What is the difference between physical and chemical changes in Chemistry?

In Chemistry, a physical change involves a change in the physical properties of a substance, such as its shape, state, or appearance, without altering its chemical composition. Examples include melting ice or dissolving sugar in water. A chemical change, on the other hand, results in the formation of one or more new substances with different chemical properties, such as rusting iron or burning wood. This distinction is crucial in understanding chemical reactions, as chemical changes involve breaking and forming chemical bonds.

How can you identify a chemical change during a reaction?

A chemical change can be identified by several indicators, including the production of gas (bubbling or fizzing), a change in color, the formation of a precipitate (a solid that forms from a solution), or a change in temperature or light. These signs suggest that a new substance has been formed, which is a key characteristic of chemical reactions. Understanding these indicators is essential for Cambridge IGCSE Chemistry students to distinguish between physical and chemical changes.

Why is the conservation of mass important in chemical changes?

The conservation of mass is a fundamental principle in Chemistry, stating that mass is neither created nor destroyed in a chemical reaction. This means that the total mass of the reactants equals the total mass of the products. This principle is crucial for balancing chemical equations and understanding chemical reactions, as it ensures that all atoms present in the reactants are accounted for in the products. Cambridge IGCSE students must grasp this concept to accurately describe and predict the outcomes of chemical changes.

Can a physical change be reversed, and how does this compare to chemical changes?

Physical changes are often reversible, meaning the original substance can be recovered. For example, water can be frozen into ice and then melted back into water. In contrast, chemical changes are usually irreversible under normal conditions, as they involve the formation of new substances. For instance, burning paper results in ash and gases, which cannot be turned back into paper. Understanding the reversibility of changes helps Cambridge IGCSE students differentiate between physical and chemical processes.

What role do energy changes play in physical and chemical changes?

Energy changes are integral to both physical and chemical changes. In physical changes, energy is often absorbed or released during state changes, such as melting or boiling. In chemical changes, energy is involved in breaking and forming chemical bonds, often resulting in exothermic reactions (releasing energy) or endothermic reactions (absorbing energy). Recognizing these energy changes is important for Cambridge IGCSE Chemistry students to understand the dynamics of chemical reactions and the energy transformations involved.

Why is "Saying every colour change is chemical" a common mistake in Physical and Chemical Changes?

Why it happens: Common rule of thumb. How to avoid it: Some physical changes (e.g. dissolving CuSO₄, hydrating cobalt chloride) also change colour. Look for OTHER signs too. Source: 0620/42 — recurring

Why is "Calling dissolving a chemical reaction" a common mistake in Physical and Chemical Changes?

Why it happens: It looks dramatic. How to avoid it: Dissolving is usually physical — solute is recovered by evaporation.

Why is "Thinking mass changes during a chemical reaction" a common mistake in Physical and Chemical Changes?

Why it happens: Mass increase in open systems. How to avoid it: Atoms are conserved. Mass changes only because gases enter or leave the system.

Why is "Listing 'something happened' as evidence" a common mistake in Physical and Chemical Changes?

Why it happens: Imprecise. How to avoid it: Examiners want SPECIFIC: gas evolved, precipitate formed, permanent colour change, energy change.

Chemical ReactionsCambridge IGCSE Chemistry (0620)

Physical and Chemical Changes

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Physical and Chemical Changes — Cambridge IGCSE 0620 Chemistry Extended (2026)

Physical: same substance, different state. Chemical: NEW substance with new properties. Tell them apart by looking for evidence of new substances.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

7.1 — Distinguish between physical and chemical changes.
7.1 — Identify evidence that a chemical reaction has taken place.

Physical vs chemical change

Physical: nothing new. Chemical: new substance. Look for the give-aways.

Physical change. A change of state, shape, or form WITHOUT producing a new substance.

Melting (ice → water), boiling (water → steam), dissolving sugar in tea.
The substance can usually be RECOVERED by reversing the change (re-freezing the water; evaporating the tea to get sugar back).
No new chemical bonds formed or broken (in most cases).

Chemical change (chemical reaction). Produces ONE OR MORE NEW SUBSTANCES with different properties from the starting materials.

Burning, rusting, baking a cake, photosynthesis.
Bonds are broken and formed. Atoms rearrange.
Often hard to reverse (would need another chemical reaction).

Evidence of a chemical change.

Gas evolved (bubbles, fizz, smell).
Precipitate formed (insoluble solid appears in solution).
Temperature change (warms or cools).
Colour change (esp. when no dilution effect).
Light or sound (e.g. magnesium burning brightly; sodium-water fizzing).
Often a new smell.

Tip. Even one of these signs strongly suggests a chemical change. Two or more is essentially conclusive.

Physical: no new substance.
Chemical: new substance with new properties.
Evidence: gas, precipitate, $\Delta T$ , colour, light/sound, smell.
Atoms rearrange in chemical changes; bonds break/form.

Mass conservation; energy is exchanged

Total mass before = total mass after. Energy moves between system and surroundings.

Mass conservation. In ANY chemical reaction (closed system), the total mass of reactants = total mass of products. Atoms are neither created nor destroyed — they're just rearranged.

Worked qualitative. A piece of magnesium ribbon with mass $0.24\,\text{g}$ is burned in air. The product (magnesium oxide) has mass $\sim 0.40\,\text{g}$ . Where did the extra mass come from? Oxygen from the air combined with magnesium → MgO. Total mass of MgO = mass of Mg + mass of O₂ that reacted.

Worked qualitative (apparent loss). A candle burns and seems to lose mass. Where did the mass go? Mostly into CO₂ and H₂O vapour, which escape into the air. If you trapped them, the total mass before and after would balance.

Energy. Chemical reactions usually release or absorb energy. The system's internal energy changes; mass is unaffected (for IGCSE chemistry — relativistic mass-energy effects are negligible).

Cambridge tip. Conservation of mass is a foundational law. When asked to verify a balanced equation, count atoms on both sides AND check that the total mass calculations work out from $A_r$ values.

Mass conserved in all reactions.
Apparent mass change usually means gases came in or escaped.
Energy is what's exchanged; mass is unchanged.

How it’s examined

Physical/chemical change is a 2-3 mark item on most Paper 2s (classify a list, identify evidence). Examiner reports flag students treating any colour or temperature change as automatic evidence of chemical change — dilution and warming are PHYSICAL.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (7.1); 0620/22 May/Jun 2024 — Q1 (physical vs chemical); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Physical and Chemical Changes

Step-by-step solutions to past-paper-style questions on physical and chemical changes, written exactly the way a tutor would explain them at the board.

1Classify changes
Core• classify
Question
Classify each as physical or chemical: melting iron, boiling water, burning magnesium ribbon, mixing sand with water.
Step-by-step solution
1. Step 1
  Physical: state change only; reversible.
2. Step 2
  Chemical: new substance formed; usually irreversible.
Answer
Physical: melting iron, boiling water, mixing sand. Chemical: burning Mg.
2Signs of a chemical reaction
Core• Adapted from 0620/22 May/Jun 2024 Q6• evidence
Question
Give four observations that suggest a chemical reaction has occurred.
Step-by-step solution
1. Step 1
  (a) Effervescence (gas given off).
2. Step 2
  (b) Precipitate forms.
3. Step 3
  (c) Permanent colour change.
4. Step 4
  (d) Temperature change (heat released or absorbed).
Answer
Effervescence; precipitate; permanent colour change; temperature change.
3Mass conservation in a chemical change
Extended• conservation
Question
Explain why the total mass does not change during a chemical reaction.
Step-by-step solution
1. Step 1
  Atoms are rearranged, not created or destroyed.
2. Step 2
  Total mass of reactants = total mass of products (in a closed system).
Answer
Atoms are conserved → mass is conserved.
4Apparent mass change in an open system
Extended• open system
Question
When magnesium ribbon burns in air, the mass of the product (MgO) is greater than the original mass of magnesium. Explain.
Step-by-step solution
1. Step 1
  $\text{Mg}$ combines with $\text{O}_{2}$ from the air.
2. Step 2
  Product mass = original Mg + mass of oxygen added.
Answer
Mass of oxygen from the air is added to the magnesium → product is heavier.

Key Definitions and Keywords — Physical and Chemical Changes

Definitions to memorise and the exact keywords mark schemes credit for physical and chemical changes answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Physical change
Examiner keyword
A change involving no new chemical substance — only state, shape or size. Usually reversible.
Chemical change
Examiner keyword
A change in which one or more new substances are formed. Usually accompanied by an energy change.
Conservation of mass
Examiner keyword
Total mass of reactants equals total mass of products (in a closed system).
Effervescence
Examiner keyword
Production of bubbles of gas — common observation of a chemical reaction.

Common Mistakes and Misconceptions — Physical and Chemical Changes

The traps other students keep falling into on physical and chemical changes questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying every colour change is chemical
0620/42 — recurring
Why it happens
Common rule of thumb.
How to avoid it
Some physical changes (e.g. dissolving CuSO₄, hydrating cobalt chloride) also change colour. Look for OTHER signs too.
✕Calling dissolving a chemical reaction
Why it happens
It looks dramatic.
How to avoid it
Dissolving is usually physical — solute is recovered by evaporation.
✕Thinking mass changes during a chemical reaction
Why it happens
Mass increase in open systems.
How to avoid it
Atoms are conserved. Mass changes only because gases enter or leave the system.
✕Listing 'something happened' as evidence
Why it happens
Imprecise.
How to avoid it
Examiners want SPECIFIC: gas evolved, precipitate formed, permanent colour change, energy change.

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Physical and Chemical Changes — frequently asked questions

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Physical and Chemical Changes

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Classify changes

2Signs of a chemical reaction

3Mass conservation in a chemical change

4Apparent mass change in an open system

Physical change

Chemical change

Conservation of mass

Effervescence

✕Saying every colour change is chemical

✕Calling dissolving a chemical reaction

✕Thinking mass changes during a chemical reaction

✕Listing 'something happened' as evidence

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Physical and Chemical Changes — frequently asked questions