Changes Around Us — Class 6 Physical Science
'Change is the only constant — the world around us is always transforming, from ice melting to paper burning.'
1. Introduction
Everything around us is constantly changing. Ice melts, milk curdles, paper burns, plants grow. Some changes can be reversed (melted wax can be refrozen), while others cannot (a burnt paper cannot be restored).
Why This Chapter Matters
- Understand the nature of changes around us
- Distinguish between reversible and irreversible changes
- Introduction to physical and chemical changes (in higher classes)
- Practical: cooking, safety, preservation
2. Types of Changes
Changes can be classified in two main ways:
Classification 1: By Reversibility
| Type | Can It Be Reversed? | Examples |
|---|---|---|
| Reversible | YES — the original substance can be recovered | Ice ↔ Water, stretching a rubber band |
| Irreversible | NO — the original substance cannot be recovered | Burning wood, cooking an egg |
Classification 2: By Nature (Introduction)
| Type | What Changes | Examples |
|---|---|---|
| Physical | Form or state changes, NOT the substance itself | Melting, freezing, cutting |
| Chemical | Substance changes into a NEW substance | Rusting, burning, curdling |
3. Reversible Changes
A change that can be undone — the original substance can be recovered.
Examples
Melting of ice
- Ice (solid) → Water (liquid) on heating
- Water → Ice on cooling (reversible!)
- 'This is a REVERSIBLE change because water can be frozen back to ice.'
Dissolving sugar in water
- Sugar dissolves → you get sugar solution
- Evaporate water → you get sugar crystals back
- Reversible
Stretching a rubber band
- Rubber band stretches when pulled
- Goes back to original shape when released
- Reversible
Inflating a balloon
- Balloon expands with air
- Deflate → balloon returns to original
- Reversible
Melting wax
- Candle wax melts on heating
- Cools and solidifies again
- Reversible
Table of Reversible Changes
| Change | How to Reverse |
|---|---|
| Ice → Water | Cool to freeze |
| Water → Ice | Heat to melt |
| Water → Steam | Cool to condense |
| Sugar in water | Evaporate water |
| Stretched band | Release |
4. Irreversible Changes
A change that cannot be undone — the original substance cannot be recovered.
Examples
Burning of paper
- Paper + fire → Ash + smoke + gases
- You CANNOT get the original paper back
- Irreversible
Cooking an egg
- Raw egg (liquid) → Cooked egg (solid)
- You CANNOT turn a cooked egg back to raw
- Irreversible
Curdling of milk
- Milk + acid/rennet → Curd (paneer/curd) + whey
- You CANNOT get milk back from curd
- Irreversible
Rusting of iron
- Iron + oxygen + water → Rust (iron oxide)
- The rusted nail cannot become a new nail
- Irreversible
Baking a chapati
- Dough → Cooked chapati
- You cannot get dough back
- Irreversible
Recognizing Irreversible Changes
- Heat is often involved (but not always)
- A NEW substance is formed
- The change is permanent
5. Physical vs Chemical Changes (Introduction)
Physical Changes
- Only the appearance or state changes
- The substance remains the SAME chemically
- Usually reversible
| Physical Change | Explanation |
|---|---|
| Melting chocolate | Same chocolate, just a different state |
| Cutting vegetables | Same vegetable, just smaller pieces |
| Folding paper | Same paper, different shape |
| Dissolving salt | Salt is still salt (recover by evaporation) |
Chemical Changes
- A NEW substance is formed
- Usually irreversible
- Often involves heat, light, or gas production
| Chemical Change | New Substance Formed |
|---|---|
| Burning wood | Ash, CO₂, water vapour |
| Cooking food | Cooked food (different from raw) |
| Rusting iron | Iron oxide (rust) |
| Curdling milk | Curd (different from milk) |
Comparison Table
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance? | NO | YES |
| Reversible? | Usually YES | Usually NO |
| Examples | Melting, cutting | Burning, rusting |
| Molecular change | No change | Atoms rearrange |
6. Detailed Examples from Daily Life
Example 1: Ice Cream Melting
- Melting is a physical, reversible change
- You can refreeze melted ice cream (though texture changes slightly)
Example 2: Making Lassi/Buttermilk
- Churning curd to get buttermilk (majjiga)
- Physical change (just separating components)
- But the curd itself was formed by a chemical change!
Example 3: Lighting a Matchstick
- Chemical change — the head burns to form new substances
- Irreversible — you cannot unburn a match
Example 4: AP Context — Making Gongura Pickle
- Mixing gongura leaves with salt, spices, oil
- Some changes are physical (cutting leaves)
- Some are chemical (fermentation over time)
- The final pickle is very different from the original leaves
Example 5: Ripening of Mangoes
- Raw mango → Ripe mango
- Chemical changes inside (starch → sugar)
- Irreversible — ripe mango cannot become raw again
7. Common Mistakes — Fix Them Now
| # | Mistake | Correction |
|---|---|---|
| 1 | Thinking all heating changes are irreversible | Melting is reversible. Burning is not. Heat alone doesn't determine reversibility. |
| 2 | Calling dissolving a chemical change | Dissolving is usually PHYSICAL — no new substance is formed. |
| 3 | Saying cutting vegetables is a chemical change | Cutting changes shape and size only — the substance (vegetable) is still the same. |
| 4 | Thinking curdling is a physical change | Curdling is CHEMICAL — milk changes into curd, a different substance. |
| 5 | Believing all irreversible changes require heat | Curdling happens with lemon juice (no heat needed). |
8. Exam Focus
Marks Blueprint
| Question Type | Marks | Topic |
|---|---|---|
| MCQ | 1 | Reversible vs Irreversible |
| Short answer | 2 | Classify a change with reason |
| Short answer | 2 | Physical vs Chemical change |
| Long answer | 3 | Examples of changes around us |
Quick Self-Test (5 Questions)
Q1: Is melting of wax reversible or irreversible?
<details><summary>Answer</summary>Reversible — melted wax can cool and solidify back to solid wax.</details>Q2: Why is burning of wood an irreversible change?
<details><summary>Answer</summary>Wood converts to ash, CO₂, and gases — new substances are formed that cannot be turned back into wood.</details>Q3: Classify: making a paneer from milk.
<details><summary>Answer</summary>Irreversible chemical change — milk changes into paneer (a new substance).</details>Q4: Is dissolving salt in water a physical or chemical change?
<details><summary>Answer</summary>Physical change — salt can be recovered by evaporating water. No new substance is formed.</details>Q5: Give two examples of reversible changes.
<details><summary>Answer</summary>Ice melting (and refreezing), stretching a rubber band (and releasing).</details>9. Chapter Summary
- Reversible: original substance CAN be recovered (melting, dissolving, stretching)
- Irreversible: original substance CANNOT be recovered (burning, cooking, curdling)
- Physical change: no new substance; usually reversible (melting, cutting)
- Chemical change: new substance formed; usually irreversible (rusting, burning)
- Recognising changes: look for new substances, reversibility, gas/heat/colour change
'Understanding changes helps us predict what will happen when we heat, cool, mix, or process materials — a fundamental skill in science and daily life.'
