6.1 Physical and chemical changes

2026 Syllabus Objectives

  • Core: Identify physical and chemical changes, and describe the differences between them

What is a Chemical Reaction? 🔬

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Chemistry deals with how substances react with each other. Chemical reactions range from the very simple through to the interconnecting reactions that keep our bodies alive. But what distinguishes a chemical reaction from a simple physical change?

Key Definition: A chemical reaction (change) is a change in which a new substance is formed.

Key Definition: A physical change is a change in the physical state of a substance or the physical nature of a situation that does not involve a change in the chemical substance(s) present.

Physical Changes ❄️

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Understanding Physical Changes

Physical changes involve alterations in the physical state or physical form of a substance, but the chemical identity remains unchanged. For example:

  • Ice, snow, and water may look different, but they are all made of water molecules (H2O\mathrm{H_2O})
  • They are different physical forms of the same substance existing under different conditions of temperature and pressure
  • One form can change into another if those conditions change

Another example: Dissolving sugar in ethanol or water is a physical change. It produces a solution, but the substances can easily be separated again by distillation.

Key Characteristics of Physical Changes 📌

1. No New Substances Formed

  • The substances present remain chemically the same
  • No new chemical substances are created
  • Only the physical form or state changes

2. Usually Easy to Reverse

  • Physical changes are often easy to reverse
  • The original substances can be recovered
  • Any mixtures produced are usually easy to separate

3. Energy Changes Involved

  • Physical changes can involve heat energy
  • Changing physical state involves heat being taken in or given out

Energy in Physical Changes ⚡

Physical changes involve energy transfers:

Endothermic Physical Changes (absorbing heat):

  • Melting a solid takes in heat
  • The heat taken in overcomes the forces holding the structure together
  • Evaporation is also endothermic

Exothermic Physical Changes (releasing heat):

  • Condensation gives out heat
  • Freezing gives out heat
  • These are the reverse of melting and evaporation

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