Combustion:
Combustion is a chemical process in which a substance reacts with oxygen and releases heat and light energy.
Example:
Burning wood
Burning petrol or diesel
Burning candle wax
Burning cooking gas
During combustion, new substances such as carbon dioxide, water vapour, smoke, and ash may be formed.
Combustible and Non-Combustible Substances:
| Combustible | Non-combustible |
| Burn easily in air. | Do not burn easily. |
| Examples: Wood, paper, petrol, coal, LPG | Examples: Stone, glass, sand, water, brick |
Conditions Required for Combustion (Fire Triangle):
Three conditions are needed for combustion:
- Fuel – The substance that burns (wood, coal, petrol, LPG).
- Oxygen – Supports burning. Removing oxygen stops the fire.
- Heat – Raises the fuel to its ignition temperature.
Remove any one of these three conditions and the fire goes out.
Ignition Temperature:
Weathering:
Ignition temperature is the minimum temperature at which a substance catches fire.
- Paper has a low ignition temperature, so it burns easily.
- Wood has a higher ignition temperature, so it takes longer to catch fire.
Rusting:
Rusting is a slow chemical change in which iron reacts with oxygen and moisture to form rust (reddish-brown iron oxide).
Example:
Rusted gate
Rusted bicycle chain
Rusted nails
Conditions Required:
- Oxygen
- Moisture (water)
Both are necessary for rusting.
Slow Natural Changes:
A slow natural change occurs naturally and takes months, years, or even thousands of years.
Example:
Weathering of rocks
Soil formation
Erosion
Formation of valleys and caves
Weathering is the breaking down of rocks into smaller pieces at the same place by natural agents.
Agents of Weathering:
- Wind
- Water
- Temperature changes
- Ice
- Plant roots
Importance:
- Helps soil formation
- Releases minerals
- Supports plant growth
- Shapes landscapes
Erosion:
Erosion is the wearing away and carrying away of soil and rock particles by wind, water, glaciers, or sea waves.
Difference Between Weathering and Erosion:
| Weathering | Erosion |
| Breaks rocks. | Carries away broken materials. |
| Occurs at the same place. | Materials move to another place. |
| Helps soil formation. | May reduce soil fertility |
Prevention of Soil Erosion:
- Plant more trees.
- Avoid deforestation.
- Build terraces on hills.
- Grow grasses and shrubs.
Desirable and Undesirable Changes:
| Description | Desirable Changes | Undesirable Changes |
| Meaning | Useful or beneficial changes. | Harmful or unwanted changes. |
| Examples |
|
|
Burning of a Candle – Both Physical and Chemical Change:
Physical Changes
- Wax melts into liquid.
- Liquid wax changes into vapour.
- Melted wax solidifies after cooling.
- No new substance is formed.
Chemical Changes
- Wax vapour burns in oxygen.
- Forms carbon dioxide and water vapour.
- Produces heat and light.
- New substances are formed.
Therefore, a burning candle shows both physical and chemical changes.