Physical and chemical changes are the most basic distinction in chemistry. CBSE Class 9 and 10 introduce them; NEET rarely tests it directly but the concepts underlie everything.
Core Concepts
Physical change
No new substance is formed. Reversible in most cases. Only physical properties change — state, shape, size. Examples — melting ice, boiling water, cutting paper, dissolving salt.
The key test for a physical change: can you get the original substance back by a simple physical method? If you dissolve salt in water, you can evaporate the water and get the salt crystals back. That means no new substance was formed — just a mixture.
At the molecular level, a physical change rearranges how molecules are positioned relative to each other (liquid water becomes ice — same H2O molecules, different arrangement) but does not break or form chemical bonds between different atoms.
Chemical change
New substance(s) formed. Usually irreversible. Chemical properties change. Often accompanied by energy change (heat, light, sound), colour change, gas evolution or precipitate formation.
At the molecular level, chemical bonds are broken and new bonds are formed. When iron rusts, Fe-Fe metallic bonds break, O=O bonds break, and new Fe-O bonds form. The product (rust) has completely different properties from the starting materials.
Signs of chemical change
Evolution of gas (bubbles). Change of colour. Formation of precipitate. Change of temperature. Change of smell. Emission of light.
Let us connect each sign to a real example:
| Sign | Example | Reaction |
|---|---|---|
| Gas evolution | Baking soda + vinegar | |
| Colour change | Rusting of iron | Fe (grey) → Fe2O3 (brown) |
| Precipitate | AgNO3 + NaCl | |
| Temperature change | CaO + water | Quicklime reacts vigorously, temperature rises |
| Light emission | Burning magnesium | Bright white flame |
Not every sign guarantees a chemical change. Boiling water produces bubbles (gas evolution) but it is a physical change. Always check whether a new substance is formed.
Reversible and irreversible
Physical changes are usually reversible (water to ice back to water). Chemical changes are mostly irreversible (burning wood). Some exceptions — reversible chemical reactions exist in equilibrium systems.
This distinction matters for higher classes: in Class 11, you will study chemical equilibrium where reactions go both forward and backward simultaneously. The decomposition of to and is reversible under the right conditions — cool the products and add CO2 at high pressure, and CaCO3 re-forms. But at the Class 9-10 level, we treat most chemical changes as irreversible for simplicity.
Examples in daily life
Cooking is chemical (new substances formed). Rusting is chemical. Freezing is physical. Dissolving sugar in water is physical. Photosynthesis is chemical. Sweating is physical.
Energy changes in reactions
Every chemical change involves energy. Exothermic reactions release energy — burning of fuels, neutralisation of acids with bases, respiration. Endothermic reactions absorb energy — photosynthesis, thermal decomposition of CaCO3, dissolving ammonium chloride in water (the test tube feels cold).
The energy can be in the form of heat, light, sound, or electrical energy. Burning magnesium releases both heat and light. A battery converts chemical energy to electrical energy.
Worked Examples
The wax melts (physical) and then vaporises and burns (chemical). Two different changes happen in sequence at different parts of the flame.
Iron (Fe) combines with oxygen and water to form hydrated iron oxide (rust). The rust has totally different properties — brown, brittle, non-magnetic. A new substance is formed.
Physical. The sugar molecules (sucrose, ) are dispersed among water molecules but not broken apart. You can recover the sugar by evaporating the water. No new substance is formed. However, dissolving an acid in water (like HCl) involves ionisation — that is a chemical change because new species (H+ and Cl-) are formed.
If you leave a bottle of ethanol open, it evaporates — physical change (liquid to gas, same molecule). If you light the ethanol, it burns — chemical change (). Same substance, two very different changes depending on conditions.
When you knead dough with water — physical (mixing). When you roll the dough — physical (shape change). When you put it on the tawa — chemical (proteins denature, starch gelatinises, Maillard browning creates new flavour compounds). The brown spots on a cooked chapati are new substances that did not exist in the raw dough.
Detailed Comparison Table
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance | Not formed | Formed |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | Small or none | Usually significant |
| Bond breaking | No (intermolecular forces may change) | Yes (chemical bonds broken and formed) |
| Mass change | No change | No change (law of conservation of mass) |
| Examples | Melting, boiling, dissolving, cutting | Burning, rusting, cooking, fermentation |
| How to confirm | Recover original substance by physical means | Cannot easily recover original substance |
Common Mistakes
Confusing dissolving with reacting. Dissolving salt is physical; salt can be recovered by evaporating water.
Saying all chemical changes are irreversible. Some are reversible under different conditions.
Writing that change of state is chemical. It is physical.
Assuming colour change always means chemical change. Mixing two coloured liquids can produce a new colour (physical mixing), and some chemical reactions have no colour change.
Saying that breaking glass is a chemical change because it cannot be reversed. Breaking glass is physical — no new substance is formed. The pieces are still glass. “Irreversible” and “chemical” are not the same thing.
Exam Weightage and Revision
This topic carries 2-3 marks in CBSE Class 9 and 10 exams. Questions are typically: classify the following as physical or chemical change, give reasons. NEET does not test it as a standalone topic but uses the concept in thermodynamics and equilibrium questions.
| Exam | How It Appears | Marks |
|---|---|---|
| CBSE Class 9 | ”Classify these changes” with 5-6 examples | 2-3 |
| CBSE Class 10 | ”Give evidence that burning is a chemical change” | 2 |
| NEET | Embedded in thermodynamics or equilibrium | Indirect |
| JEE | Not directly tested | - |
The CBSE marking scheme awards full marks only if you give the reason, not just the classification. “Rusting is chemical” gets half marks. “Rusting is chemical because a new substance (iron oxide) with different properties is formed” gets full marks.
Practice Questions
Q1. Classify the following as physical or chemical change: (a) Making curd from milk (b) Tearing paper (c) Burning paper (d) Dissolving common salt in water (e) Photosynthesis
(a) Chemical — Lactobacillus converts lactose to lactic acid, a new substance. (b) Physical — no new substance, just smaller pieces of the same paper. (c) Chemical — paper burns to produce CO2, water vapour, and ash — all new substances. (d) Physical — NaCl dissolves but can be recovered by evaporation. (e) Chemical — CO2 and water are converted to glucose and oxygen, entirely new substances.
Q2. A student says “evaporation of water is a chemical change because the water disappears.” Correct this statement.
The water does not disappear — it changes from liquid to gas (water vapour). The same H2O molecules exist in the gas phase. No new substance is formed. The water vapour can be condensed back to liquid water, confirming it is a physical change. The student confused “disappearing from sight” with “being destroyed.”
Q3. When you add lemon juice to baking soda, bubbles form. Is this physical or chemical? Explain.
Chemical change. Lemon juice (citric acid) reacts with baking soda (NaHCO3) to produce CO2 gas (the bubbles), water, and sodium citrate. New substances are formed. You cannot get back the baking soda by any simple physical method.
Q4. Is the digestion of food a physical or chemical change? Or both?
Both. Chewing food is physical (breaking into smaller pieces). The action of enzymes is chemical — amylase breaks starch into maltose, pepsin breaks proteins into peptides, lipase breaks fats into fatty acids and glycerol. These are all new substances formed by breaking chemical bonds. Digestion is a mix of physical and chemical changes happening together.
FAQs
Why is dissolving salt physical but dissolving acid in water chemical?
When NaCl dissolves, the ions (Na+ and Cl-) that already existed in the crystal simply disperse into water. No new bonds form. When HCl gas dissolves in water, it ionises: . The H-Cl bond breaks and a new bond (H3O+) forms. New species are created.
Does mass change during a chemical reaction?
No. The law of conservation of mass (Lavoisier) states that mass is neither created nor destroyed. If a reaction seems to lose mass (burning wood becomes lighter ash), it is because gases (CO2, H2O vapour) escaped. If you burned wood in a sealed container, the total mass would remain the same.
Can a physical change be irreversible?
Yes. Cutting a log into pieces is a physical change (no new substance), but you cannot easily put the pieces back together. Grinding wheat into flour is another example — physical but practically irreversible.
What about radioactive decay — is it physical or chemical?
Neither, strictly speaking. Radioactive decay changes the nucleus of an atom (protons and neutrons), which is a nuclear change. Chemical changes involve only electrons (outer shell). Physical changes involve no atomic-level changes at all. Nuclear changes are a third category.
Remember four clues of chemical change — colour, gas, precipitate, heat. Any one of these usually means chemical.
Changes are the simplest chemistry to observe and the foundation for everything that comes later in the subject.