Evidence of a Chemical Reaction (WJEC GCSE Chemistry)

Evidence of a Chemical Reaction

• 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