Three Things a Fire Needs, and What Happens If You Remove One
Learn the word equation for rusting, four ways to stop it, the three conditions combustion needs, and how weathering slowly turns rock into soil.
What conditions does rusting need?
Iron rusts only when it is in contact with both air (oxygen) and moisture. Remove either one and rusting stops — which is the principle behind every method of protecting iron.
This page covers everything in the CBSE Class 7 Science chapter's second half: the rusting equation and how to prevent it, combustion and the three conditions it needs, and the very slow changes of weathering and erosion.
This page covers everything in the CBSE Class 7 Science chapter's second half: the rusting equation and how to prevent it, combustion and the three conditions it needs, and the very slow changes of weathering and erosion.
How do you write rusting as a word equation?
Rusting is a chemical change in which iron combines with oxygen and water to form a new reddish-brown substance:
iron + oxygen + water → rust (hydrated iron oxide)
Rust is flaky and weak, so a rusted object loses its strength — which is why rusting of bridges, gates and machinery matters far beyond appearance.
The standard experiment proves both conditions are needed. Take three test tubes with an iron nail in each: one with a dry nail in dry air, one with a nail in boiled water sealed under a layer of oil (water but no air), and one with a nail half in ordinary water and half in air. Only the third nail rusts.
For example, iron gates rust fastest during the monsoon, when air and moisture are both plentiful, and much more slowly in a dry summer.
The misconception to fix: rust is not dirt sitting on the iron. The iron itself has chemically become something new, which is why scrubbing removes the rust but leaves the metal thinner.
iron + oxygen + water → rust (hydrated iron oxide)
Rust is flaky and weak, so a rusted object loses its strength — which is why rusting of bridges, gates and machinery matters far beyond appearance.
The standard experiment proves both conditions are needed. Take three test tubes with an iron nail in each: one with a dry nail in dry air, one with a nail in boiled water sealed under a layer of oil (water but no air), and one with a nail half in ordinary water and half in air. Only the third nail rusts.
For example, iron gates rust fastest during the monsoon, when air and moisture are both plentiful, and much more slowly in a dry summer.
The misconception to fix: rust is not dirt sitting on the iron. The iron itself has chemically become something new, which is why scrubbing removes the rust but leaves the metal thinner.
How do you prevent rusting?
Every method works by keeping air or moisture away from the iron surface.
Painting forms a sealed coat, and is used on gates, grilles, railings and bridges.
Oiling or greasing is used where parts must keep moving, such as cycle chains, hinges, tools and machine parts, since paint would clog them.
Galvanising coats the iron with a layer of zinc, used for roofing sheets, buckets and water pipes. It survives scratches better than paint because the zinc itself resists corrosion.
Using stainless steel avoids the problem entirely, since stainless steel does not rust — which is why kitchen utensils and surgical instruments are made from it.
Choose the method to suit the object: paint a gate, grease a chain, galvanise a roofing sheet, and make a cooking vessel from stainless steel.
Note that stainless steel is the costliest option, which is why it is used where rusting would be dangerous or unhygienic rather than everywhere.
Painting forms a sealed coat, and is used on gates, grilles, railings and bridges.
Oiling or greasing is used where parts must keep moving, such as cycle chains, hinges, tools and machine parts, since paint would clog them.
Galvanising coats the iron with a layer of zinc, used for roofing sheets, buckets and water pipes. It survives scratches better than paint because the zinc itself resists corrosion.
Using stainless steel avoids the problem entirely, since stainless steel does not rust — which is why kitchen utensils and surgical instruments are made from it.
Choose the method to suit the object: paint a gate, grease a chain, galvanise a roofing sheet, and make a cooking vessel from stainless steel.
Note that stainless steel is the costliest option, which is why it is used where rusting would be dangerous or unhygienic rather than everywhere.
What three conditions does combustion need?
Combustion is a chemical change in which a substance reacts with oxygen and gives out heat and light. It needs three things together:
1. a combustible substance — something that can burn, such as paper, wood, kerosene or LPG
2. a supply of oxygen — normally from the air
3. heating up to the ignition temperature — the lowest temperature at which the substance catches fire
Remove any one and burning stops, which is exactly how fires are put out. Water cools the fuel below its ignition temperature; a blanket or sand cuts off the oxygen; and a firebreak removes the combustible material.
So a candle goes out when you cover it with a glass because the oxygen inside is used up, and a wet matchstick will not light because it cannot reach its ignition temperature.
The boundary case worth knowing: a substance below its ignition temperature will not burn no matter how much oxygen surrounds it. That is why paper sitting in open air does not spontaneously catch fire.
1. a combustible substance — something that can burn, such as paper, wood, kerosene or LPG
2. a supply of oxygen — normally from the air
3. heating up to the ignition temperature — the lowest temperature at which the substance catches fire
Remove any one and burning stops, which is exactly how fires are put out. Water cools the fuel below its ignition temperature; a blanket or sand cuts off the oxygen; and a firebreak removes the combustible material.
So a candle goes out when you cover it with a glass because the oxygen inside is used up, and a wet matchstick will not light because it cannot reach its ignition temperature.
The boundary case worth knowing: a substance below its ignition temperature will not burn no matter how much oxygen surrounds it. That is why paper sitting in open air does not spontaneously catch fire.
What is the difference between weathering and erosion?
Both are extremely slow natural changes, and the difference is what each one does.
Weathering is the breaking down of rocks where they stand, caused by rain, wind, heat, cold and the action of plant roots. Repeated heating and cooling cracks rock, and water seeping into cracks widens them.
Erosion is the carrying away of the broken material by wind, flowing water or moving ice, depositing it elsewhere.
So weathering breaks and erosion transports. Together they turn solid rock into the soil that crops grow in, over immensely long periods.
Changes like these are also classified as desirable or undesirable. Ripening of fruit, cooking of food and formation of soil are desirable. Rusting of iron, souring of milk and the erosion of fertile topsoil from a farmer's field are undesirable.
The same process can be either, depending on where it happens: erosion that builds a fertile river delta is desirable, while erosion stripping a hillside farm is not.
Weathering is the breaking down of rocks where they stand, caused by rain, wind, heat, cold and the action of plant roots. Repeated heating and cooling cracks rock, and water seeping into cracks widens them.
Erosion is the carrying away of the broken material by wind, flowing water or moving ice, depositing it elsewhere.
So weathering breaks and erosion transports. Together they turn solid rock into the soil that crops grow in, over immensely long periods.
Changes like these are also classified as desirable or undesirable. Ripening of fruit, cooking of food and formation of soil are desirable. Rusting of iron, souring of milk and the erosion of fertile topsoil from a farmer's field are undesirable.
The same process can be either, depending on where it happens: erosion that builds a fertile river delta is desirable, while erosion stripping a hillside farm is not.
Exam tip
Exam tip: putting out a fire by naming which condition you removed
Asked how water extinguishes a fire, students answer "because water is cold" — true but incomplete.
Every extinguishing answer should name which of the three conditions has been removed. Water lowers the fuel's temperature below its ignition temperature. A sand bucket or fire blanket cuts off the oxygen supply. Clearing dry grass removes the combustible substance.
The same structure answers the reverse question. If asked why a fire keeps burning, check all three are present. And remember that water must not be used on an electrical or oil fire — water conducts electricity and oil floats on water, so there the correct method is cutting off oxygen instead.
Every extinguishing answer should name which of the three conditions has been removed. Water lowers the fuel's temperature below its ignition temperature. A sand bucket or fire blanket cuts off the oxygen supply. Clearing dry grass removes the combustible substance.
The same structure answers the reverse question. If asked why a fire keeps burning, check all three are present. And remember that water must not be used on an electrical or oil fire — water conducts electricity and oil floats on water, so there the correct method is cutting off oxygen instead.
Did you know
Why does a candle go out under an upturned glass?
The glass traps a fixed amount of air. The flame keeps using the oxygen in it, and once enough has been used the supply can no longer sustain burning.
Two of the three conditions are still there — the wax is combustible and it is well above its ignition temperature — but with the oxygen gone, the flame dies. It is the clearest demonstration that all three conditions are needed simultaneously.
Two of the three conditions are still there — the wax is combustible and it is well above its ignition temperature — but with the oxygen gone, the flame dies. It is the clearest demonstration that all three conditions are needed simultaneously.
Key takeaways
Rusting, combustion and weathering: quick revision
- Rusting needs both air and moisture: iron + oxygen + water → rust, a new flaky substance that weakens the metal.
- Prevent rusting by painting, oiling or greasing, galvanising with zinc, or using stainless steel — each keeps air and moisture off the iron.
- Combustion needs a combustible substance, a supply of oxygen and heating to the ignition temperature; removing any one stops the fire.
- Water extinguishes by cooling below ignition temperature, sand or a blanket by cutting off oxygen — and water must not be used on electrical or oil fires.
- Weathering breaks rock down where it lies and erosion carries the pieces away; together they form soil, and changes can be desirable or undesirable depending on context.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Prevent rusting by painting, oiling or greasing, galvanising with zinc, or using stainless steel — each keeps air and moisture off the iron.
- Combustion needs a combustible substance, a supply of oxygen and heating to the ignition temperature; removing any one stops the fire.
- Water extinguishes by cooling below ignition temperature, sand or a blanket by cutting off oxygen — and water must not be used on electrical or oil fires.
- Weathering breaks rock down where it lies and erosion carries the pieces away; together they form soil, and changes can be desirable or undesirable depending on context.
You will remember all of this far better after answering five questions on it than after reading it twice.