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Foam Fights Fire

Fabulous Foam Fire Extinguishers

These are tremendous, messy fun!!!
You need:

• One150 cm³ conical flask
• Two 250 cm³ beakers
• Rubber bung to fit the conical flask
• An L-shaped delivery tube with one end drawn out into a fine nozzle. The delivery tube should be fitted into the bung.
• Sodium hydrogencarbonate
• Aluminium sulphate
• Washing-up liquid
• A SINK!

Method

First make up your foaming agent. Place two spatulas of aluminium sulphate into one of the beakers. To this add 30 cm³ of water. Stir until dissolved. Now add two drops of detergent. Swirl gently to mix - WE DON'T WANT A FOAM YET!!!!

Carefully pour your foaming agent into the conical flask.

Now make up your source of carbon dioxide. Put two spatulas of sodium hydrogencarbonate into the second beaker and add 30 cm³ of water. Stir until dissolved.

Take your conical flask and beaker to a sink. Have the delivery tube ready to put into the conical flask.

Quickly pour the hydrogencarbonate into the conical. Place the bung in the conical and aim the nozzle towards the sink.

A white foam should come bubbling out of the nozzle.

Click here to see the reaction.

Why does it work?

Sodium hydrogencarbonate reacts with acids to release carbon dioxide gas. Aluminium sulphate is acidic. So when the two solutions are mixed together, carbon dioxide gas is released. The washing-up liquid helps to trap the gas as a foam.

As the pressure increases inside the conical flask, there is only one place the foam can escape through - the delivery tube.

Acid + Carbonate > Carbon dioxide + Salt

Foams are good for putting out fires because they form a blanket on top of the fire. This excludes oxygen from the flames. Without oxygen the fire goes out.

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