Physical and Chemical Changes — Transformation of Matter
"When ice melts, it is STILL water. When wood burns, it becomes ASH. The first is a physical change, the second is chemical."
1. What Is a Physical Change?
Physical change: A change in which the SUBSTANCE remains the SAME — only the physical properties (state, shape, size, colour) may change.
Characteristics of Physical Changes
- No NEW substance is formed.
- The change is usually reversible (can be reversed).
- Mass is CONSERVED.
- No change in chemical composition.
- Usually NO heat or light is produced (or minimal energy change).
Examples of Physical Changes
| Example | What Changes | Why It Is Physical |
|---|---|---|
| Melting of ice | Solid → Liquid | Water is still H₂O |
| Boiling of water | Liquid → Gas (steam) | Steam is still H₂O |
| Cutting paper | Shape and size | The paper is STILL paper |
| Dissolving sugar in water | State (solid → solution) | Sugar and water are unchanged — can recover sugar by evaporation |
| Breaking a glass | Shape | Glass is still glass (just in pieces) |
| Stretching a rubber band | Shape | Rubber is still rubber |
| Magnetising an iron nail | Magnetic property | Nail is still iron |
2. What Is a Chemical Change?
Chemical change: A change in which a NEW substance is formed with DIFFERENT properties.
Characteristics of Chemical Changes
- One or more NEW substances are formed.
- The change is usually irreversible or very difficult to reverse.
- There may be a change in COLOUR, ODOUR, or TASTE.
- Heat, light, or sound may be produced or absorbed.
- Gas may be EVOLVED, or a PRECIPITATE (insoluble solid) may form.
Examples of Chemical Changes
| Example | What Happens | Signs of Chemical Change |
|---|---|---|
| Burning of paper/wood | Paper → Ash + CO₂ + H₂O | New substance (ash), heat, light |
| Rusting of iron | Iron + O₂ + H₂O → Iron oxide (rust) | Brown colour, flaky new substance |
| Cooking an egg | Egg white solidifies | Irreversible, colour and texture change |
| Curdling of milk | Milk → Curd + whey | Sour taste, new substance formed |
| Digestion of food | Food → Simpler nutrients | New chemical substances formed |
| Baking a cake | Batter → Cake | Heat, new substance, irreversible |
| Burning of a candle wick | Wax burns → CO₂ + H₂O | New substances, heat, light |
3. Comparison Table — Physical vs Chemical Changes
| Aspect | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | NO | YES |
| Reversible? | Usually YES | Usually NO |
| Composition | Same | Changes |
| Energy change | Minimal (may involve state change energy) | Significant (heat, light often produced/absorbed) |
| Examples | Melting, freezing, boiling, cutting, dissolving | Burning, rusting, cooking, digestion |
| Mass | Conserved | Conserved (law of conservation of mass) |
'In BOTH types of changes, the total mass remains the SAME — this is the LAW OF CONSERVATION OF MASS. No atoms are created or destroyed.'
4. Rusting of Iron — A Detailed Chemical Change
Rust: A reddish-brown flaky substance that forms on iron when exposed to AIR and MOISTURE.
Chemical equation: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (hydrated iron(III) oxide)
Conditions for Rusting
'Both OXYGEN (from air) and WATER are ESSENTIAL for rusting. If either is absent, rust does not form.'
Experiment:
| Test Tube | Conditions | Result |
|---|---|---|
| Nail + Air + Water (open) | All conditions present | RUSTS |
| Nail + Boiled water + Oil layer | Boiling removes air; oil prevents air entry | NO RUST |
| Nail + Anhydrous CaCl₂ (drying agent) | No moisture | NO RUST |
Prevention of Rusting
| Method | How It Works |
|---|---|
| Painting | Creates a barrier between iron and air/moisture |
| Oiling/Greasing | Similar barrier |
| Galvanisation | Coating with ZINC (zinc reacts first — sacrificial protection) |
| Chrome plating | Coating with chromium |
| Stainless steel | Alloying iron with chromium and nickel (prevents rust) |
| Enamel coating | Durable protective layer |
AP Context: 'Ships at Visakhapatnam port and fishing boats along the AP coast RUST more quickly than inland structures — the SALTY, MOIST air accelerates rusting. Regular painting and maintenance are essential.'
5. Crystallisation — A Physical Change
Crystallisation: The process of forming PURE crystals of a substance from its SOLUTION by evaporation.
'Crystallisation is a PHYSICAL change — the substance remains CHEMICALLY the same, only its physical form (crystal shape) changes.'
Making Copper Sulphate Crystals
- Dissolve copper sulphate in hot water (saturated solution).
- Filter the solution to remove impurities.
- Cool the solution slowly.
- Blue crystals of copper sulphate (CuSO₄·5H₂O) form.
Applications
- Purification of salt: 'AP's coastal salt pans produce pure salt through crystallisation of seawater.'
- Making sugar crystals: From sugarcane juice.
- Purification of chemicals: In pharmaceutical and chemical industries.
6. Common Mistakes and Fixes
| Mistake | Why It Is Wrong | Correct Approach |
|---|---|---|
| Melting of wax is a chemical change | The wax is still wax — only state changed BURNING the wick IS chemical though | Melting is physical; burning of the wick is chemical |
| Cutting vegetables is a chemical change | The vegetable is still the same substance | Only the SIZE and SHAPE changed — no new substance |
| Rusting is a physical change | Rust is a NEW substance (iron oxide) with different properties | Rusting creates NEW substances — definitely chemical |
| All irreversible changes are chemical | Breaking a glass is irreversible but physical | Check: is a new substance formed? Breaking a glass — still glass (physical). |
| Dissolving sugar in water is chemical | Sugar molecules are still sugar — you can recover them by evaporating water | Sugar remains chemically unchanged — physical change |
7. AP SSC Exam Focus
| Topic | Marks | Question Type |
|---|---|---|
| Physical change — definition and examples | 2-3 | Identify from list |
| Chemical change — definition and examples | 2-3 | Identify from list |
| Rusting — conditions and prevention | 3-4 | Explain with experiment |
| Crystallisation | 2-3 | Describe process |
| Compare physical and chemical changes | 2-3 | Table/difference |
Quick Self-Test
Q1. Is the melting of butter a physical or chemical change? Why? A1. Physical change. Butter is still butter — only its state changed from solid to liquid.
Q2. List two conditions necessary for rusting. A2. Presence of OXYGEN (air) and MOISTURE (water).
Q3. Give an example of a chemical change where heat and light are produced. A3. Burning of a candle wick / Burning of wood.
Q4. Why is crystallisation considered a physical change? A4. No new substance is formed. The substance (e.g., copper sulphate) remains chemically the same — only the arrangement of molecules changes to form a crystal structure.
Q5. How can rusting be prevented on iron gates? A5. By PAINTING — the paint layer prevents air and moisture from reaching the iron surface.
Q6. Is boiling water a chemical change? Why? A6. No. Water (liquid) becomes steam (gas), but both are H₂O — no new substance is formed.
Q7. What happens when an iron nail is placed in copper sulphate solution? Is this physical or chemical? A7. Iron displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. Copper deposits on the nail, and the blue colour fades. This is a CHEMICAL change (new substances formed).
