2019 has been declared by UNESCO as the Year of the Periodic Table. To celebrate, we are releasing a series of blogs about our favourite elements and their importance to the chemical industry. Today’s blog is about iodine and some of the exciting reactions it can do!


Iodine & Aluminium

 iodine and aluminum gif

Reaction between iodine and aluminum. These two components were mixed together, followed by a few drops of hot water. Source: FaceOfChemistry

Reactions between iodine and group 2 metals generally produce a metal iodide. The reaction that occurs is:

2Al(s) + 3I2(s) → Al2I6(s)

Freshly prepared aluminium iodide reacts vigorously with water, particularly if its hot, releasing fumes of hydrogen iodide. The purple colour is given by residual iodine vapours.


Iodine & Zinc

 Zinc and iodine gif

Zinc and iodine react similarly to aluminium and iodine. Source: koen2all

Zinc is another metal, and when it reacts with iodine it too forms a salt – zinc iodide. The reaction is as follows:

Zn + I2→ ZnI2

The reaction is highly exothermic, so we see sublimation of some of the iodide and purple vapours, as with the aluminium reaction. Zinc iodide has uses in industrial radiography and electron microscopy. 


Iodine & Sodium

 Iodine reacting with molten sodium gif

Iodine reacting with molten sodium gives an explosive reaction that resembles fireworks. Source: Bunsen Burns

As with the other two metals, sodium reacts violently with iodine, producing clouds of purple sublimated iodine vapour and sodium iodide. The reaction proceeds as follows:

Na + I2→ 2NaI

Sodium iodide is used as a food supplement and reactant in organic chemistry.


Iodine Clock reaction

 iodine clock reaction gif

The iodine clock reaction – a classic chemical clock used to study kinetics. Source: koen2all

The reaction starts by adding a solution of potassium iodide, sodium thiosuphate and starch to a mixture of hydrogen peroxide and sulphuric acid. A set of two reactions then occur.

First, in a slow reaction, iodine is produced:

H2O2 + 2I + 2H+ → I2 + 2H2O

This is followed by a second fast reaction, where iodine is converted to iodide by the thiosulphate ion:

2S2O32− + I2 → S4O62− + 2I

The reaction changes colour to a dark blue or black.


Elephants toothpaste

 elephants toothpaste reaction gif

The elephant’s toothpaste reaction is a favourite for chemistry outreach events. Source: koen2all

In this fun reaction, hydrogen peroxide is decomposed into hydrogen and oxygen, and catalysed by potassium iodide. When this reaction is mixed with washing-up liquid, the oxygen and hydrogen gas that is produced creates bubbles and the ‘elephant’s toothpaste’ effect.

There are lot’s of fun reactions to be done with iodine and the other halogens (fluorine, bromine, chlorine). 

Iodine’s sublimation to a bright purple vapour makes it’s reactions visually pleasing, and great fun for outreach events and science classes.