Decolourising and deodorising

Class practical

A form of carbon, known as activated charcoal, which has undergone special treatment, is capable of removing coloured and volatile material from mixtures by a process of adsorption. This has wide application in the organic chemistry laboratory and in industry, such as the removal of objectionable taste and odours from drinking water.
 

Lesson organisation

The experiment is conveniently carried by groups of two or three and will take about 45 minutes.

Health & Safety and Technical  notes

Read our standard health & safety guidance

Wear eye protection.

Activated charcoal, C(s) - see CLEAPSS Hazcard.

Malt vinegar - see CLEAPSS Hazcard.

1 Fountain pen ink (“washable” variety) is the best type of ink to use. A dilute solution of potassium manganate(VII) could be used instead of ink or food flavouring.

2 Juice from sauerkraut or pickles could be used instead of malt vinegar.

Procedure

a Fold a piece of filter paper, place it in a funnel, and put the stem of the funnel into a test-tube in a test-tube rack.

b Add about five spatulas of activated charcoal to the funnel.

c Add one drop of ink or food colouring to 100 cm³ of water in a beaker.

d Carefully pour some of the coloured water on to the charcoal in the filter paper. Note whether the drops of liquid in the test-tube have lost the original colour.

e Repeat the activity with another test-tube, this time pour 100 cm³ of malt vinegar through the charcoal. Note whether the filtered liquid has lost some of its original strong smell.

Teaching notes

Students need to be warned that activated charcoal powder is extremely messy and difficult to remove from clothing.

The vinegar still smells after filtration, but noticeably less so.

Background Theory
Heating wood to a very high temperature in the absence of air makes charcoal. When it is heated to an even higher temperature - about 930 °C - impurities are driven from its surface and it becomes “activated charcoal”. This activated charcoal can remove impurities in either the gaseous or liquid state from many solutions. It does so by the process of adsorption, by attracting these molecules to the surface of the charcoal.

Adsorption by charcoal is also used to remove unburned hydrocarbons from car exhausts, harmful gases from the air, and unwanted colours from certain products.

Students may find the difference between adsorption and absorption confusing. Adsorption is a process in which a gas, liquid, or a dissolved substance is gathered on the surface of another substance – eg charcoal. Absorption is a process in which a liquid is soaked up, as with blotting paper. It is taken in completely and mixes with the absorbing material – eg absorbent cotton.

Health & Safety checked April 2008