?? 1. Changes Happen Everywhere!
Look around you � things keep changing all the time! Day turns to night. Water becomes ice. A raw egg becomes
a boiled egg. A seed becomes a plant. These are all examples of changes.
A change is any alteration in the size, shape, colour, state, or composition of
a substance or object.
Everyday Examples of Changes:
- Ice melting into water ?????
- Milk turning sour ?????
- Wood burning to ash ?????
- A bud opening into a flower ?????
- Rusting of iron ?????
- A paper being torn ??
- Dissolving sugar in water ?????
?? 2. The Big Question: Can the Change Be Reversed?
The most important way to classify changes is: Can we get back the original substance after the
change? This gives us two types of changes:
? Reversible Changes
The change can be undone � the original substance can be obtained back.
- Ice ? Water (melt) ? Ice (freeze back)
- Rubber band stretched ? released back
- Paper folded ? unfolded
- Water ? vapour ? water (water cycle)
- Dissolving sugar ? evaporate to get sugar back
- Inflating a balloon ? deflate back
? Irreversible Changes
The change CANNOT be undone � the original substance cannot be obtained back.
- Wood burning ? ash (cannot become wood again)
- Egg boiling (cannot become raw again)
- Milk ? curd (cannot become milk again)
- Paper burning ? ash
- Rusting of iron (hard to reverse)
- Cooking food (raw ? cooked cannot reverse)
?? Image Prompt
A split educational poster with two panels: Left panel (green background, labeled
"Reversible Changes") shows: an ice cube melting to water with a double-headed arrow showing it can be
frozen back, and a stretched rubber band with arrows showing the reverse. Right panel (red/pink
background, labeled "Irreversible Changes") shows: wood burning to ash (one-way arrow), and an egg in a
frying pan becoming fried (one-way arrow). Bold text labels, clean flat-design illustration for Class 6
students.
Fig. 6.1 � Reversible and Irreversible Changes
?? 3. Types of Changes � More Ways to Classify
A. Physical Change vs Chemical Change
Another important way to classify changes:
| Property |
Physical Change |
Chemical Change |
| Composition |
No new substance formed; same material just changes shape/size/state |
New substance(s) with different properties are formed |
| Reversibility |
Usually reversible |
Usually irreversible |
| Examples |
Cutting paper, melting ice, dissolving sugar, stretching rubber |
Burning wood, rusting, cooking, souring of milk, digestion |
Physical change = only shape/size/state changes ? original substance IS still there
Think: P for Physical = Previous substance present
Chemical change = new substance is formed with different properties
Think: C for Chemical = substances Change into something
Completely different
B. Fast and Slow Changes
Fast changes: Happen very quickly � in seconds or minutes.
Examples: Lightning, explosion, burning of paper, evaporation of alcohol
Slow changes: Take hours, days, months, or years.
Examples: Rusting of iron (days-months), weathering of rocks (thousands of years), child growing (years)
C. Desirable and Undesirable Changes
Desirable changes: Changes we WANT to happen (useful/beneficial)
Examples: Ripening of fruits, cooking food, making paper from wood pulp, churning milk to get butter
Undesirable changes: Changes we do NOT want to happen (harmful/wasteful)
Examples: Rusting of iron, spoiling of food, forest fires, rotting of wood
??? 4. Changes Caused by Heat
Heat is one of the commonest causes of change. When we heat a substance, it can change in many ways:
- Expansion: Most solids, liquids, and gases expand (become bigger) when heated and
contract (become smaller) when cooled. That's why thermometers work, and why
railway tracks have gaps!
- Change of state:
- Solid ? Liquid (melting) � e.g., ice melts, butter melts
- Liquid ? Gas (evaporation) � e.g., water boils
- New substance formed: Burning of wood, cooking of egg (irreversible chemical
changes)
?? 5. Important Examples � Classification
| Change |
Reversible? |
Physical/Chemical |
Fast/Slow |
| Ice melting |
? Reversible |
Physical |
Fast/Medium |
| Wood burning |
? Irreversible |
Chemical |
Fast |
| Rusting of iron |
? Irreversible |
Chemical |
Slow |
| Milk souring |
? Irreversible |
Chemical |
Slow |
| Cutting vegetables |
? Irreversible |
Physical |
Fast |
| Paper folded |
? Reversible |
Physical |
Fast |
| Paper torn |
? Irreversible |
Physical |
Fast |
| Sugar dissolving |
? Reversible (by evaporation) |
Physical |
Medium |
| Cooking (egg boiling) |
? Irreversible |
Chemical |
Medium |
| Inflation of balloon |
? Reversible |
Physical |
Fast |
Cutting paper/bread/vegetables is a physical change (no new substance formed � it's still
paper/bread) but it is irreversible (cannot join it back). So a physical change is NOT
always reversible!
?? 6. Real-Life Applications � Why This Matters
- Blacksmith's work: A blacksmith heats iron ? makes it malleable (physically changes
shape) ? cools it ? it hardens. This is a reversible physical change.
- Salt from seawater: Evaporation separates salt � a physical change, partly
reversible.
- Why rivers carry clay to sea: Weathering (slow physical/chemical change breaks
rocks into smaller particles over thousands of years).
- Candle burning: The wax melts (physical, reversible) AND burns (chemical,
irreversible) � so candle burning involves BOTH types of changes!
?? 7. Quick Revision
- Reversible change = can be undone; original substance obtained back (ice melting)
- Irreversible change = CANNOT be undone; original substance cannot be recovered
(burning, rusting)
- Physical change = only size/shape/state changes; no new substance (cutting,
melting)
- Chemical change = new substance formed with different properties (rusting, burning,
souring)
- Fast change = lightning, burning. Slow change = rusting,
weathering
- Desirable = ripening, cooking. Undesirable = rusting, food
spoiling
- Heat causes expansion, change of state, or new substance (burning)
- Melting of ice ? reversible physical change; Burning of wood ? irreversible chemical
change
- Candle burning involves BOTH physical (wax melting) and chemical (wax burning)
changes
