Fight fire! Fill up with foam

The difficulties in putting out big oil fires in the open air is that the wind blows away the carbon dioxide gas, allowing oxygen to reach the fire, keeping it burning.

This experiment should take around 70 minutes.

Task

Make as much foam as possible (measured in a very large container) using:

1. Any combination of the 3 liquids (you are allowed a maximum volume of 20 cm3 of each).
2. Any combination of the 3 solids (in this case you are allowed a maximum of 6 spatula fulls of each).

These can be mixed in any order, but must not be shaken or stirred. 

Equipment

• Eye protection
• Large measuring cylinders (500, 1000 cm³) or empty 1 litre plastic lemonade bottles with tops cut off
• Pestles and mortars
• Spatulas
• Sodium hydrogencarbonate
• Sulfuric acid (2 mol dm^-3)
• Aluminium sulfate
• Washing up liquid
• Washing powder
• Water
• Food colouring–optional (see possible approaches below). 

Health, safety and technical notes

• Read our standard health and safety notes here.
• Wear eye protection.
• Wear lab coats or aprons, if desired. 
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Sulfuric acid, 2 M H₂SO₄ (aq), is corrosive. See CLEAPSS Hazcard HC098a.
• Aluminium sulfate causes eye damage at solutions of over 3% and is an irritant over 1%. See CLEAPSS Hazcard HC002b.

Possible approaches

A foam is a colloidal system in which a gas is dispersed in a liquid.

Construction of a table to record results would be useful and aid systematic working.

The foam is formed when a sodium hydrogencarbonate solution is mixed with a solution containing a detergent and aluminium sulfate (or any weak acid). Carbon dioxide gas is produced, which is trapped by the detergent.

Trialled with a group of mixed ability second years, students were not given washing powder, just liquid detergent, to reduce variables. 500 cm³ measuring cylinders were fine for most students, but one or two needed to use the litre measuring cylinders. (It might be possible to use 500 cm³ beakers, but they would really be too wide.)

The foam makes it very difficult to read the calibrations on the measuring cylinder – colouring the mixture might help.

Students were good at keeping a record of the amounts they used – which has to be quoted for their ‘best’ result to count. The greatest volume of foam was 800 cm³. 

Possible extension

Students could go on to design a ‘foam launcher’, and make a fire extinguisher that operates. They will need to consider the technical problem of delivering the foam to put out fire. 

Name some other examples of foams and their useful applications, eg shaving and whipped cream.

Notes

Any remaining acid should be neutralised with weak alkali before being washed to waste.

Conkers make wonderful foam!

1. Boil conkers up with water
2. Peel conkers, mash them up, put in 250 cm³ beaker a third full of water. 
3. Boil for 5/10 minutes. DECANT SOLUTION.
4. To make the froth: add sodium hydrogencarbonate (approximately a dessert spoon) and aluminium sulfate (sufficient acidity).
5. Frothing agent obtained is the same as that found in shaving foams and fire extinguishers (a C10 alcohol).