What Is Fire?
Many people think of fire as a thing — something you can hold or pour. But fire is actually a chemical reaction, not a substance. Specifically, fire is a type of combustion reaction — a rapid reaction between a fuel and oxygen that releases heat and light.
When a candle burns, the wax is not simply disappearing. It is reacting with oxygen from the air to produce carbon dioxide (CO₂) and water vapour (H₂O). The energy stored in the chemical bonds of the wax is released as heat and light — what we see and feel as fire.
Fire is a process, not a material. You cannot bottle fire or weigh it. What you see as a flame is a glowing zone of hot gas where combustion is actively happening.
The Fire Triangle
For fire to exist, three things must be present at the same time:
- Fuel — a substance that can burn (wood, petrol, natural gas, paper, wax)
- Oxygen — from the air (fire needs about 16% oxygen; normal air is 21%)
- Heat (Ignition energy) — enough energy to start the reaction
This is called the fire triangle. Remove any one side and the fire goes out. This principle is the basis of all fire-fighting methods.
| Side of Triangle | Examples |
|---|---|
| Fuel | Wood, petrol, methane, candle wax |
| Oxygen | Air (21% O₂), pure oxygen cylinders |
| Heat / Ignition | Match, spark, sunlight through a lens |
The fire triangle is the most important concept in fire safety. Every fire-extinguishing method works by attacking one or more sides of the triangle.
Flames and Light Emission
A flame is the visible, glowing part of a fire. It forms because combustion produces extremely hot gases. These hot gases contain particles (atoms and molecules) that have been excited by the heat — they absorb energy and then release it as light.
Different parts of a flame have different temperatures. A typical candle flame reaches about 1,400 °C at its hottest point (the outer cone), while the inner part near the wick is cooler, around 600–800 °C.
Flame Colour and Temperature
The colour of a flame tells us a great deal about its temperature and the substances burning:
| Flame Colour | Approximate Temperature | Cause |
|---|---|---|
| Red / Orange | 700–1,000 °C | Carbon soot particles glowing; incomplete combustion |
| Yellow | 1,000–1,200 °C | Hot soot and carbon particles |
| Blue | 1,200–1,600 °C | Complete combustion; very hot gas |
| White | Above 1,600 °C | Extremely high temperature (e.g. welding torches) |
A blue flame, like on a gas stove, indicates complete combustion — the fuel is burning fully and efficiently. An orange or yellow flame often means incomplete combustion, producing soot (carbon particles) as a byproduct.
Real world: Why a gas stove has a blue flame
Natural gas (methane) burns with a clean blue flame because it mixes well with air before igniting. This is complete combustion — very efficient and producing almost no soot. When you see an orange or yellow flame on a gas stove, it usually means the gas-to-air mixture is off and the burner needs cleaning.
Types of Combustion: Smouldering vs. Flaming
Not all fire looks the same. There are two main types of burning:
Flaming combustion is the type we usually picture, with visible flames. It happens when gases or vapours from the fuel react rapidly with oxygen in the air. The fuel must first vaporise before it can burn with a flame.
Smouldering combustion is a slower, flameless form of burning. The surface of the solid fuel reacts directly with oxygen without producing flames. A glowing coal or an ember in a fire pit are examples. Smouldering fires are particularly dangerous because they can continue for hours and re-ignite into full flaming fires when given more air.
Temperatures of Common Fires
| Type of Fire | Temperature Range |
|---|---|
| Candle flame | 600–1,400 °C |
| Wood campfire | 600–900 °C |
| House fire | 500–1,000 °C |
| Propane torch | 1,200–1,400 °C |
| Welding arc | 3,000–6,000 °C |
Fire as Both Useful and Dangerous
Fire has been essential to human civilisation for hundreds of thousands of years. Humans use fire for:
- Cooking — heat changes the structure of food, making it safer and easier to digest
- Heating homes — fireplaces, furnaces, and boilers
- Generating electricity — burning fossil fuels in power stations
- Industry — smelting metals, making glass and ceramics
However, fire is also one of nature's most destructive forces. Out-of-control fires destroy forests, homes, and lives. The same properties that make fire useful — intense heat and rapid spread — make it extremely dangerous when uncontrolled.
Real world: Cooking as a combustion reaction
Every time you use a gas cooker, you are using a controlled combustion reaction. Natural gas burns with oxygen to release heat, which warms your pan and cooks your food. Humans have been using controlled fire for cooking for at least one million years — it is one of the defining features of our species.
Never leave open flames unattended. A small, controlled fire can become a dangerous wildfire or house fire within minutes if it reaches new fuel or is exposed to a strong wind.
Practice Problems
Name the three components of the fire triangle and explain what happens when one is removed.
Show answer
A campfire has orange-yellow flames. What does this tell you about the type of combustion taking place?