Changes Around Us – Observations Through Senses
A change is any alteration in the appearance, shape, size, position, state, temperature, smell, colour, or composition of a substance.
We observe many changes using our sense organs:
Eyes: detect changes in colour, shape, size, and position.
Nose: detects changes in smell.
Skin: detects changes in temperature and texture.
Tongue: detects changes in taste.
Ears: detect changes through sound.
Examples:
Ice melts → change in state.
A leaf turns yellow → change in colour.
Food gets spoiled → change in smell and taste.
Physical Changes
A physical change is a change in which the physical properties of a substance change, but no new substance is formed.
Physical properties include shape, size, state, and appearance.
Examples
Cutting paper, Inflating a balloon, Breaking chalk, Melting ice, etc.
Activity 5.2 – Paper, Balloon and Chalk
Observe the following:
1. Paper:
When a paper is folded or cut, its shape or size changes, but it is still paper.
2. Balloon:
When air is blown into a balloon, its shape and size change, but the balloon material remains the same.
3. Chalk:
When a piece of chalk is broken, it becomes smaller pieces, but the pieces are still chalk.
Conclusion
In all these cases:
Original substance → Change in shape/size → Same substance
Therefore, these are physical changes.
Chemical Changes – Formation of New Substances
A chemical change is a change in which one or more new substances with different properties are formed.
The new substance usually has properties different from the original substances.
Common signs of a chemical change
A chemical change may involve:
• Change in colour
• Formation of a gas
• Formation of a solid precipitate
• Change in temperature
• Production of light or sound
• Formation of a new smell
Activity 5.3 – Lime Water Test:-
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The calcium carbonate formed is a white solid that makes the lime water appear milky.
Therefore, this is a chemical change.
Activity 5.4 – Vinegar + Baking Soda:-
When vinegar is mixed with baking soda, bubbles are produced.
Vinegar contains acetic acid, while baking soda is sodium hydrogen carbonate.
Acetic acid + Sodium hydrogen carbonate → Sodium acetate + Water + Carbon dioxide
Observation
Bubbles/fizzing → gas is produced
The gas produced is carbon dioxide.
How can we confirm the gas?
Pass the gas through lime water.
Lime water → turns milky
This confirms the presence of carbon dioxide.
Rusting – A Chemical Change
Rusting requires:
Iron + Oxygen + Water/Moisture → Rust
Both oxygen and moisture are important for ordinary rusting.
Example
An iron nail left exposed to moist air gradually develops a reddish-brown coating of rust.
Prevention of rusting:
• Painting
• Oiling or greasing
• Galvanisation
• Coating with suitable protective materials
Combustion
Combustion is a chemical process in which a substance reacts with oxygen, usually producing heat and often light.
Examples
• Burning wood
• Burning a candle
• Burning LPG
Burning of Magnesium Ribbon
When a magnesium ribbon is heated, it burns with a bright white flame and forms a white powder of magnesium oxide.
Reaction:
Magnesium + Oxygen → Magnesium oxide
2Mg + O₂ → 2MgO
Observations
• Magnesium burns with a bright white flame.
• Heat and light are produced.
• white solid is formed.
Fire Triangle
For combustion to continue, three things are necessary. They are represented by the fire triangle:
1. Fuel 2. Oxygen 3. Heat
Fuel + Oxygen + Heat → Fire
If any one of these three is removed, the fire can be extinguished.
Example
Water can cool a burning material and remove the heat needed for combustion.
Ignition Temperature
The ignition temperature of a substance is the lowest temperature at which it catches fire and starts burning.
Example
Paper does not catch fire at room temperature. When heated sufficiently, it reaches its ignition temperature and starts burning.
Both Physical and Chemical Changes – Burning of a Candle
Burning a candle is an interesting example because both physical and chemical changes occur at the same time.
Physical change
The wax near the flame melts.
Solid wax → Liquid wax
No new substance is formed during melting.
Therefore, melting of wax is a physical change.
Chemical change
The wax vapour reacts with oxygen and undergoes combustion.
It produces new substances such as:
Carbon dioxide
Water vapour
Therefore, burning of wax is a chemical change.
Reversible Change
A change that can be reversed to obtain the original substance or state is called a reversible change.
Examples
Ice → water → ice
Folding paper
Stretching a rubber band
Irreversible Change
A change that cannot easily be reversed to obtain the original substance or state is called an irreversible change.
Examples
Burning paper
Cooking food
Rusting of iron
Desirable Changes
Changes that are useful or beneficial to us are called desirable changes.
Examples
Cooking food, Ripening of fruits, Germination of seeds, etc.
Undesirable Changes
Changes that are harmful, unwanted, or cause damage are called undesirable changes.
Examples
Rusting of iron, Spoilage of food, Decay of materials, etc.
Slow Natural Changes
Some changes occur naturally and take a very long time. These are called slow natural changes.
Examples include:
Weathering of rocks, Erosion, Formation of soil, etc.
Weathering of Rocks
Weathering is the gradual breaking down of rocks into smaller pieces due to natural agents.
Causes of weathering-
Temperature changes
Water
Wind
Ice
Plant roots
Chemical reactions
Example
Water enters cracks in a rock. Repeated heating and cooling or freezing and thawing can gradually widen the cracks, causing the rock to break into smaller pieces.
Erosion
Erosion is the wearing away and carrying away of soil or rock particles from one place to another by natural agents.
Agents of erosion-
Running water, Wind, Glaciers, Sea waves
Example
Heavy rain and flowing water can carry soil from one place to another.
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