Physical and Chemical Changes

Definition and Key Differences

Physical change

• A change that affects only the physical state, shape, or appearance of a substance, without changing its chemical composition.
• No new substances are formed.
• The change is usually reversible by physical means (e.g., heating, cooling, dissolving, evaporation).
• Examples:
  • Melting of ice to water (H₂O(s) → H₂O(l))
  • Evaporation of ethanol
  • Dissolving sugar in water

Characteristics of physical changes:

• Involve changes in state (solid ↔ liquid ↔ gas)
• No chemical bonds are broken or formed between atoms/molecules
• No change in the chemical formula of the substance
• Energy changes are relatively small (mainly changes in kinetic energy of particles)

Chemical change

• A change in which new substances with different chemical properties are formed.
• Atoms are rearranged, and chemical bonds are broken and formed.
• Usually irreversible by simple physical means.
• Examples:
  • Combustion of magnesium in air: 2Mg + O₂ → 2MgO
  • Rusting of iron: Fe + O₂ + H₂O → Fe₂O₃·xH₂O
  • Decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂

Characteristics of chemical changes:

• New substances are produced
• Often involve noticeable energy changes (heat, light, sound)
• Change in colour, smell, or taste may occur
• Formation of a precipitate or evolution of a gas may be observed
• Usually difficult or impossible to reverse without another chemical reaction

Energy Considerations in Physical vs. Chemical Changes

• Physical changes:
  • Only involve changes in particle arrangement and movement
  • Energy changes are small (e.g., melting ice requires latent heat)
  • No change in potential energy due to bond rearrangements
• Chemical changes:
  • Involve breaking and forming of chemical bonds
  • Can be exothermic (release heat) or endothermic (absorb heat)
  • Enthalpy change (ΔH) is more significant compared to physical changes

Particle Theory Explanation

Physical change:

• Particles are the same before and after the change
• Only the distance between particles and their movement changes
• Example: In melting, particles gain enough energy to overcome some intermolecular forces and slide past each other, but the particles themselves remain the same

Chemical change:

• Particles (atoms or ions) rearrange to form new combinations
• Bonds are broken in reactants and new bonds form in products
• Example: In combustion of methane:
  CH₄ + 2O₂ → CO₂ + 2H₂O
  Carbon-hydrogen and oxygen-oxygen bonds are broken, carbon-oxygen and hydrogen-oxygen bonds are formed

Indicators of a Chemical Change

A chemical change has likely occurred if one or more of the following are observed:

1. Change in colour (e.g., copper turning green when forming copper carbonate)
2. Gas formation (bubbles, fizzing, odour change)
3. Formation of a precipitate (solid appears in a liquid mixture)
4. Change in temperature (heat given out or absorbed)
5. Emission of light or sound (e.g., fireworks)
6. Permanent change in properties (e.g., hardness, taste, smell)

Examples Table: Physical vs Chemical Changes

Key Points to Remember for Exams

• Physical change: No change in chemical identity; easily reversible; small energy change.
• Chemical change: New substances form; usually irreversible; bonds broken and formed; significant energy change.
• Test tip: Always check if the chemical formula of the substance changes — if yes, it’s a chemical change.