How a fire is put out
A fire needs three things at once: fuel, oxygen and heat. These form the fire triangle. A fire extinguisher stops a fire by removing at least one side of the triangle, for example by cooling the fuel, smothering the oxygen, or both. Extinguishers are only for small fires; a large or spreading fire needs evacuation and the fire brigade (call 999 in Malaysia). Every laboratory should also have a fire blanket nearby, which smothers a small fire on clothing or a burner by cutting off oxygen.
Classes of fire and matching extinguishers
Fires are grouped by what is burning, and each extinguisher suits certain classes.
| Type | Best for |
|---|---|
| Water | Wood, paper, cloth |
| Carbon dioxide (CO2) | Electrical and flammable liquids |
| Dry powder | Many types, including flammable liquids |
| Wet chemical | Cooking oil and fat fires |
Key idea
Never use a water extinguisher on an electrical fire or on burning oil. Water conducts electricity and can cause a shock, and it makes hot oil erupt violently. Use CO2 for electrical fires and wet chemical for oil fires.
Using an extinguisher: PASS
The steps are remembered as PASS: Pull the safety pin; Aim the nozzle at the base of the flames; Squeeze the handle; Sweep from side to side. Always keep your back to a clear exit so you can escape if the fire grows.
Example
A Bunsen burner tips over and sets paper alight on the bench. A student takes the water or dry powder extinguisher, pulls the pin, aims at the base of the flames, squeezes and sweeps until the paper stops burning, then reports the incident to the teacher.
Remember
Aim at the base of the fire, not the top of the flames, because that is where the fuel is. Check that extinguishers are serviced and not expired, and know where the nearest one is before you start work.