Physical Change

Summary: A physical change is a change in which no new chemical substance is formed — the particles themselves remain unchanged and only their arrangement, motion, or energy is altered. Physical changes are usually easily reversible and do not involve the breaking or making of chemical bonds.

Tags: physical-change states-of-matter igcse-chemistry reversibility phase-change chemical-change particle-theory

Created: 2026-07-18

A physical change is a transformation in which the appearance, state, shape, or physical properties of a substance are altered, but the chemical composition and identity of the substance remain the same — no new chemical substance is produced and the particles (atoms, molecules, or ions) themselves do not change, only their arrangement, spacing, or energy. In a physical change, no chemical bonds are broken or formed; this distinguishes it fundamentally from a Chemical Change, where bonds are both broken and made and one or more new substances with different chemical properties are produced as a result. Physical changes are typically — though not universally — easily reversible, meaning the original substance can be recovered by a simple physical process such as cooling, filtration, or evaporation, without requiring a chemical reaction. Common examples include all changes of state (Melting, Boiling, freezing, condensation, sublimation), dissolving a solute in a solvent (such as salt dissolving in water, where Na⁺ and Cl⁻ ions are merely separated by water molecules but remain chemically unchanged), cutting or grinding a solid into smaller pieces, magnetising and demagnetising iron, and stretching or bending a material without breaking it. The key diagnostic test for a physical change is: if you can recover the original substance by a simple physical technique — such as distillation, filtration, or cooling — without any chemical transformation, then the process is a physical change. The Particle Theory of matter provides the explanatory framework: in a physical change, the particles gain or lose kinetic energy and move closer together or further apart, but their fundamental nature is unaltered — a water molecule (H₂O) is still a water molecule whether it exists as ice, liquid water, or steam.

Distinguishing Physical Changes from Chemical Changes

The distinction between physical changes and chemical changes is one of the foundational concepts in IGCSE Chemistry. It is tested frequently, both as direct recall and in the context of experimental observations.

Key Diagnostic Question

When analysing an unfamiliar change, ask: “Are the substances present at the end chemically identical to the substances present at the start?”

• If yes — physical change.
• If no — chemical change.

For example, when ice melts, the substance at the end (liquid water, H₂O) is chemically identical to the substance at the start (solid ice, H₂O) — therefore it is a physical change. When magnesium burns in air, the silvery metal and colourless oxygen gas are replaced by a white powder (magnesium oxide, MgO) that has entirely different properties — therefore it is a chemical change.

The Particle Model and Physical Change

The Particle Theory (kinetic particle theory) explains all physical changes in terms of the behaviour of particles:

1. All matter is made of particles (atoms, molecules, or ions) that are in constant random motion.
2. The particles themselves do not change during a physical change — only their arrangement and energy.
3. Temperature reflects the average kinetic energy of the particles.

How the Particle Model Explains Changes of State

A crucial point: in each of these changes, the chemical identity of the particles remains unaltered. When water boils, the H₂O molecules are still H₂O molecules in the gas phase — no O—H covalent bonds are broken. Only the intermolecular forces (hydrogen bonds between water molecules) are overcome. The breaking of chemical bonds (intramolecular forces) happens only during chemical changes, and this distinction is a common examination question at IGCSE level (see Bond Breaking vs. overcoming intermolecular forces).

Examples of Physical Changes

1. Changes of State

All six phase transitions are physical changes:

• Melting: Solid to liquid (e.g. ice → water)
• Freezing: Liquid to solid (e.g. water → ice)
• Boiling/Evaporation: Liquid to gas (e.g. water → steam)
• Condensation: Gas to liquid (e.g. steam → water)
• Sublimation: Solid directly to gas (e.g. solid carbon dioxide (dry ice) → CO₂ gas; iodine crystals → iodine vapour)
• Deposition: Gas directly to solid (e.g. water vapour → frost on a cold surface)

In every case, the substance is chemically the same before and after the change. States of Matter provides a full treatment of the properties of solids, liquids, and gases.

2. Dissolving

When a solute dissolves in a solvent, the solute particles (ions or molecules) separate and disperse among the solvent particles, but they remain chemically unchanged.

Example: dissolving sodium chloride in water

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

The Na⁺ and Cl⁻ ions were present in the solid lattice and are still present in the solution — they have simply been separated by water molecules. The original salt can be recovered by evaporating the water, a simple physical process. This makes dissolving a physical change.

Important nuance: Some sources classify dissolving as a physical change because no new substance is formed, while others note that the underlying process (breaking the ionic lattice) involves energy changes that border on the chemical. In IGCSE Chemistry, dissolving an ionic compound in water is generally taught as a physical change because the ions retain their chemical identity. However, dissolving a reactive metal in acid is a chemical change because a reaction occurs (e.g. magnesium dissolving in hydrochloric acid produces hydrogen gas and magnesium chloride — new substances).

3. Magnetising and Demagnetising Iron

When a piece of iron is magnetised, the magnetic domains (small regions within the metal where atomic magnetic fields are aligned) become oriented in the same direction. No new substance is formed — the iron is still iron, chemically unchanged. The process can be reversed by heating, hammering, or applying an alternating magnetic field. This is a physical change.

4. Cutting, Grinding, and Crushing

Breaking a solid into smaller pieces is a physical change. The surface area increases and the appearance changes, but the chemical composition remains identical. A lump of chalk (calcium carbonate, CaCO₃) ground into a powder is still calcium carbonate. Similarly, cutting paper, sawing wood, or crushing a rock are all physical changes.

5. Stretching, Bending, and Deforming

Elastic deformation of materials like rubber bands or metal springs is a physical change — the material returns to its original shape when the force is removed, and its chemical identity is unaltered. Plastic (permanent) deformation is also a physical change, though it is not reversible.

6. Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of particles. No chemical reaction occurs — the particles simply spread out and mix. When a drop of ink disperses through water, or when the smell of perfume travels across a room, these are physical changes. The ink molecules and the perfume molecules remain chemically the same; they have only become more spread out. Diffusion is a physical process that can be explained entirely by the kinetic particle model.

Physical Properties and Physical Changes

A physical property is any property of a substance that can be observed or measured without changing its chemical composition. When a physical change occurs, some physical properties may alter while others remain constant:

The constancy of chemical formula and fixed melting/boiling points is a powerful indicator that a change is physical rather than chemical.

Energy and Physical Changes

Physical changes involve energy transfers, but these energy changes are generally smaller than those associated with chemical reactions:

• Melting and boiling are endothermic physical changes (energy absorbed from surroundings).
• Freezing and condensation are exothermic physical changes (energy released to surroundings).

The energy absorbed when one kilogram of a substance changes state at constant temperature is called the specific latent heat:

• Specific latent heat of fusion (L_f): energy required to melt 1 kg of a solid at its melting point. For water, L_f ≈ 334 kJ/kg.
• Specific latent heat of vaporisation (L_v): energy required to boil 1 kg of a liquid at its boiling point. For water, L_v ≈ 2260 kJ/kg.

The fact that L_v is much larger than L_f for water reflects that boiling separates particles completely, overcoming nearly all intermolecular attractions, whereas melting only loosens the particles from fixed positions but leaves them in contact.

Temperature Remains Constant During a Change of State

When a pure substance is heated and undergoes a change of state, the temperature remains constant until the change is complete, even though heating continues. This is because the energy supplied is used to overcome intermolecular forces (latent heat) rather than to increase the kinetic energy of the particles. This is observable as a horizontal plateau on a heating curve and is a classic demonstration that a change of state is a physical process: the particles themselves are not changing, only being rearranged.

Reversibility: A General (but Not Universal) Rule

Physical changes are usually easily reversible, and this reversibility is a useful heuristic for classification:

Exceptions: Not all physical changes are easily reversible. Cutting a piece of paper into small pieces is a physical change, but gluing it back together does not restore the original sheet (and the gluing may itself be a chemical process). Similarly, the permanent deformation of a metal beyond its elastic limit is a physical change that cannot be undone without melting and recasting. Reversibility is therefore a useful guide but not a strict criterion — the definitive test is whether the chemical identity of the substance has changed.

Physical Change vs. Chemical Change: Worked Comparisons

The following comparisons illustrate the distinction in concrete terms. Each pair shows processes that might seem superficially similar but are fundamentally different types of change.

IGCSE Core vs. Supplement

Core (all students)

• Know that physical changes involve no new substance being formed.
• Recognise changes of state as physical changes.
• Distinguish between physical and chemical changes from given examples.
• Understand that physical changes are usually reversible.

Supplement (extended candidates)

• Apply the particle model to explain physical changes in terms of particle arrangement, motion, and energy.
• Explain why temperature remains constant during a change of state.
• Interpret heating and cooling curves, identifying the plateaus where physical changes (state changes) occur.
• Use the concept of reversibility as evidence for classification, while recognising its limitations.
• Relate the energy changes in physical processes to latent heat and intermolecular forces.

Related Pages

• Chemical Change — the complementary concept; changes where new substances are formed
• States of Matter — solids, liquids, and gases; the states involved in physical changes
• Melting — the physical change from solid to liquid
• Boiling — the physical change from liquid to gas at the boiling point
• Diffusion — the physical process of particles spreading through random motion
• Particle Theory — the kinetic model that explains physical changes at the particle level
• Bond Breaking — the endothermic process that distinguishes chemical changes from physical changes
• Mixtures — physical combinations of substances, separable by physical means
• Distillation — a physical separation technique based on differences in boiling points
• Filtration — a physical separation technique for insoluble solids from liquids
• Crystallisation — a physical method of recovering a dissolved solid by evaporation
• Pure Substances and Impurities — physical changes such as boiling point elevation can indicate the presence of impurities

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus, Section 6.1: Physical and Chemical Changes
• Harwood, R. & Lodge, I., Cambridge IGCSE Chemistry Coursebook, 5th Edition, Cambridge University Press, 2021, Chapter 2: The Nature of Matter, pp. 14—20
• Gallagher, R. & Ingram, P., Complete Chemistry for Cambridge IGCSE, 3rd Edition, Oxford University Press, 2016, Chapter 1: States of Matter, pp. 6—12
• Clegg, A. et al., Cambridge IGCSE Chemistry Study and Revision Guide, Hodder Education, 2017, Section 2: The Particulate Nature of Matter
• Earl, B. & Wilford, L.D.R., Cambridge IGCSE Chemistry, 3rd Edition, Hodder Education, 2021, Chapter 2: The Particulate Nature of Matter
• Cambridge Assessment International Education, “Teacher Guide: Chemistry 0620,” Section 6: Physical and Chemical Changes

Common Misconceptions