IGCSE Chemistry Edexcel 2.8 Chemical Tests

2.8 Chemical Tests

2.8.1 Tests for Gases

  • Many reactions produce gases that need to be identified.
  • Hydrogen Test:
    • A lighted splint is used.
    • Hydrogen burns with a squeaky pop.
  • Oxygen Test:
    • A glowing splint is used.
    • Oxygen relights a glowing splint.

2.8.2 Flame Tests

  • Metal ions produce characteristic colors when heated strongly in a flame.
  • The flame test is used to identify metal ions based on the flame color they produce.
  • Procedure:
    1. Dip a loop of unreactive metal wire (nichrome or platinum) in concentrated acid, then hold it in the blue flame of a Bunsen burner until there is no color change. This cleans the wire loop and avoids contamination.
    2. Dip the loop into the solid sample and place it at the edge of the blue Bunsen flame.
  • Avoid letting the wire get so hot that it glows red, as this can be confused with a flame color.
  • The sample needs to be heated strongly, so a blue Bunsen burner flame should be used.

2.8.3 Tests for Cations

  • Metal cations in aqueous solution can be identified by the color of the precipitate they form on the addition of sodium hydroxide (NaOH) and ammonia.
  • If only a small amount of NaOH is used, the metal hydroxide precipitates.
  • If a precipitate forms from NaOH, the hydroxide is insoluble in water.
  • Even a slight cloudiness or color change can indicate a positive test result.

2.8.4 Tests for Anions

  • There will be a test for the metal cation part of the molecule and another test for the anion part.

2.8.5 Tests for Water

  • Water can be identified using a chemical test and/or a physical test.
  • Chemical Test:
    • Anhydrous copper(II) sulfate turns from white to blue on the addition of water.
    • Equation: CuSO4(s) + 5H2O(l) \rightarrow CuSO4.5H2O(s)
  • Physical Test:
    • Check the boiling point of the water sample to see if a sample of water is pure
    • Pure water boils at exactly 100^{\circ}C
    • Any impurities present will usually raise the boiling point.
  • It is extremely hazardous to taste anything in the lab, and water is not the only colorless liquid with no taste or smell.