Identify physical and chemical changes, and describe the differences between them

Physical and Chemical Changes – IGCSE Chemistry 0620 (Section 6.1)

Learning Objectives

• Identify whether a given change is a physical change or a chemical change.
• Explain the key differences between the two types of change using the exact Cambridge syllabus terminology.
• Interpret experimental observations and decide which type of change has taken place.

1. Syllabus Terminology (Core 6.1)

• Physical change – a change in form, size, shape or state of matter in which the chemical identity (formula) of the substance is unchanged.
• Chemical change – a transformation that produces one or more new substances with different chemical formulas and properties.
• Reversible – the original material can be recovered by a simple physical process (e.g., melting ↔ freezing).
• Irreversible – the original material cannot be recovered under the conditions of the experiment (many chemical reactions are irreversible, although some are reversible under different conditions, see Section 6.3).
• Mass conserved – the total mass of the system remains the same for both physical and chemical changes; only the form of the matter may differ.

2. What Is a Physical Change?

A physical change alters the form, size, shape or state of matter without changing the chemical formula of the material. No new substances are produced.

• Changes of state: melting, freezing, evaporation, condensation, sublimation.
• Changes of shape or size: cutting, crushing, bending, grinding.
• Mixtures that do not react: sand + salt, sugar dissolved in water (NaCl stays NaCl when dissolved).

Physical changes are usually reversible by simple physical means and the total mass is conserved. A temperature change can accompany a physical change (e.g., melting of ice) but it does **not** indicate a new substance has formed.

3. What Is a Chemical Change?

A chemical change (chemical reaction) results in the formation of one or more new substances with different chemical formulas and properties. Bonds are broken and/or formed, and the original substances are transformed.

• Formation of a gas, solid precipitate, or colour change.
• Release or absorption of energy that is **intrinsic to the reaction** (heat, light, sound) – not caused merely by external heating or cooling.
• Often (but not always) irreversible under the experimental conditions; many reactions become reversible only when the appropriate conditions are applied (e.g., equilibrium reactions).

Typical Evidence of a Chemical Change

• Evolution of a gas (bubbles, odour, effervescence).
• Formation of a solid precipitate.
• Colour change.
• Temperature change **without** an external heat source.
• Emission of light, sound or other forms of energy.

4. Checklist for Classifying a Change

1. Does the substance retain its original chemical formula? (e.g., NaCl stays NaCl when dissolved) → Physical change.
2. Is a new substance produced with a different formula? → Chemical change.
3. Is any characteristic evidence of a chemical change observed (gas, precipitate, colour, temperature change without external heating, light/sound)? → Chemical change.
4. Can the original material be recovered by a simple physical method (filtering, evaporation, melting, etc.)? → Physical change.

5. Comparison Table

6. Worked‑Examples for Practice

• Ice melting to water – Physical
• Burning a candle – Chemical
• Dissolving table‑salt (NaCl) in water – Physical (NaCl remains NaCl)
• Mixing hydrochloric acid with sodium hydroxide – Chemical (HCl + NaOH → NaCl + H₂O)
• Crushing a glass bottle – Physical
• Rusting of iron – Chemical (4Fe + 3O₂ → 2Fe₂O₃)
• Adding silver nitrate to sodium chloride solution – Chemical (AgCl precipitate)
• Evaporating water from a salt solution – Physical (water changes state, salt recovered unchanged)
• Zinc metal placed in water – Physical (no reaction, metal unchanged)
• Zinc metal added to dilute HCl – Chemical (Zn + 2HCl → ZnCl₂ + H₂↑)

7. Summary of Key Points

1. Both physical and chemical changes conserve mass; only chemical changes alter the composition of the system.
2. A physical change does not change the chemical formula of the substance.
3. A chemical change produces one or more new substances with different formulas.
4. Look for the characteristic evidence of a chemical change: gas evolution, precipitate, colour change, temperature change without external heating, light or sound.
5. Physical changes are usually reversible by simple physical methods; chemical changes are often irreversible unless special conditions (e.g., equilibrium) are applied.