Topic 9

• Ion


  A charged atom, molecule or particle
• Tests for specific ions must be unique so that the test gives an easily observed result which is specific to the ion that is present
• Carrying out a flame test to test for ions

  1. Clean a nichrome wire using hydrochloric acid

  2. Turn the Bunsen burner onto the blue flame

  3. Dip the wire in a solution of the substance being tested

  4. Place the wire in the flame and record the colour
• Flame test on lithium ions

  Crimson flame
• Flame test on sodium ions

  Yellow flame
• Flame test on potassium ions

  Lilac flame
• Flame test on calcium ions

  Orange-red flame
• Flame test on copper ions

  Blue-green flame
• The wire must be cleaned before carrying out a flame test to remove any unwanted ions that might obscure the colour of the flame
• A flame test cannot be used when a compound contains a mixture of metal ions as the flame colours of the ions will blend together so the individual flame colours won't be seen, meaning the ions can't be identified
• Precipitate


  An insoluble solid suspended in a liquid
• Testing for aqueous metal ions without using the flame test

  Add sodium hydroxide solution to the metal ions and observe the colour of the precipitate
• Precipitate formed when sodium hydroxide reacts with aluminium ions


  White (initially), with excess NaOH the precipitate re-dissolves to form a colourless solution
• When sodium hydroxide reacts with ammonium ions, a pungent smelling gas (NH3) is produced which will turn damp red litmus paper blue
• Distinguishing between solutions containing aluminium ions and calcium ions

  Add excess sodium hydroxide. Both will form white precipitates initially. The precipitate formed from aluminium ions will re-dissolve to form a colourless solution.
• Testing for carbonate ions

  Add a few drops of HCl to the sample in a test tube, connect this test tube to a test tube of limewater. If carbonate ions are present, they will react with the acid to produce carbon dioxide which will turn the limewater cloudy when it is bubbled through.
• The chemical equation for the reaction between HCl and Na2CO3 is: Na2CO3 + 2HCl → CO2 + 2NaCl + H2O
• Testing for sulfate ions

  Add HCl to remove any CO3^2- ions, then add a couple of drops of barium chloride. If sulfate ions are present a white precipitate of barium sulfate will form.
• The chemical equation for the reaction between BaCl2 and MgSO4 is: BaCl2 + MgSO4 → BaSO4 + MgCl2
• Carrying out a test for halide ions

  Add a few drops of nitric acid to react with any carbonate ions, then add a couple of drops of silver nitrate and observe the colour of the precipitate.
• Precipitate formed when silver nitrate is added to a chloride solution


  White precipitate of silver chloride
• Precipitate formed when silver nitrate is added to a bromide solution


  Cream precipitate of silver bromide
• Precipitate formed when silver nitrate is added to an iodide solution


  Yellow precipitate of silver iodide
• Instrumental methods of analysis

  Methods of analysis that use machinery such as mass spectrometry and gas chromatography
• What colour precipitate is formed when silver nitrate is added to a bromide solution?

  Cream precipitate of silver bromide
• What colour precipitate is formed when silver nitrate is added to an iodide solution?

  Yellow precipitate of silver iodide
• What is meant by the phrase instrumental methods of analysis?

  Methods of analysis that use machinery such as mass spectrometry and gas chromatography
• Instrumental methods used for the analysis of elements or compounds

  • Mass spectrometry
  • Gas chromatography
• Benefits of using instrumental methods for analysis

  • Accurate
  • Sensitive
  • Quick to carry out
• What is a flame photometer?

  A device used in inorganic analysis. It can be used to identify or determine the concentration of metal ions
• How can a flame photometry be used to identify metal ions?

  An emission spectrum is produced by the flame photometer. Each metal ion produces a unique spectrum so comparing the unknown spectrum to reference spectra can identify the ion
• How can a flame photometry be used to determine the concentration of metal ions?

  Take readings using a flame photometer of the metal ions at different concentrations. Plot a calibration curve. Take a reading of the unknown sample and compare to the curve
• What is the major advantage of flame photometry compared to simple flame testing?

  Flame photometry can be used to analyse a mixture of ions whereas flame tests can only be used to identify one ion at a time
• Name the first four alkanes and write their molecular formulae

  • Methane - CH4
  • Ethane - C2H6
  • Propane - C3H8
  • Butane - C4H10
• What type of bond are formed between carbon and hydrogen atoms in alkanes?

  Covalent bonds
• What is the displayed formula of ethane?
• Why are alkanes saturated hydrocarbons?

  Saturated - all C-C bonds are single bonds. Hydrocarbon - only contain carbon and hydrogen atoms
• Name the first four alkenes and write their molecular formulae

  • Ethene - C2H4
  • Propene - C3H6
  • Butene - C4H8
  • Pentene - C5H10
• Why are alkenes unsaturated hydrocarbons?

  They are compounds made up of only carbon and hydrogen atoms and they contain double carbon bonds, C=C
• What is a functional group?

  The group of atoms responsible for the main chemical properties of a compound