soluble in water and they are likely rendered more soluble here due to the presence of the potassium chloride. Lugol’s solution, a solution of iodine in potassium iodide, is well documented.

The intensity of the colour depends on the concentration of iodine molecules present. It’s the same brown colour that we see in antiseptic iodine solutions bought at the pharmacy., although such antiseptic solutions are likely to be Povidone-iodine which contains a complex formed from povidone (polyvinylpyrollidone) and iodine.


In the above reaction the chlorine is said to displace the iodide ions from solution, forming chloride ions and iodine molecules. This happens because chlorine is more reactive than iodine.

 Chlorine, chloride ..  the difference between an "ine" and "ide"

Chlorine, chloride .. the difference between an “ine” and “ide”

Chlorine is above iodine in group 7 and this displacement reaction follows the trend observed in the group 7 elements. Fluorine is the most reactive halogen at the top and others show decreasing reactivity as you go down the group. So any halogen will displace halide ions from solution of any member of the group below it.

So what other halogen/halide displacement reactions might we observe in group 7?

Here’s an experiment where chlorine water, bromine water and aqueous iodine solution were added drop-wise using a pipette to solutions of sodium chloride, sodium bromide and potassium iodide. See if you can tell which combinations produce a displacement reaction. You need to look for a change in colour or an increase in intensity of any colour already present.

Halogen / halide displacement reactions

Halogen / halide displacement reactions


The formula of calcium fluoride is CaF2. It is made up of calcium ions, Ca2+ and fluoride ions, F-.

The formula of calcium fluoride is ...

The formula of calcium fluoride is …

Calcium fluoride, or fluorite, is an interesting mineral from which the term fluorescence was coined. Fluorescence is the phenomenon by which a substance emits light after absorbing electromagnetic radiation of a different wavelength. In the case of the sample of fluorite shown below, it emits blue light after absorbing certain wavelengths of ultraviolet radiation. Different samples of fluorite may emit different colours.

Pale green in daylight, blue in U.V. light

Pale green in daylight, blue in U.V. light

Fluorine is the most reactive element in group 7, chlorine the second element in the group is the next most reactive and so on. Thus, the reactivity of the halogens is said to decrease on going down the group.
What would you predict would happen if chlorine Cl2, were added to an aqueous solution of potassium iodide, KI?