Changes Around Us: Physical and Chemical · Lesson 3 of 6
Rusting, Combustion, and the Fire Triangle
“Explore new substances formed by rusting and burning, and discover the three conditions a fire needs.”
• Explain why rusting iron and burning magnesium are chemical changes. • Describe combustion and recognise familiar combustible substances. • Explain why oxygen is needed to keep a candle burning. • Use ignition temperature and the fire triangle to explain when burning starts and stops. • Distinguish firefly light from the flame and heat of ordinary burning.
New substances appear in rusting and burning
Chemical changes do not all look alike. Some occur slowly and leave a coloured layer on a surface. Others are rapid, produce a bright light, and release heat. Rusting iron and burning magnesium help us recognise this variety while keeping the same classification rule: look for new substances.
An iron nail may develop a brown coating after exposure to air and moisture. This coating is rust, described at this level as iron oxide. It differs from the original iron, so rusting is a chemical change. Simply bending the nail would change its shape; rusting changes part of the material into something new.
The slow chemical change in which iron reacts in the presence of oxygen and moisture to form rust, a coating containing iron oxides.
When magnesium ribbon burns in air, it gives out an intense white light and leaves white magnesium oxide. The white powder is not merely magnesium in smaller pieces. It is a new substance formed when magnesium reacts with oxygen. The powder also has basic properties, as introduced in the earlier study of acids and bases; “basic” means showing the properties of a base.
Problem
A bicycle handle develops brown deposits. Explain why this is chemical rather than just a change of appearance.
- 1.Identify the original material as iron or an iron-containing part.
- 2.The deposits are rust, a substance formed by reaction rather than iron merely changing shape.
- 3.Formation of iron oxide makes rusting a chemical change. The brown colour is an observation that helps us notice it.
Flames, heated glass, and burning magnesium must be handled by a teacher or responsible adult. Do not look directly at burning magnesium’s intense light or attempt the burning demonstrations on your own.
Combustion needs oxygen
Magnesium burns by reacting with oxygen, and familiar fuels also need oxygen to burn. The oxygen comes from the surrounding air in these examples. We can investigate its role by comparing a candle with access to fresh air and a candle under an inverted glass.
A chemical reaction in which a substance reacts with oxygen and gives out heat and/or light; ordinary burning is a familiar example.
A substance that can undergo combustion. When used as a source of energy, it is called a fuel.
Wood, paper, cotton, and kerosene are familiar combustible materials. Having a combustible material nearby does not mean it is already burning. The conditions of the reaction matter. First, consider whether enough oxygen is available to sustain it.
A teacher lights two similar candles and covers one with a glass tumbler. The open candle continues burning. The covered candle burns briefly and then goes out. The glass traps a limited amount of air and prevents a continuous supply of fresh air. As oxygen is used, the supply becomes insufficient to keep the flame going.
The burning wax also forms carbon dioxide: carbon in the wax reacts with oxygen from the air. In the teacher’s gas-test demonstration, lime water exposed to the collected gas turns milky. This connects combustion to our earlier investigation of new substances. The milkiness tests for carbon dioxide; the open-versus-covered comparison shows the importance of a continuing oxygen supply.
Problem
Why does a candle under an inverted glass not go out at the exact moment the glass is placed over it?
- 1.There is already air, including oxygen, inside the glass.
- 2.The candle can burn for a short time using that limited supply.
- 3.The glass stops fresh air replacing the oxygen being used. When oxygen becomes insufficient, the flame goes out.
A fuel must reach its ignition temperature
Paper is combustible, and a sheet on your desk is surrounded by air. Yet it does not catch fire simply by being there. Fuel and oxygen are therefore not the entire explanation. The paper must also be heated enough for burning to begin.
The minimum temperature at which a substance catches fire under the relevant conditions.
In one supervised demonstration, a lighted match provides heat to paper held with tongs. In the other, a magnifying glass concentrates sunlight onto a small spot on paper. The spot heats up; after enough heating, the paper can smoke and ignite. An existing flame is one way to supply heat, but the sunlight demonstration shows that a flame is not the only way.
The magnifying glass concentrates the incoming light onto a smaller area. It does not create a new fuel or supply oxygen. Its role is to raise the paper’s temperature to the level at which combustion can start. Looking at the Sun through a lens is unsafe; this activity belongs in an adult-controlled demonstration.
The fire triangle collects these three requirements: a combustible substance or fuel, oxygen, and enough heat to reach ignition temperature. If one requirement is missing, sustained burning cannot continue. A hot object without fuel cannot maintain this combustion; fuel without enough oxygen goes out; fuel and oxygen at too low a temperature do not start burning.
Problem
Explain why paper can lie in air without burning but ignite when sufficiently heated.
- 1.Paper supplies the combustible material, and air supplies oxygen in both situations.
- 2.At ordinary room temperature, the paper has not reached its ignition temperature.
- 3.A sufficient heat source provides the missing condition. All three fire-triangle requirements can then act together.
Connect the science to safety and living things
A fire blanket illustrates the oxygen side of the triangle: covering a suitable fire can cut off the supply of air. A clothing fire helps show to show why limiting oxygen can extinguish flames, and warns that synthetic cloth may melt and stick to skin. Treat this as a scientific explanation of trained emergency action; children should call for adult help and follow the school’s emergency procedures.
Fireflies provide a different connection. They produce light through a chemical process called bioluminescence, meaning light production by a living organism. Their light gives off very little heat, so it is often described as light without noticeable heating. A glowing firefly is not a tiny piece of fuel burning with an ordinary flame. Chemical changes can release energy in different ways.
A sheet of paper in air already has fuel and oxygen. It still needs enough heat to reach ignition temperature. Also, “oxygen supports burning” does not mean oxygen is the fuel: it reacts with the combustible substance.
Quiz
Why is rusting a chemical change?
What remains as a new solid after magnesium burns?
Why does a covered candle eventually go out?
Which set correctly gives the fire-triangle requirements?
What does focused sunlight supply in the paper investigation?
Which statement about fireflies is correct?
Practice Problems
- Explain the difference between bending an iron nail and rusting it.
- Write the magnesium word equation. Which term names the new solid, and which terms refer to released energy?
- Describe the open-and-covered-candle comparison using investigation, observation, and conclusion.
- Explain why lime water can be used to identify a product of burning wax.
- Use the fire triangle to explain why paper in air does not normally catch fire at room temperature.
- Compare a match and focused sunlight: what essential condition do both provide?
- Explain the science behind cutting off a fire’s air supply, and state why synthetic cloth is unsuitable in the clothing-fire example.
- Why should the firefly example not be treated as an ordinary flame?
Key Takeaways
• Rusting forms iron oxides; burning magnesium forms magnesium oxide. Both are chemical changes. • Combustion involves reaction with oxygen and release of heat and/or light. • A continuing oxygen supply is needed to sustain ordinary burning. • The fire triangle consists of fuel, oxygen, and enough heat to reach ignition temperature. • A flame is one source of ignition heat; concentrated sunlight is another. • Firefly bioluminescence shows that chemical light production need not involve noticeable heating or an ordinary flame.