Fire Safety in the Laboratory

Introduction

Fire is a chemical reaction. In the laboratory we use it on purpose in Bunsen burners and in controlled experiments, but an uncontrolled fire is one of the most serious dangers. Understanding the chemistry of fire explains both how fires start and how they are put out. This page introduces the fire triangle, the common types of extinguisher and what students should — and should not — do if a fire starts.

Core explanation

The fire triangle. A fire needs three things at the same time: fuel (something that can burn, such as paper, wood, ethanol or gas), heat (enough to raise the fuel to its ignition temperature) and oxygen (usually from the air). Combustion is a reaction between the fuel and oxygen that releases energy as heat and light. The heat from the reaction keeps the fire going by heating more fuel. Remove any one side of the triangle and the fire goes out.

Putting out fires by breaking the triangle.

- Remove the fuel : turn off the gas supply to a burner or move flammable materials away. - Remove the heat : cool the fire, for example with water — but only where water is safe to use. - Remove the oxygen : smother the fire, for example with a fire blanket, a heat-resistant mat over a small beaker fire, or a carbon dioxide extinguisher that pushes air away.

Types of extinguisher. Different fires need different extinguishers. Water extinguishers cool burning solids such as paper and wood but must never be used on burning oils or flammable liquids (the water can spread the burning liquid) or on electrical equipment (risk of electric shock). Carbon dioxide extinguishers smother fires and are suitable for electrical fires and small fires involving flammable liquids. Foam extinguishers form a blanket over burning liquids. Dry powder extinguishers work on many fire types. Extinguishers are colour-coded or labelled to show their type; in many countries the label band colour indicates the contents.

What students should do. In a school laboratory, the main responsibilities of students are to prevent fires and to raise the alarm. Keep flammable liquids away from flames, keep benches clear of paper, tie back hair and never leave a lit burner unattended. If a fire starts: alert the teacher immediately, turn off the gas if it is safe, move away from the fire and follow evacuation instructions. Teachers deal with small, contained fires, such as a small flame in a beaker, often by smothering. If someone's clothing catches fire, they should stop, drop and roll , and others can smother the flames with a fire blanket.

Step-by-step reasoning

To decide how a fire could be put out:

1. Identify the fuel, the heat source and the oxygen supply. 2. Decide which side of the triangle can be removed most safely. 3. Choose a method: turn off fuel, smother (remove oxygen) or cool (remove heat). 4. Check that the method is safe for the fuel — no water on burning liquids or electrical fires. 5. If in any doubt, raise the alarm and leave; do not put yourself in danger.

Visual explanation

Fire involving Suitable Unsuitable Side of triangle removed --- --- --- --- Paper, wood Water, powder — Heat (cooling) Ethanol in a beaker Heat-resistant mat, fire blanket, CO₂ Water Oxygen (smothering) Electrical equipment CO₂, powder (after isolating power) Water Oxygen Clothing Stop, drop and roll; fire blanket Running Oxygen

Real-world analogy

Think of a campfire. It keeps burning while there is wood (fuel), air flowing around it (oxygen) and heat from the flames. Campers put it out by letting the wood burn away, by throwing sand over it to cut off air, or by pouring water on it to cool it. Every method removes one side of the triangle.

Real-world example

Kitchen oil fires are a common cause of house fires. Throwing water onto burning oil is extremely dangerous because the water instantly turns to steam and throws burning oil into the air. Safety advice is to turn off the heat and cover the pan with a lid or fire blanket if it is safe to do so — removing both heat and oxygen.

Why?

Why does a fire blanket put out a fire? The blanket is made of a fire-resistant material that does not burn. When it covers the fire, it stops fresh air from reaching the fuel. Once the oxygen near the flames is used up, the combustion reaction cannot continue, and the fire goes out even though fuel and heat are still present.

Common misconception

Many people think water puts out every kind of fire. Water is dangerous on burning liquids such as oil or ethanol, because it can spread them, and on electrical fires, because it conducts electricity. The right extinguisher depends on what is burning.

Worked example

Question: A small amount of ethanol in an evaporating basin catches fire near a Bunsen burner. How should the teacher deal with it, and why?

Reasoning: The fuel is a flammable liquid; water could spread it. The fire is small and contained.

Answer: Turn off the gas to the Bunsen burner, then smother the fire by covering the basin with a heat-resistant mat or fire blanket, which cuts off the oxygen. Water should not be used because it could spread the burning ethanol.

Quick check

1. What are the three sides of the fire triangle? Answer: Fuel, heat and oxygen.

Exam focus

Exam answers about fire should refer to the fire triangle and name which side is removed. When choosing extinguishers, explain why water is unsuitable for electrical and flammable-liquid fires, and state that students should raise the alarm rather than tackle fires themselves.

Advanced insight

Some fires cannot be put out by removing oxygen from air, because certain reactive metals such as magnesium burn so fiercely that they react with carbon dioxide and even water. Special dry powder extinguishers designed for metal fires are used in industry. This shows that fire safety depends on understanding the specific chemistry involved.

Summary

A fire needs fuel, heat and oxygen — the fire triangle. Removing any one puts the fire out: turn off the fuel, cool with water where safe, or smother to remove oxygen. Use the correct extinguisher: never water on burning liquids or electrical fires. Students should prevent fires, raise the alarm, turn off gas if safe, move away and follow evacuation instructions; if clothing catches fire, stop, drop and roll.

Practice questions

1. Explain how a carbon dioxide extinguisher puts out a fire. Answer: It pushes away the air and surrounds the fire with carbon dioxide, removing the oxygen the fire needs. 2. Why should water not be used on an electrical fire? Answer: Water conducts electricity, so there is a risk of electric shock. 3. What should you do if your clothing catches fire? Answer: Stop, drop and roll (and others can smother the flames with a fire blanket). 4. Give one way to prevent fires in the laboratory. Answer: Any one of: keep flammable liquids away from flames; never leave a lit burner unattended; keep paper away from burners; tie back hair.