What Are Physical and Chemical Changes?
Every day, things around us change. Ice melts in your glass, bread turns brown on a tawa, iron gates rust after the monsoon. Some of these changes are reversible — you can get the original substance back. Others are permanent — the original substance is gone forever.
This is the core idea: physical changes alter the form or appearance of a substance without changing what it actually is, while chemical changes produce entirely new substances with different properties.
The question Class 7 students always ask is: “How do I know which type it is?” We’ll build that instinct here, with clear rules and worked examples.
This topic is directly asked in CBSE Class 7 Science Chapter 6. Expect 2–3 marks in your school exams — usually a “classify the following” table or a short-answer question explaining a specific change.
Key Terms and Definitions
Physical Change — A change in which the physical properties (shape, size, state, colour) of a substance change, but its chemical composition stays the same. No new substance is formed.
Example: Cutting a piece of paper. You now have smaller pieces of paper, but it’s still paper (cellulose). Nothing new was made.
Chemical Change — A change in which one or more new substances are formed, with different chemical properties from the original. This is usually irreversible.
Example: Burning paper. The paper turns to ash and carbon dioxide. Ash is chemically nothing like paper — you cannot get the paper back.
Reversible Change — A change that can be undone to recover the original substance.
Irreversible Change — A change that cannot be undone.
Physical changes are usually reversible, and chemical changes are usually irreversible. But watch out — this is a general rule, not an absolute one. We’ll cover the exceptions in the Common Mistakes section.
Reactants — The original substances that undergo a chemical change.
Products — The new substances formed after a chemical change.
Indicators of a chemical change — signs that tell us a new substance has been formed: change in colour, production of gas (bubbles), change in temperature, formation of a precipitate (solid from a liquid), change in smell.
How to Identify: Physical vs Chemical Change
The Four Key Questions
When you see a change, ask yourself these in order:
1. Is a new substance formed? If yes → very likely a chemical change.
2. Is the change reversible? If yes → physical change. (You can get the original back.)
3. Are there any signs of a chemical reaction? Heat/light produced, gas given off, colour change, smell change, precipitate formed → chemical change.
4. Does the substance retain its identity? If the substance is still chemically the same thing → physical change.
Physical Changes — Concepts and Examples
Types of Physical Changes
Change of State — Solid → Liquid → Gas (or reverse). The substance stays the same; only the arrangement of particles changes.
| Change | Example |
|---|---|
| Melting | Ice → water |
| Freezing | Water → ice |
| Evaporation | Water → water vapour |
| Condensation | Water vapour → water |
| Sublimation | Camphor → camphor vapour |
Change in Shape or Size — Cutting, bending, crushing. Examples: Cutting vegetables, breaking glass, rolling chapati dough flat.
Change in Appearance — Dissolving a substance in water. Example: Salt dissolving in water. The salt is still salt — evaporate the water and you get the salt back.
Dissolution in water is a physical change because the dissolved substance can be recovered by evaporation. This is a very common question — students often confuse dissolving with reacting.
Chemical Changes — Concepts and Examples
Signs That Tell You a Chemical Change Has Happened
A chemical change almost always shows at least one of these signs:
- New substance formed — always
- Colour change — e.g., iron turns reddish-brown on rusting
- Gas produced — e.g., bubbles when vinegar + baking soda
- Heat or light produced — e.g., burning wood
- Change in smell — e.g., food spoiling
- Formation of precipitate — e.g., white solid when mixing certain solutions
Common Chemical Changes in Class 7
Rusting of Iron
Iron reacts with oxygen in the presence of moisture to form iron oxide — a completely different reddish-brown substance. You cannot turn rust back into iron at home. Irreversible, new substance formed → chemical change.
Burning
Burning always involves a chemical change. The products (CO₂, water vapour, ash) are chemically nothing like the original wood.
Cooking Food Raw egg → boiled egg. The proteins in the egg change their structure permanently. You cannot “un-boil” an egg — chemical change.
Milk turning sour Bacteria act on lactose in milk, producing lactic acid. The smell, taste, and composition change → chemical change.
Baking soda + vinegar These react to produce carbon dioxide gas (bubbles), water, and sodium acetate. The fizzing is a classic sign of chemical change.
Solved Examples
Example 1 — Easy (CBSE Level)
Q: Is melting of wax a physical or chemical change? Give two reasons.
Solution: Melting of wax is a physical change.
Reason 1: The chemical composition of wax does not change — it is still wax in liquid form.
Reason 2: The change is reversible — when the liquid wax cools, it solidifies back into wax.
No new substance is formed, so it cannot be a chemical change.
Example 2 — Easy (CBSE Level)
Q: When iron filings are mixed with sulphur powder, is a physical or chemical change observed? What if we heat the mixture?
Solution: Mixing iron and sulphur: Physical change. The two substances are just mixed together. We can separate them using a magnet — iron is attracted to the magnet, sulphur is not. Both substances retain their individual properties.
Heating iron and sulphur together: Chemical change. They react to form iron sulphide (FeS), a new grey substance. Iron sulphide is not attracted to a magnet and has completely different properties. The change is irreversible.
This is a classic CBSE experiment — the iron + sulphur demonstration directly shows the difference between mixing (physical) and reacting (chemical).
Example 3 — Moderate (CBSE, short answer type)
Q: Classify the following as physical or chemical changes: (a) Tearing of paper (b) Burning of paper (c) Dissolving sugar in water (d) Curdling of milk (e) Melting of ice cream
Solution:
| Change | Type | Reason |
|---|---|---|
| (a) Tearing paper | Physical | No new substance; paper is still paper |
| (b) Burning paper | Chemical | New substances formed (ash, CO₂); irreversible |
| (c) Dissolving sugar | Physical | Sugar recoverable by evaporation; composition unchanged |
| (d) Curdling of milk | Chemical | New substance (lactic acid) formed; irreversible |
| (e) Melting ice cream | Physical | Change of state; composition unchanged; reversible |
Example 4 — Slightly Harder (CBSE Class 7, reasoning type)
Q: Crystallisation of salt from seawater is used to obtain salt. Is this a physical or chemical change? Justify.
Solution: Crystallisation is a physical change.
When seawater evaporates, the water (solvent) escapes as vapour, and the salt (solute) is left behind as crystals. The salt was always there — it was just dissolved. No new substance is formed. The chemical composition of salt (NaCl) does not change throughout this process.
This is also how salt pans work along the Indian coastline — a large-scale physical change used commercially.
Exam-Specific Tips
CBSE Class 7 Board Pattern
Questions come in three forms:
- Classify the following (1 mark each) — give the type and one reason
- Short answer (2–3 marks) — explain with signs of chemical change
- Diagram/activity based — describe what you observe and classify
For any classification question, always state (a) the type and (b) one clear reason. “Melting of ice is a physical change because no new substance is formed and the change is reversible” — that’s a full-mark answer.
CBSE Marking Scheme Tip: In a 2-mark question asking to “distinguish” or “give differences,” give exactly two clear points in tabular form. Examiners reward neatness and clarity.
Important activities from NCERT Chapter 6:
- Iron + sulphur heating experiment (physical mixing vs chemical reaction)
- Burning of magnesium ribbon (chemical — white ash = magnesium oxide)
- Crystallisation of copper sulphate (physical change — and it’s a beautiful blue crystal!)
Common Mistakes to Avoid
Mistake 1: “Reversible = physical, irreversible = chemical” — always.
This is mostly true but not always. Dissolving salt in water is reversible and physical. But stretching a rubber band (physical) might become irreversible if stretched too far. The first question to ask is always: “Is a new substance formed?” — not “Can it be reversed?”
Mistake 2: Confusing “dissolving” with “reacting”
Salt dissolving in water → physical change (no new substance). Zinc dissolving in hydrochloric acid → chemical change (hydrogen gas is produced, new substances form).
The test: can you get the original substance back by a simple physical method? If yes, it’s physical.
Mistake 3: Thinking all colour changes mean chemical change
Adding a few drops of ink to water changes the colour — but this is a physical change (mixing/dissolving). A colour change is a sign of chemical change, not proof. Look for other signs too.
Mistake 4: Burning of candle — saying it’s only physical
Burning a candle involves both. The wax melting is a physical change (state change). The wax burning and producing CO₂ and H₂O is a chemical change. CBSE sometimes asks “what type of changes occur when a candle burns?” — say: both physical (melting) and chemical (burning).
Mistake 5: Rusting and burning — confusing which needs oxygen and which needs water
Rusting needs both oxygen and water (moisture). That’s why painting iron or keeping it dry prevents rust. Burning needs oxygen but not water. In a CBSE question about “conditions for rusting,” both conditions must be stated.
Practice Questions
Q1. Give one example each of a physical change and a chemical change that you can observe in your kitchen.
Physical change: Cutting vegetables (no new substance formed; the vegetable is still the same vegetable, just in smaller pieces).
Chemical change: Cooking/frying an egg. The egg white turns white and solidifies permanently. The proteins change structure. You cannot get the raw egg back — irreversible, new substance properties.
Q2. Why is dissolution of sugar in water considered a physical change, but fermentation of sugar is a chemical change?
Dissolution: Sugar dissolves in water but remains sugar (sucrose, C₁₂H₂₂O₁₁). If we evaporate the water, we get the sugar back. No new substance. → Physical change.
Fermentation: Yeast acts on sugar and converts it to alcohol and carbon dioxide. These are entirely new substances with different properties. The sugar is consumed and cannot be recovered. → Chemical change.
The key difference: dissolution is reversible and produces no new substance; fermentation is irreversible and produces new substances.
Q3. You heat a piece of copper in a flame. It turns black. Is this a physical or chemical change? Explain.
This is a chemical change.
Copper reacts with oxygen in the air when heated:
The black substance is copper oxide — a completely new compound. The colour change and formation of a new substance (copper oxide) confirm this is a chemical change. The copper itself is gone (it has been converted).
Note: If you cool a thin copper wire gently (not strongly heated), the black layer can sometimes be reduced back — but for CBSE Class 7, treat this as a chemical change.
Q4. Magnesium ribbon burns with a bright white flame and leaves a white powder. What type of change is this? Write the word equation.
Chemical change.
Signs of chemical change present: bright light and heat produced, new white substance (magnesium oxide) formed, irreversible. The magnesium ribbon is consumed — you cannot get it back from the ash.
This is a standard NCERT Class 7 experiment. In your board exam, mention at least two signs of chemical change for full marks.
Q5. A student says: “Freezing of water is a chemical change because water becomes a completely different-looking substance (solid ice).” Is the student correct? Explain.
The student is incorrect.
Freezing of water is a physical change. Here’s why:
- The chemical formula remains H₂O — whether it’s liquid water or solid ice, the molecules are the same.
- The change is completely reversible — melt the ice and you get water back.
- No new substance is formed — ice is just water in solid state.
The student’s error is confusing “looks different” with “chemically different.” Physical changes can make substances look very different (ice looks nothing like water!) but the chemical composition is unchanged.
Q6. Why does rusting of iron take longer in a dry desert climate than in a coastal city like Mumbai?
Rusting requires both oxygen and water (moisture):
A dry desert has very low humidity — very little water vapour in the air. Without sufficient moisture, the rusting reaction proceeds very slowly.
Mumbai has high humidity (especially during monsoon). Plenty of moisture is available, so the reaction happens quickly.
This is why iron structures near the sea rust much faster — salt water (from sea spray) actually speeds up the rusting process even further, because salt water is a better conductor and accelerates the electrochemical reaction.
Q7. Classify: (a) Inflating a balloon (b) Milk souring (c) Folding paper into an origami crane (d) Digestion of food (e) Making butter from cream (churning)
| Change | Type | Key Reason |
|---|---|---|
| (a) Inflating a balloon | Physical | Air fills the balloon; no new substance |
| (b) Milk souring | Chemical | Lactic acid formed; new substance, irreversible |
| (c) Folding paper (origami) | Physical | Paper is still paper; shape changes only |
| (d) Digestion of food | Chemical | Complex food molecules broken into simpler ones (glucose, amino acids); new substances formed |
| (e) Churning cream | Physical | Fat globules clump together mechanically; no new chemical substance |
Q8. An iron nail is placed in copper sulphate solution. After some time, the solution becomes light blue/green and the nail gets a reddish coating. What type of change is this? Identify all signs of chemical change.
This is a chemical change — a displacement reaction.
Iron is more reactive than copper. It displaces copper from copper sulphate solution.
Signs of chemical change observed:
- Colour change: The blue copper sulphate solution turns light green (iron sulphate is green).
- New solid formed: Reddish copper deposits on the iron nail.
- Irreversible: You cannot reverse this without a separate chemical process.
- New substances formed: Iron sulphate (in solution) and copper (on nail).
This is also a NCERT Class 7 activity — always mention the colour change of the solution AND the deposition on the nail in your answer.
Frequently Asked Questions
Is cooking of food a physical or chemical change?
Cooking food is a chemical change. Heat causes proteins to denature, starches to gelatinise, and new flavour compounds to form. A raw potato and a boiled potato are chemically different — the boiled one cannot become raw again. This is true for all cooking methods.
Can a change be both physical and chemical at the same time?
Yes. Burning a candle is the classic example. The wax melting is physical (wax changes state but is still wax). The wax burning is chemical (wax reacts with oxygen to form CO₂ and H₂O). CBSE Class 7 does ask this — answer: “burning of candle involves both types of changes.”
Is rusting a slow chemical change?
Yes. Rusting is a slow chemical change (also called a slow reaction or gradual chemical change). Unlike burning (which is fast), rusting happens gradually over days, weeks, or years. But it is still a chemical change — new substance (iron oxide) is formed, and it is irreversible.
Why is photosynthesis a chemical change?
Plants convert carbon dioxide and water into glucose and oxygen using sunlight. These are entirely new substances:
New substances formed, irreversible under normal conditions → chemical change.
Is dissolving a gas in water (like making soda) a physical or chemical change?
Dissolving CO₂ in water under pressure (carbonated drinks) is largely a physical change — the CO₂ can be released back when you open the bottle (pressure decreases). However, a tiny amount reacts with water to form carbonic acid (H₂CO₃) — that’s a chemical change. For Class 7, treat carbonation as a physical change.
How do I remember the signs of chemical change quickly in an exam?
Use this mnemonic: CGPST
- Colour change
- Gas produced
- Precipitate formed
- Smell change
- Temperature change (heat or light given off)
If you observe any of these during a change, suspect a chemical change and look for new substance formation.
Is digestion a physical or chemical change?
Digestion involves both. The physical breakdown of food (chewing, churning in stomach) is physical. The chemical breakdown by enzymes (amylase breaking starch into sugars, pepsin breaking proteins into amino acids) is chemical. For Class 7, NCERT treats digestion as a chemical change overall, because the key outcome — new substances with different properties — happens chemically.
Why can’t we reverse chemical changes at home?
Chemical changes form new substances with different bonds and different properties. Reversing them would require breaking those new chemical bonds and reforming the original ones — which needs specific conditions (high temperature, pressure, catalysts, or other chemicals) that we don’t have at home. Burning paper, rusting iron, souring milk — these are all thermodynamically favourable in one direction. The reverse process doesn’t happen on its own.