✅ Physical and Chemical Changes – Complete Summary with NCERT Solution PDF (Class 7 Science)
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| Changes Around Us: Physical and Chemical (AI Generated Image) |
Every day, we observe many changes around us. Ice melts, fruits ripen, iron rusts, dough becomes chapati, and clothes dry in the sun. Some of these changes are slow, some fast, some useful, and some harmful. In this chapter, we learn how to classify these changes into physical and chemical changes, along with reversible and irreversible changes, and many real-life applications.
✅ What is a Physical Change?
A physical change is a change in which only the appearance, shape, size or state of a substance changes, but no new substance is formed.
The material remains the same even after the change.
✅ Real-life examples
- Melting of ice – Ice changes into water, but the substance (H₂O) remains the same.
- Boiling of water – Water becomes steam but can again be cooled to get water back.
- Cutting paper, chopping vegetables, breaking chalk – The shape changes, but the substance remains unchanged.
- Dissolving salt or sugar in water – The salt can be obtained back by evaporation.
✅ Key Features of Physical Change
✔ No new substance is made
✔ Properties remain almost same
✔ Usually reversible
✔ Only state, size, color, or shape changes
Examples from daily life: blowing up a balloon, making dough balls, folding paper, drying clothes, freezing water.
✅ What is a Chemical Change?
A chemical change produces one or more new substances with new properties. Once the change happens, it usually cannot be reversed easily.
✅ Signs of a chemical change
✔ New colour appears
✔ New smell forms
✔ Gas bubbles appear
✔ Heat or light is released
✔ A solid forms or residue remains
✅ Example 1: Blowing air into lime water
- Lime water turns milky when carbon dioxide mixes with it.
- A new substance, calcium carbonate, is formed.
- Therefore, it is a chemical change.
✅ Example 2: Vinegar + Baking Soda
- Bubbles are produced.
- Carbon dioxide gas is formed.
- Passing this gas into lime water turns it milky.
→ A new substance is formed → Chemical change.
✅ Combustion (Burning) – A Chemical Change
Combustion is a reaction where a substance burns in oxygen and produces heat, light, and new substances.
Example: Burning of magnesium ribbon
- Magnesium burns with a bright white flame
- Forms white ash called magnesium oxide
- It is a chemical change because new substances form
✅ What is needed for burning?
Burning needs three things:
- Fuel – the substance that burns (wood, wax, oil, kerosene)
- Oxygen – from air
- Heat – to reach ignition temperature
If any of these is missing, burning stops.
For example, covering a burning candle with a glass jar stops its flame because oxygen supply cuts off.
✅ Reversible & Irreversible Changes
Changes that can be reversed are reversible, and changes that cannot be undone are irreversible.
| Reversible Changes | Irreversible Changes |
| | |
| Water → ice → water | Burning paper |
| Water → steam → water | Rusting |
| Folding paper | Ripening of fruits |
| Stretching a rubber band | Making popcorn |
Physical changes are mostly reversible.
Chemical changes are usually irreversible.
✅ Are All Changes Useful?
Not all changes are desirable.
✅ Useful changes
✔ Cooking food
✔ Making curd from milk
✔ Baking bread
✔ Composting waste
❌ Undesirable changes
✘ Rusting of iron
✘ Spoiling of food
✘ Air pollution from burning fuels
Sometimes, an undesirable change becomes useful.
Example: Rotten food becomes compost, which helps plants grow.
✅ Slow Natural Changes
Some natural changes take a very long time.
✅ Weathering of Rocks
Rocks break into smaller pieces due to heat, cold, rain, wind, or plant roots.
Sometimes chemical reaction with water forms new colored layers.
This leads to formation of soil over thousands of years.
✅ Erosion
Wind and flowing water carry soil and sand from one place to another.
Rocks near rivers become smooth because of continuous rubbing.
Sediments later harden to form new rocks.
Weathering and erosion are slow physical and chemical changes that shape Earth’s surface.
✅ Short Revision (In a Nutshell)
Physical change → no new substance formed
Chemical change → new substance formed
Combustion needs fuel, heat, and oxygen
Some changes can be reversed, some cannot
Natural changes like weathering and erosion form soil
~~The END~~
Chapter 5 Changes Around US: Chemical and Physical
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