The formation of a sol

In this experiment, students observe as the teacher prepares and boils a small quantity of iron(III) chloride solution. As the solution is heated, hydrolysis produces hydrogen chloride and the solution becomes cloudier as iron(III) hydroxide forms as a precipitate. Students then watch as some of the original iron(III) chloride solution is added to hot water, producing a cherry-red colour with the formation of iron(III) oxide as a sol (a colloidal suspension).

This demonstration is easy to set up and should take around 10 minutes. The experiment requires a high degree of precision for complete success. It should be carried out in a fume cupboard because some hydrogen chloride gas is formed during the procedure.

Equipment

Apparatus

• Eye protection
• Access to a fume cupboard (see note 4 below)
• Beaker or conical flask, 1 dm³
• Test tube
• Boiling tube
• Teat pipette
• Bunsen burners, x2
• Tripod and gauze
• Heat resistant mat
• Filter funnel and filter paper (optional)
• Cellophane sheet, small piece for filtering (optional)

Chemicals

• Solid hydrated iron(III) chloride, about 8 g
• Deionised (or distilled water), about 500 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and carry out the demonstration in a fume cupboard.
• Iron(III) chloride,  FeCl₃.6H₂O(s), (HARMFUL) – see CLEAPSS Hazcard HC055C.
• A small amount of hydrogen chloride gas, HCl(g) (TOXIC, CORROSIVE – see CLEAPSS Hazcard HC049) is produced when the concentrated iron(III) chloride solution is boiled.

Procedure

1. Heat about 500 cm³ of deionised water in the beaker until it boils. Turn the heat down so that the water simmers gently.
2. Prepare a small quantity (about 10 cm³) of concentrated iron(III) chloride solution by dissolving the iron(III) chloride in about three times its volume of deionised water.
3. Transfer a few cm³ of the solution to a boiling tube and heat it until it boils. Ask students to write down what they see as the solution boils.
4. Turn off the heat under the beaker of water.
5. Use the teat pipette to add some of the original concentrated iron(III) chloride solution to the hot water a few drops at a time. Ask students to write down what they see.
6. If there is time, it might be instructive to attempt to filter this solution using filter paper and then cellophane.

Teaching notes

As the concentrated solution of iron(III) chloride is heated, hydrolysis occurs producing hydrogen chloride, some of which will escape as a gas. The dark brown solution turns murkier as a precipitate of iron(III) hydroxide forms. This hydrolysis is reversible:

FeCl₃(aq) + 3H₂O(l) → Fe(OH)₃(s) + 3HCl(aq)

When concentrated iron(III) chloride solution is added to the large beaker of hot water, a cherry-red colour is seen, due to the formation of colloidal iron(III) oxide.

A colloidal system is one with components of one or two phases – gas, liquid or solid. In a relatively straightforward system there are two components, one of which is being dispersed and the other known as the continuous medium.

Colloids can be classified according to the phases involved:

• Liquid in gas: often known as an aerosol, eg mist
• Solid in gas: this is a smoke, eg dusty air, smoke from a bonfire
• Gas in liquid: this is a foam, eg whipped cream
• Liquid in liquid: this is an emulsion, eg blood, mayonnaise, milk
• Solid in liquid: this is a sol (colloidal suspension), eg paint, starch ‘solution’, the iron(III) oxide seen in this demonstration
• Gas in solid: this is usually known as a solid foam, eg pumice, expanded polystyrene foam
• Liquid in solid: this is a gel, eg gelatine, jelly, many types of cheese
• Solid in solid: this is a solid sol, eg coloured glass

Notice that ‘gas in gas’ is not on the list. This is because all gases are ‘soluble’ in each other and thoroughly mix to give a homogeneous system.

Students can make a table classifying the various types of colloidal system, researching their own list of examples using the internet.

Colloids bridge the gap between solutions and suspensions. They represent a type of mixture intermediate between a homogeneous mixture (also called a solution) and a heterogeneous mixture, and have properties that are also intermediate between the two.

The size of dispersed phase particles in a colloid range from one nanometre (1 x 10^–9 m) to one micrometre (1 x 10^–6 m).

While precipitates can be held back in filter paper, sols cannot. This is because their particles are smaller than the size of the pores in the paper. However, many kinds of membrane, such as cellophane, restrict the passage of dispersed colloidal particles more than they restrict the passage of dissolved ions or molecules. In this demonstration, the sol passes through a filter paper but should be trapped by a cellophane film.

Another feature of colloidal systems is that they often exhibit what is known as the Tyndall Effect, which involves the scattering of light by particles in the colloid.

Students could investigate the Tyndall Effect by researching on the internet.