Fire Extinguishers

Public summary: 

What does a fire need to burn, and how can we put it out? Find out how we can put out a candle with a cupful of gas!

Demonstrating the power of a carbon dioxide fire extinguisher.
Useful information
Kit List: 

NB: this experiment does not have a box. It normally lives in mini explosions as it uses bicarb and acid.
Candle (Tea light)
Deep walled heat-proof container (e.g. ceramic mug)
Plastic beaker
Bicarbonate of soda
Cloth (useful to be able to wipe-up spillages)

Packing Away: 

The liquid vinegar/bicarb reaction products may be poured down a drain/sink.

Frequency of use: 

Setup note
This experiment will only work in very still conditions (somewhere not too draughty indoors) since if there is any wind the carbon dioxide will blow away rather than extinguishing the flame.

Light a tea light (small candle in a metal holder).
Mix vinegar and bicarb in a film cannister.
Carefully, without spilling any of the liquid, pour the carbon dioxide over the candle. The flame should go out.
Take advantage of the shape of the container the candle may be in to trap the CO2.

Explanation (NB, some kids will have seen the mini explosions experiment and so will have more idea what's going on than others)

Light the tealight.
- Discuss burning. What is needed for something to burn? Fire triangle- fuel, heat, oxygen.
How can we put out a fire?
(Kids may talk about fire extinguishers... if they do, can draw that into how different extinguishers work- CO2, water, powder.)
Take away the fuel (like turning off a gas flame on a hob).
Take away the heat (cool things down, part of how water puts out fires... takes energy to heat the water so less is available to carry on the fire).
Take away oxygen (smothering the flame, what we're going to do here).

We're going to try putting out a fire by depriving it of oxygen using a gas called carbon dioxide which is heavier than air, and in which flames can't burn (used in fire extinguishers).
Make carbon dioxide by reacting vinegar (like on chips) with sodium bicarbonate/bicarbonate of soda/bicarb which they may have used for cooking. Discuss where the CO2 comes from (locked up in carbonate in solid, let out by vinegar or lemon juice (acids), or by heating as in cake baking). Reaction is: Acid + Carbonate -> Salt + Water + Carbon Dioxide

Pour CO2 over the flame... it should go out (will work better if candle has burnt down a bit in the holder, as it will hold some CO2). If it doesn't try again, you could discuss the effect of draughts on mixing oxygen back into the carbon dioxide.

This is the idea behind CO2 fire extinguishers however there's an added benefit with extinguishers, they store the CO2 as liquid using the pressure in the cylinder, when it turns from liquid to gas this also dissipate heat. It also means the cone of a CO2 extinguisher gets very cold, in fact people get frostbite and other injuries from using them incorrectly and holding this cone.

Other types of fire extinguishers are powder or foam which work by smothering the fire and cutting off the oxygen supply to the fire. Water extinguishers remove the heat from fires by propelling liquid water and it taking away heat when it turns to steam.

Risk Assessment
Date risk assesment last checked: 
Sat, 18/01/2020
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Mon, 20/01/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Vinegar and bicarb mixed and CO2 produced and used to put out a candle (tealight).

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Tealight + matches Fire risk All 4 4 16 Use a tealight rather than a more standard candle: short, stable candle contained within metal holder. Make sure tealight is on a ceramic plate or something that will not burn.
Do not leave lit candle unattended.
Keep tight control of matches. Ensure no flammable materials are near the lighted candle. Know where the nearest fire extinguisher is.
In case of fire follow local procedure (see venue RA).
1 4 4
Tealight Risk of burns. Demonstrator mainly - Kids shouldn't be holding the candle / matches. 2 3 6 Don't let kids get hold of candle / matches. Don’t leave the tealight lit for extended periods.
Run burns under cold tap for at least 10 minutes. Call first aider in event of injury.
1 2 2
Vinegar Vinegar could get into eyes/broken skin. Irritant to eyes and cuts. All 3 2 6 Ensure children have no exposed cuts before starting the demonstration. Minimal contact with vinegar anyway.
Don't let kids lean in too close to vinegar + bicarb reaction (to avoid possibility of bubbles splashing into eyes etc).
If vinegar gets in eyes, we have eyewash in the first aid box. The demonstrator must know where the eyewash is located and allow a trained first aider/parent to perform eyewash.
2 2 4