Evidence of a Chemical Reaction (WJEC GCSE Chemistry: Combined Science)

• The progress of a chemical reaction can be monitored by:
  • Colour changes
  • Temperature changes
  • Effervescence (fizzing)

Colour change

• One example of a reaction that shows a colour change is the metal displacement reaction of silver and copper
  • Orange-brown copper metal is added to a colourless solution of silver nitrate 
  • As the reaction proceeds the copper displaces the silver from the solution
  • This causes two colour changes:
    1. The solid inside the beaker changes from orange-brown to silver
    2. The solution changes from colourless to blue

The metal displacement reaction of silver and copper

The reaction shows two colour changes; one in the colour of the solution and one in the colour of the solid

• Other reactions, from the syllabus, that show a clear colour change include:
  • The halide ion test 
    • From colourless to cream / white / yellow depending on the halide ion
    • This also includes a change of state as a solid / precipitate is formed
  • The specified practical of hydrochloric acid and sodium thiosulfate
    • From colourless to yellow
    • This also includes a change of state as a solid / precipitate is formed AND a pungent smell being released
  • The specified practical on the thermal stability of carbonates
    • For example, green copper(II) carbonate form black copper(II) oxide as it decomposes 

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 quicklime with water
    • 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