Changes Around Us:
We observe many changes around us every day. Some changes affect only the appearance of a substance, while others produce new substances.
| Type of Change | Example |
| Size | Cutting vegetables, crushing chalk |
| Shape | Folding paper, bending a wire |
| State | Ice melting into water, water freezing into ice |
| Colour | Iron rusting, burning paper |
| Smell | Milk turning into curd, food spoiling |
Not every change forms a new substance. Some changes affect only the appearance, while others completely change the substance.
A Substance May Change in Appearance but Remain the Same (Physical Change):
A physical change is a change in which only the size, shape, or state of a substance changes, but no new substance is formed.
Characteristics of Physical Changes:
- Only the appearance changes.
- No new substance is formed.
- The substance remains the same.
- Usually reversible.
Example:
Folding paper, Tearing paper, Cutting vegetables, Melting ice, Freezing water, Boiling water, Crushing chalk, Inflating a balloon
A Substance May Change in Appearance and Not Remain the Same (Chemical Change):
A chemical change is a change in which one or more new substances are formed with different properties.
Characteristics of Chemical Changes:
- A new substance is formed.
- Usually irreversible.
- New substances have different properties.
- Often accompanied by: Heat, Light, Gas, Colour change, Smell
Example:
Burning paper, Burning wood, Rusting of iron, Cooking food, Curdling of milk, Vinegar \(+\) baking soda
Rusting – A Chemical Change:
Rusting is a slow chemical change.
How does rusting occur?
Iron reacts with:
- Oxygen
- Moisture (water)
to form a new substance called rust (iron oxide).
Observation:
A reddish-brown coating appears on iron.
Conditions necessary for Rusting:
Oxygen (air)
Moisture (water)
Preventing Rusting:
- Painting
- Greasing
- Galvanising
- Coating with another metal
Combustion (Burning):
Combustion is a chemical change in which a substance burns in the presence of oxygen and produces heat and light.'
Conditions Necessary for Combustion:
A substance burns only when:
- It has fuel.
- Oxygen is available.
- It reaches its ignition temperature.
Ignition Temperature:
The lowest temperature at which a substance starts burning.
Example:
Burning paper, Burning wood, Burning LPG, Burning magnesium ribbon
When a person's clothes catch fire:
Wrap the person in a thick cotton blanket.
Why?
- The blanket cuts off the oxygen supply.
- Without oxygen, combustion stops.
- The fire goes out.
Do Not use:
A synthetic blanket, because it may melt and stick to the skin, causing severe burns.
Fascinating Fact – Fireflies:
Fireflies produce light through a chemical reaction inside their bodies.
Interesting Facts:
- The light is called bioluminescence.
- It produces very little heat.
- Fireflies use this light to communicate and attract mates.