Physical & Chemical Changes (Cambridge International IGCSE Science)

Spec point — C3.1.1

Physical & chemical changes

Physical changes

• Physical changes do not produce any new chemical substances

• These changes are often:

  • Easy to reverse
  • Relatively easy to separate

• Examples of physical changes include:

  • Changing state, e.g. melting / solid → liquid
  • Making a mixture from 2 or more substances
  • Dissolving a solute in a solvent

Chemical changes

• During chemical changes / reactions, new chemical substances are formed that have very different properties to the reactants

• Most chemical changes are difficult to reverse

• There may be signs that a new substance has formed, such as:

  • Colour changes
  • Temperature changes
  • Effervescence (fizzing)

Colour change

• One example of a reaction that shows a colour change is the halide ion test

  • A colour change from colourless to cream / white / yellow depending on the halide ion is seen
  • This also includes a change of state as a solid / precipitate is formed

Temperature change

• Chemical reactions that give a temperature change can be grouped into two categories:

  • Those that give out heat / thermal energy - exothermic
  • Those that take in heat / thermal energy - endothermic

Exothermic reactions

• Some examples of reactions that cause an increase in temperature are:

  • The reaction of calcium oxide with water to form calcium hydroxide is highly exothermic
  • The reaction of sodium with water is exothermic and effervesces

Endothermic reactions

• Reactions that cause a decrease in temperature are less commonly talked about than exothermic reactions

• Examples include:

  • Photosynthesis

    • Light energy is absorbed during the process of converting carbon dioxide and water into glucose and oxygen

  • When solid ammonium chloride is dissolved in water

    • Heat / thermal energy is absorbed from the surroundings, causing the temperature to decrease
    • This reaction is commonly used in cold packs

Effervescence

• Effervescence, or fizzing, is another sign of a chemical reaction

• Chemical reactions that cause effervescence often involve acids:

  • The reaction of alkali metals with water

    • The reaction of the alkali metals, such as sodium, with water releases hydrogen gas
    • Metal + water → metal hydroxide + hydrogen

• There are other signs of a chemical reaction including:

  • A change of physical state, e.g. liquid → solid
  • Light being produced
  • A smell being produced
  • A change in pH