---
title: "Burning: What a Fire Needs"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 5
exercises: 11
source: https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 5 — Burning: What a Fire Needs

A campfire at dusk. It needed dry wood, a match to start it, and the wind that blows through it keeps it bright. After an hour, the logs have shrunk to glowing embers and white ash, and grey smoke has drifted away into the sky. Where has the wood gone? What does a fire need, and what does it leave behind?

**You already know.**

[Air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) is a mixture of gases: about four fifths nitrogen and one fifth oxygen, with a little argon and carbon dioxide. A flame takes oxygen from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) around it; a candle under a jar goes out when too little oxygen is left ([Chapter 4](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#ch-g4-air-a-mixture-of-gases)).

![A campfire: wood, air and a match — and afterwards ash and smoke.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/img-6939f57d30e3.jpg)

*A campfire: wood, [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and a match — and afterwards ash and smoke.*

## 5.1 Fuels

**Definition 5.1 (Fuel).**

A *fuel* is a material that can burn and give out heat and light: wood, paper, coal, candle wax, the gas of a cooker, petrol, heating oil.

**Example 5.2 (Solid, liquid and gas fuels).**

Wood, paper, coal and candle wax are solid [fuels](#def-g4-what-a-fire-needs-fuel). Petrol and heating oil are liquid [fuels](#def-g4-what-a-fire-needs-fuel). The gas of a kitchen cooker and the gas in a lighter are gas [fuels](#def-g4-what-a-fire-needs-fuel).

**Remark 5.3 (Not everything burns).**

Stone, sand, glass and water do not burn: they are not [fuels](#def-g4-what-a-fire-needs-fuel). That is why a fireplace is built of stone or brick, and why sand and water are used to put fires out.

## 5.2 The fire triangle

**Definition 5.4 (Fire triangle).**

A fire needs three things at the same time: a [fuel](#def-g4-what-a-fire-needs-fuel), oxygen from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), and heat to start it. These three things are drawn as the three sides of a triangle, the *fire triangle*. If one side is missing, there is no fire.

![The fire triangle: fuel, oxygen and heat, all three together.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/fig-374fb826a128.svg)

*The [fire triangle](#def-g4-what-a-fire-needs-fire-triangle): [fuel](#def-g4-what-a-fire-needs-fuel), oxygen and heat, all three together.*

**Example 5.5 (Lighting a match).**

A match head is rubbed on the side of its box. The rubbing heats it enough to start it [burning](#def-g4-what-a-fire-needs-burning): the heat side of the triangle. The wood of the match is the [fuel](#def-g4-what-a-fire-needs-fuel), and the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) brings the oxygen.

**Example 5.6 (Blowing on embers).**

Glowing embers that seem almost dead burst into flame again when someone blows on them: the blowing brings more [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), and so more oxygen, to the [fuel](#def-g4-what-a-fire-needs-fuel) that is still hot.

## 5.3 Burning makes new substances

**Definition 5.7 (Burning and combustion).**

*Burning* is what happens when a [fuel](#def-g4-what-a-fire-needs-fuel) takes oxygen from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and gives out heat and light. Chemists call a burning a *combustion*. During a combustion the [fuel](#def-g4-what-a-fire-needs-fuel) and the oxygen are used up, and new substances appear: smoke, ash, carbon dioxide and water vapour.

**Proposition 5.8 (A burning cannot be undone).**

The ash and smoke of a fire do not turn back into wood, however long we wait. [Burning](#def-g4-what-a-fire-needs-burning) makes new substances that were not there before; the [fuel](#def-g4-what-a-fire-needs-fuel) is gone for good.

**In the lab — A cold glass over a candle.**

The teacher holds a cold, dry glass beaker upside down for a moment above a candle flame. The inside of the glass mists up with tiny droplets of water: water vapour made by the [burning](#def-g4-what-a-fire-needs-burning) candle has turned back into liquid on the cold glass. A [burning](#def-g4-what-a-fire-needs-burning) candle makes water, and carbon dioxide too, which can be detected with a test met in a later chapter.

![A cold glass held above a candle mists up: burning wax makes water vapour.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/fig-664b4e0a9567.svg)

*A cold glass held above a candle mists up: [burning](#def-g4-what-a-fire-needs-burning) wax makes water vapour.*

**Example 5.9 (What a log becomes).**

A log of a few kilograms burns down to a small heap of ash. Most of the wood has not vanished: together with oxygen from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), it has become smoke, carbon dioxide and water vapour, which are gases and drift away into the sky.

## 5.4 Putting a fire out

**Method 5.10 (Putting out a fire: take away one side).**

To put out a fire, take away one side of the [fire triangle](#def-g4-what-a-fire-needs-fire-triangle):

- **take away the oxygen** : cover the fire with a fire blanket, a lid or sand, so that [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) cannot reach it;
- **take away the heat** : pour water on [burning](#def-g4-what-a-fire-needs-burning) wood or paper, which cools it below the point where it can burn;
- **take away the [fuel](#def-g4-what-a-fire-needs-fuel)** : turn off the gas tap, or clear the dry plants around a forest fire.

![Three ways to put out a fire, each removing one side of the triangle (crossed out): the oxygen, the heat or the fuel.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/fig-5966f82bab01.svg)

*Three ways to put out a fire, each removing one side of the triangle (crossed out): the oxygen, the heat or the [fuel](#def-g4-what-a-fire-needs-fuel).*

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/img-5d565db1e9d0.jpg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/img-0e1650c55b84.jpg)

*A fire blanket over a pan: the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) can no longer reach the fire.*

*Firefighters spray water to cool a [burning](#def-g4-what-a-fire-needs-burning) shed.*

## 5.5 Fire safety

**Remark 5.11 (Never water on a pan of burning oil).**

If the oil in a frying pan catches fire, nobody should throw water on it: the water boils at once under the oil and throws [burning](#def-g4-what-a-fire-needs-burning) oil out of the pan. The gas or the hotplate is turned off and the pan is covered with a lid or a fire blanket, by an adult.

**Safety.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/fig-7323bebe7717.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/fig-33d325f1c428.svg)

The gas in a lighter or a camping cartridge is butane. Its flame pictogram (GHS02) means: extremely flammable, keep away from flames and sparks. The gas bottle pictogram (GHS04) means: a gas kept under pressure.

**Proposition 5.12 (What to do if there is a fire).**

Smoke is the first danger of a fire: it is hot, it hides the way out and it is poisonous to breathe. A smoke alarm on the ceiling beeps loudly as soon as smoke reaches it. If there is a fire: get out, keep low under the smoke, close the doors behind you, and call the fire brigade from outside. Never go back in.

![A smoke alarm: it sounds as soon as smoke reaches it.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-what-a-fire-needs/img-5260fdf556a2.jpg)

*A smoke alarm: it sounds as soon as smoke reaches it.*

## 5.6 Exercises

**Exercise 5.1 ★.**

Name the three sides of the [fire triangle](#def-g4-what-a-fire-needs-fire-triangle).

**Solution of Exercise 5.1.**

[Fuel](#def-g4-what-a-fire-needs-fuel), oxygen (from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air)) and heat (to start the fire).

**Exercise 5.2 ★.**

Which of these are [fuels](#def-g4-what-a-fire-needs-fuel): wood, sand, candle wax, glass, petrol, water, paper?

**Solution of Exercise 5.2.**

Wood, candle wax, petrol and paper are [fuels](#def-g4-what-a-fire-needs-fuel). Sand, glass and water do not burn.

**Exercise 5.3 ★.**

A fire blanket is thrown over a small fire. Which side of the [fire triangle](#def-g4-what-a-fire-needs-fire-triangle) does it take away?

**Solution of Exercise 5.3.**

The oxygen: the blanket stops the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) from reaching the fire.

**Exercise 5.4 ★.**

Firefighters spray water on a [burning](#def-g4-what-a-fire-needs-burning) shed. Which side of the [fire triangle](#def-g4-what-a-fire-needs-fire-triangle) does the water take away?

**Solution of Exercise 5.4.**

The heat: the water cools the [burning](#def-g4-what-a-fire-needs-burning) wood so much that it can no longer burn.

**Exercise 5.5 ★.**

Name three new substances made when wood burns.

**Solution of Exercise 5.5.**

Any three of: ash, smoke, carbon dioxide, water vapour.

**Exercise 5.6 ★.**

Why does blowing on glowing embers make them flare up again?

**Solution of Exercise 5.6.**

Blowing brings more [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), so more oxygen, to the hot [fuel](#def-g4-what-a-fire-needs-fuel), and it burns again.

**Exercise 5.7 ★.**

A cold glass held above a candle mists up. Which new substance has the candle made?

**Solution of Exercise 5.7.**

Water: the [burning](#def-g4-what-a-fire-needs-burning) wax makes water vapour, which turns into droplets on the cold glass.

**Exercise 5.8 ★.**

What should you do first if a smoke alarm goes off and you see smoke?

**Solution of Exercise 5.8.**

Get out, keeping low under the smoke and closing the doors behind you, then call the fire brigade from outside.

**Exercise 5.9 ★.**

Look at the figure with the three small triangles. Which side is crossed out when the gas is turned off?

**Solution of Exercise 5.9.**

The [fuel](#def-g4-what-a-fire-needs-fuel) side: with the gas turned off, the fire has nothing left to burn.

**Exercise 5.10 ★★.**

To stop a forest fire, firefighters sometimes cut down a wide strip of trees in front of it. Which side of the [fire triangle](#def-g4-what-a-fire-needs-fire-triangle) are they taking away?

**Solution of Exercise 5.10.**

The [fuel](#def-g4-what-a-fire-needs-fuel): with no trees in the strip, the fire runs out of [fuel](#def-g4-what-a-fire-needs-fuel) when it reaches it.

**Exercise 5.11 ★★.**

A log weighs much more than its ash. Has part of the log disappeared? Explain where it has gone.

**Solution of Exercise 5.11.**

No. Most of the wood, together with oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), has become new substances that are gases — carbon dioxide, water vapour and smoke — and they have gone into the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air). Only the ash stays behind.