gas and ion testing

• what four gases are commonly tested for?

  chlorine, oxygen, carbon dioxide and hydrogen
• chlorine test


  chlorine bleaches damp litmus paper, turning it white (if you use blue litmus paper it may turn red for a moment first though - that's because a solution of chlorine is acidic)
• oxygen test


  if you put a glowing split inside a test tube containing oxygen, the oxygen with relight the glowing splint
• carbon dioxide test


  Bubbling carbon dioxide through (or shaking carbon dioxide with) an aqueous solution of calcium hydroxide (lime water) causes the solution to turn cloudy
• hydrogen test


  if you hold a burning splint at the open end of a test tube containing hydrogen, you'll get a "squeaky pop" (the noise comes from the hydrogen burning quickly in the oxygen in the air to form water)
• anions


  negatively charged ions
• tests of anions often give...

  precipitates
• test for carbonate ions

  add dilute acid - if carbon dioxide gas is produced that creates bubbles and turns the limewater cloudy then carbonate ions are present
• test for sulphate ions

  add dilute hydrochloric acid followed by barium chloride solution - if white precipitate forms then sulphate ions are present
• test for halide ions

  add dilute nitric acid followed by silver nitrate solution
• halide ions


  the ions that are produced when atoms of chlorine and other halogens gain electrons
• halide ions test results

  - chloride: white precipitate of silver chloride

  - bromide: cream precipitate of silver bromide

  - iodide: yellow precipitate of silver iodide
• cations


  positively charged ions
• flame test for metal ions

  clean a nichrome or platiumum wire loop by rubbing with fine emery paper and then holding it in a blue flame from a Bunsen burner (the flame may change colour for a bit, but once it's blue again, the loop is clean) then dip the loop into the sample you want to test; record the colour of the flame
• lithium ions produce a... flame

  crimson
• sodium ions produce a... flame

  yellow
• potassium ions produce a... flame


  lilac
• calcium ions produce a... flame

  orange-red
• copper ions produce a... flame

  green
• flame test drawbacks


  - if the sample tested contains a mixture of metal ions, the flame colours of some ions may be hidden by the colours of others

  - the results for one sample can vary depending on the person who carries out the experiment because the colours observe may be subjective

  - low concentration the sample being tested may not produce a clear and definite colour
• metal hydroxide test


  add sodium hydroxide - if you get a coloured insoluble hydroxide you can tell which metal was in the compound
• calcium ions form a ... precipitate

  white
• copper (II) ions form a ... precipitate


  blue
• iron (II) ions form a ... precipitate


  green
• iron (III) ions form a ... precipitate


  brown
• aluminium ions form a ... precipitate

  white at first, but in excess NaOH to form a colourless solution
• magnesium ions form a ... precipitate

  white
• each metal ion gives a characteristic...

  line spectrum
• flame emission spectroscopy


  an instrumental method used to analyse metal ions in solutions
• flame emission spectroscopy process

  1) sample placed in flame

  2) as ions heat up, electrons become move to higher energy levels. when electrons drop back to original energy levels, they transfer energy as light

  3) light passes through spectroscope, which can detect different wavelengths of light to produce a line spectrum
• line spectrum example

  
• what does the combination of wavelengths emitted by an ion depend on?

  charge and its electron arrangement
• the intensity of the spectrum indicates...

  the concentration of the ion in solution
• line spectra can be used to...

  identify ions in solution and calculate their concentrations
• why is flame emission spectroscopy more useful than flame tests?

  because it can be used to identify different ions in mixtures and flame tests only work for substances that contain a single metal ion
• machines can analyse...


  unknown substances
• instrumental analysis


  tests that use machines (such as flame emission spectroscopy)
• advantages of using machines

  - very sensitive as they can detect even the tiniest amounts of substances

  - very fast and tests can be automated

  - very accurate