# Combustion

> The fire triangle, complete and incomplete combustion of fuels, testing the products, and burning metals.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/combustion/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The camp stove
On a school camp, Leo's group cooked dinner on a small gas stove. When the air hole at the bottom was open, the flame was a neat, roaring **blue**. The pot stayed clean and the water boiled fast.

Then someone knocked the air hole shut. The flame turned a lazy, flickering **yellow**. The water took ages to heat, and the bottom of the pot was soon coated in black soot.

Their teacher explained: "Same fuel, different amount of oxygen. With lots of oxygen you get complete combustion. Without enough, you get incomplete combustion, which makes soot and a poisonous gas called carbon monoxide. That is why you never use a camp stove inside a closed tent."

**In short:** Combustion is the fast reaction of a fuel with oxygen that gives out heat and light. Complete combustion of a hydrocarbon, with plenty of oxygen, makes carbon dioxide and water, while incomplete combustion makes poisonous carbon monoxide or soot.

## What is combustion?

**Combustion** is a reaction where a substance (the **fuel**) reacts quickly with **oxygen**, releasing energy as **heat and light**. Burning is combustion.

Combustion is **exothermic**: it gives out energy. That is why we burn fuels for cooking, heating, cars and power stations.

## The fire triangle

A fire needs three things. Remove any one and the fire goes out.

*Diagram: Fire triangle with fuel, oxygen and heat on the three sides*
*Take away heat, oxygen or fuel and the fire stops.*

| Remove | How firefighters do it |
|---|---|
| **Heat** | Spray water to cool the fire |
| **Oxygen** | Fire blanket, foam, or a carbon dioxide extinguisher to smother it |
| **Fuel** | Turn off the gas, clear a firebreak in the bush |

### Watch out: Never use water on an oil or electrical fire
Water on burning cooking oil makes the oil explode outwards in a fireball. Water conducts electricity. Use a fire blanket or the correct extinguisher instead.

## Hydrocarbons

Most fuels are **hydrocarbons**: compounds made of **only hydrogen and carbon**. Examples: methane $\mathrm{CH_4}$ (natural gas), propane $\mathrm{C_3H_8}$ (barbecue gas), butane $\mathrm{C_4H_{10}}$ (lighter fuel), octane $\mathrm{C_8H_{18}}$ (in petrol).

## Complete combustion

With **plenty of oxygen**, a hydrocarbon burns completely to make **carbon dioxide and water** only.

$$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$

$$\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}$$

Signs: a clean **blue** flame, lots of heat, no soot.

## Incomplete combustion

With **not enough oxygen**, there is not enough to turn all the carbon into $\mathrm{CO_2}$. Instead it makes:

- **Carbon monoxide** ($\mathrm{CO}$): a colourless, odourless, **poisonous** gas. It stops blood carrying oxygen.
- **Carbon** (C): black **soot**.
- Water is still made.

$$\mathrm{2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O}$$
$$\mathrm{CH_4 + O_2 \rightarrow C + 2H_2O}$$

Signs: a smoky **yellow** flame, less heat, soot.

| | Complete | Incomplete |
|---|---|---|
| Oxygen supply | Plenty | Limited |
| Flame | Blue | Yellow, smoky |
| Products | $\mathrm{CO_2}$ + $\mathrm{H_2O}$ | $\mathrm{CO}$ and/or C (soot) + $\mathrm{H_2O}$ |
| Energy released | More | Less |

### Key idea: Why incomplete combustion is dangerous
Carbon monoxide cannot be seen or smelled. Faulty gas heaters and barbecues used indoors can fill a room with it. Homes use **carbon monoxide alarms** for this reason. Always burn fuels with good airflow.

## Testing the products

You can prove that burning a hydrocarbon makes water and carbon dioxide.

- **Water**: collect the liquid that forms on a cold surface. It turns **blue cobalt chloride paper pink** (or white anhydrous copper sulfate **blue**).
- **Carbon dioxide**: bubble the gases through **limewater**. It turns **milky**.

## Burning metals and other elements

Combustion is not only for fuels. When an element burns in oxygen, it forms an **oxide**.

$$\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}$$

Magnesium burns with a **dazzling white light**, so bright you should not look at it directly. That is why it was used in old camera flashes and is still used in fireworks.

| Element | Product | What you see |
|---|---|---|
| Magnesium | Magnesium oxide (white powder) | Brilliant white flame |
| Carbon | Carbon dioxide | Glows orange |
| Sulfur | Sulfur dioxide | Blue flame, choking smell |
| Hydrogen | Water | Squeaky pop |

## Combustion and the environment

- Burning fossil fuels releases **carbon dioxide**, a **greenhouse gas** that is causing climate change.
- Fuels containing sulfur release **sulfur dioxide**, which causes **acid rain**.
- Incomplete combustion releases **soot** particles that harm lungs.

**Mind map**

```text
Combustion
  Needs
    Fuel
    Oxygen
    Heat
  Complete
    Plenty of oxygen
    CO₂ + H₂O
    Blue flame
  Incomplete
    Limited oxygen
    CO (poison) and soot
    Yellow flame
  Tests
    Water: cobalt chloride pink
    CO₂: limewater milky
  Other fuels
    Metals make oxides
    Mg: white light
  Environment
    CO₂: greenhouse
    SO₂: acid rain
```

### Year 11: Balancing any hydrocarbon combustion
For a hydrocarbon $\mathrm{C_xH_y}$ burning completely:

1. Carbon: $x\,\mathrm{CO_2}$.
2. Hydrogen: $\frac{y}{2}\,\mathrm{H_2O}$.
3. Oxygen: count O on the right, halve it for $\mathrm{O_2}$. If it is a fraction, double every coefficient.

Butane, $\mathrm{C_4H_{10}}$: $4\,\mathrm{CO_2}$, $5\,\mathrm{H_2O}$, O on right $= 8 + 5 = 13$, so $\tfrac{13}{2}\,\mathrm{O_2}$. Doubled:

$$\mathrm{2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O}$$

**Alcohols** such as ethanol, $\mathrm{C_2H_5OH}$, burn the same way. Remember the O already inside the fuel: $\mathrm{C_2H_5OH} + \mathrm{3O_2} \rightarrow \mathrm{2CO_2} + \mathrm{3H_2O}$.

### Year 12: Measuring the energy from a fuel
In a **calorimetry** experiment, a fuel heats a known mass of water. The energy absorbed by the water is

$$q = mc\Delta T$$

with $m$ the mass of water in g, $c = 4.18 \text{ J g}^{-1}\,^\circ\text{C}^{-1}$ and $\Delta T$ the temperature rise.

Burning 0.46 g of ethanol ($M = 46 \text{ g/mol}$) heats 200 g of water by 12.0 °C.

$$q = 200 \times 4.18 \times 12.0 = 10\,032 \text{ J} \approx 10.0 \text{ kJ}$$
$$n = \frac{0.46}{46} = 0.010 \text{ mol} \qquad \Delta H_c = -\frac{10.0}{0.010} = -1.0 \times 10^{3} \text{ kJ/mol}$$

The accepted value is $-1367$ kJ/mol. The experiment is lower because heat escapes to the air and the can, and some combustion is incomplete. These are **systematic errors**: they always make the result too small.

## Quick summary

- Combustion is a fast, exothermic reaction of a fuel with oxygen.
- Fire triangle: **fuel, oxygen, heat**. Remove one to stop a fire.
- Hydrocarbons contain only hydrogen and carbon.
- **Complete** combustion (plenty of oxygen): $\mathrm{CO_2}$ + water, blue flame.
- **Incomplete** combustion (limited oxygen): poisonous **carbon monoxide** and/or **soot** + water, yellow flame.
- Tests: water turns cobalt chloride paper pink, carbon dioxide turns limewater milky.
- Elements burn in oxygen to form oxides. Magnesium burns with a bright white light.

## Quiz

Try these questions from the chapter quiz.

1. Which three things make up the fire triangle?
   - A. Fuel, water, heat
   - B. Fuel, oxygen, heat
   - C. Oxygen, carbon dioxide, heat
   - D. Fuel, oxygen, light
   - Answer: B. Fuel, oxygen, heat. All three are needed for a fire.

2. A fire blanket puts out a fire mainly by removing:
   - A. Heat
   - B. Fuel
   - C. Oxygen
   - D. Light
   - Answer: C. Oxygen. It smothers the fire, cutting off the air.

3. A hydrocarbon is a compound containing:
   - A. Hydrogen and oxygen only
   - B. Hydrogen and carbon only
   - C. Carbon and oxygen only
   - D. Hydrogen, carbon and oxygen
   - Answer: B. Hydrogen and carbon only. Hydro (hydrogen) plus carbon.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/combustion/#quiz