Experimental design

  • Apparatus: stop-watch (time), thermometer (temperature), balance (mass), and burette, volumetric pipette, measuring cylinder and gas syringe (volume). A burette, pipette or gas syringe is more accurate than a measuring cylinder.
  • Compare methods by their advantages and disadvantages, such as accuracy, speed, cost and how much gas or heat is lost.
  • A solvent dissolves a solute. A solution is a mixture of one or more solutes dissolved in a solvent. A saturated solution contains the maximum concentration of solute at a specified temperature.
  • A residue is what remains after evaporation, distillation, filtration or a similar process. A filtrate is the liquid or solution that has passed through a filter.

Acid–base titrations

  • A titration finds the volume of one solution needed to react exactly with a fixed volume of another.
  • Method: use a volumetric pipette to put the alkali in a flask, add a few drops of a suitable indicator, then add acid from the burette, swirling, until the indicator changes colour. This is the end-point.
  • Methyl orange is red in acid and yellow in alkali. Thymolphthalein is colourless in acid and blue in alkali.
  • Repeat until results agree closely, then use the mean titre.

Chromatography

  • Paper chromatography separates mixtures of soluble coloured substances using a suitable solvent.
  • Draw the start line in pencil, because ink would dissolve and run. Keep the start line above the solvent level.
  • Substances more soluble in the solvent travel further up the paper.
  • A pure substance gives one spot and a mixture gives more than one. An unknown is identified by matching its spot with known substances run alongside it.
  • ExtendedColourless substances are made visible using a locating agent, then separated in the same way.
  • ExtendedRf = distance travelled by the substance ÷ distance travelled by the solvent. Rf has no units and is between 0 and 1.

Separation and purification

  • Using a suitable solvent dissolves one component of a mixture so that it can be separated from the others.
  • Filtration separates an insoluble solid from a liquid.
  • Crystallisation obtains a soluble solid from its solution: heat to evaporate some solvent, cool so crystals form, then filter and dry.
  • Simple distillation separates a solvent from a solution, such as pure water from salt water.
  • Fractional distillation separates liquids with different boiling points, such as ethanol from water, using a fractionating column.
  • Choose the technique from the properties of the substances: solubility, state and boiling point.
  • A pure substance has a sharp, fixed melting point and boiling point. Impurities lower the melting point and raise the boiling point.

Tests for anions

  • Carbonate: add dilute acid, then test the gas. Carbon dioxide turns limewater milky.
  • Chloride, bromide, iodide: acidify with dilute nitric acid, then add aqueous silver nitrate. Chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate.
  • Nitrate: add aqueous sodium hydroxide, then aluminium foil, and warm carefully. Ammonia gas is produced.
  • Sulfate: acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate forms.
  • Sulfite: add acidified aqueous potassium manganate(VII). It changes from purple to colourless.

Tests for cations

  • With aqueous sodium hydroxide: Al³⁺ white precipitate, soluble in excess to a colourless solution. Ca²⁺ white precipitate, insoluble in excess. Cr³⁺ green precipitate, soluble in excess. Cu²⁺ light blue precipitate, insoluble in excess.
  • Also with sodium hydroxide: Fe²⁺ green precipitate, insoluble in excess, turning brown near the surface on standing. Fe³⁺ red-brown precipitate, insoluble in excess. Zn²⁺ white precipitate, soluble in excess to a colourless solution.
  • With aqueous ammonia: Al³⁺ white precipitate, insoluble in excess. Ca²⁺ no precipitate or a very slight white one. Cr³⁺ green precipitate, insoluble in excess. Cu²⁺ light blue precipitate, soluble in excess to a dark blue solution.
  • Also with ammonia: Fe²⁺ green precipitate, turning brown near the surface. Fe³⁺ red-brown precipitate, insoluble in excess. Zn²⁺ white precipitate, soluble in excess to a colourless solution.
  • Ammonium, NH₄⁺: warm with aqueous sodium hydroxide and ammonia gas is produced.
  • Flame tests: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.

Tests for gases

  • Ammonia turns damp red litmus paper blue. Carbon dioxide turns limewater milky.
  • Chlorine bleaches damp litmus paper. Hydrogen gives a 'pop' with a lighted splint.
  • Oxygen relights a glowing splint. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless.

Exam tips

  • Describe a test fully: reagent, observation, and conclusion. Say 'white precipitate', not just 'white'.
  • For precipitate tests, say whether it dissolves in excess reagent. That is often the difference between two ions.
  • In chromatography questions, measure from the start line to the middle of each spot and from the start line to the solvent front (Extended Rf).
  • State the order of reagents. Nitric acid is added before silver nitrate to remove carbonates that would give a false result.
  • Justify a separation method by naming the property used, such as solubility or boiling point.

Common mistakes

  • Using ink for the chromatography start line. Use pencil.
  • Calling a solution 'clear' when you mean 'colourless'. Clear means see-through and can be coloured.
  • Mixing up the ion tests, such as using silver nitrate for sulfate or barium nitrate for halides.
  • Writing that filtration separates a dissolved solid. Salt in water passes through the filter paper.
  • Using the word 'precipitate' for a gas. A precipitate is a solid formed in a solution.