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» Element Test Notes For O-Level Chemistry
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Posted on: May 4th, 2011

The following are tests of elements needed for O-Level/GCSE:

Testing for Anions ( Negative Ions ) :

    Carbonate ( CO3 2- ), add dilute hydrochloric acid ,result is effervescence , carbon dioxide produced ( test with limewater )

 

    Chloride in solution ( Cl- ) , acidify solution with dilute nitric acid then add aqueous silver nitrate , result is white ppt. of silver chloride formed. ( ppt soluble in ammonia solution )

 

    Bromide in solution ( Br- ) acidify solution with dilute nitric acid then add aqueous silver nitrate , result is cream ppt. of silver bromide formed , ( only slightly soluble in ammonia solution )

 

    Iodide in solution ( I- ) acidify with dilute nitric acid then add aqueous silver nitrate , result is yellow ppt of silver iodide , insoluble in ammonia solution OR acidify with dilute nitric acid then add lead nitrate solution , result yellow ppt. of lead iodide formed.

 

    Sulphate in solution ( SO4 2- ) acidify solution with dilute hydrochloric acid then add barium chloride solution , OR acidify solution with dilute nitric acid then add barium nitrate solution , result for both is white ppt of barium sulphate formed.

 

    Nitrate in solution ( NO3 – ) make alkaline with sodium hydroxide solution then add aluminium foil ( or Devarda’s alloy ) and warm carefully , result is Ammonia gas is given off ( test with moist red litmus ).

Testing for Metal ions by burning :

    Sodium (Na+) burns giving yellow flame.

 

    Potassium (K+) burns giving lilac flame ( pale violet ).

 

    Calcium (Ca2+) burns giving brick-red flame.

 

    Lithium (Li+) burns giving crimson flame.

 

    Copper (Cu2+) burns giving a blue-green flame.

 

    Barium (Ba2+) apple-green (chartuese , close to lime a bit I think ).

Testing for Positive ions in solutions by adding sodium hydroxide:

    Ammonium (NH4 +) , ammonia produced on warming.

 

    Copper(II) (Cu 2+) , light blue gelatinous ppt. of copper hydroxide ( insoluble in excess sodium hydroxide ).

 

    Iron(II) (Fe 2+) , green gelatinous ppt. of iron(II) hydroxide ( insoluble in excess ).

 

    Iron(III) (Fe 3+) , rust-brown gelatinous ppt. of iron(III) hydroxide ( insoluble in excess ).

 

    Calcium (Ca 2+) , white ppt of calcium hydroxide ( insoluble in excess ).

 

    Magnesium (Mg 2+) white ppt. of magnesium hydroxide ( insoluble in excess ).

 

    Zinc (Zn 2+) , white ppt. of zinc hydroxide ,soluble in excess ,giving a colourless solution.

 

    Aluminium (Al3+) white ppt. of aluminium hydroxide , soluble in excess ,giving a colourless solution.

Testing for Positive ions in solutions by adding ammonia solution:

    Ammonium (NH4 +) , —– ( I think it means no reaction ).

 

    Copper(II) (Cu 2+) , light blue gelatinous ppt. dissolves in excess ammonia , giving a deep blue solution.

 

    Iron(II) (Fe 2+) , green gelatinous ppt. of iron(II) hydroxide ( insoluble in excess ).

 

    Iron(III) (Fe 3+) , rust-brown gelatinous ppt. of iron(III) hydroxide ( insoluble in excess ).

 

    Calcium (Ca 2+) , no ppt. (or only a very slight ppt.).

 

    Magnesium (Mg 2+) white ppt. (soluble in excess).

 

    Zinc (Zn 2+) , white ppt. (soluble in excess).

 

    Aluminium (Al3+) white ppt. (insoluble in excess.

Testing for gases:

    Ammonia (NH3) , colourless and pungent smell , hold damp red litmus paper ( or universal indicator paper ) in gas , Indicator paper turns blue..

 

    Carbon dioxide (CO2) , colourless and odourless , bubble gas through lime water ( calcium hydroxide solution ) , white ppt. of calcium carbonate formed ( solution turns milky ).

 

    Chlorine (Cl2) , pale green and choking smell , hold damp litmus paper ( or universal indicator paper ) in gas , indicator paper is bleached white ( blue litmus will turn red first ).

 

    Hydrogen (H2) , colourless and odourless , hold a lighted splint in gas , hydrogen burns with a squeaky ‘pop’.

 

    Oxygen (O2) , colourless and odourless , hold a ‘glowing’ wooden splint in gas . the splint relights.

 

    Sulphur Dioxide (SO2) , colourless and choking smell , dip a piece of filter paper in pottasium dichromate(vi) solution and hold it in gas .solution turns pale green ( SO2 is a reducing agent ).

Testing for purity of substance :

    Solids : Testing for melting point , if it is the correct value ( like the data says ) therefore it is pure , if not hence it is impure. e.g. Ice at 0 C°

 

    Liquids : Testing for boiling point , if it is the correct value ( like the data says ) therefore it is pure , if not hence it is impure. e.g. Water at 100 C°

Drying agents :

    Concentrated Sulphuric acid : used to dry all gases except ammonia.

 

    Anhydrous calcium chloride : used to dry all gases except ammonia which forms a complex with calcium chloride.

 

    Calcium oxide : used to dry ammonia and neutral gases.

Testing for water and acidity :

    pH Testing (1) :

 

    • Acidity and Alkalinity are found by Indicators ( Usually Universal Indicator and Litmus paper )

 

    • For a more accurate result , use pH Meter
    Testing for water :

 

    • By adding anhydrous Copper(II) Sulphate , which water will turn from white to blue

 

    • By adding anhydrous Cobalt(II) chloride , which water will turn from blue to pink ( or deep rose )

Organic Analysis : ( Yet still under construction )

    Testing for unsaturated hydrocarbon ( Like Alkenes )

 

    • Liquid : By adding Bromine water to it and shaking.
    • Gas : By passing the compound through Bromine water.

 

    If it is an unsaturated hydrocarbon , it will turn the orange-brown Bromine water colourless
    Food tests : ( Testing for nutrients)
    • Detecting starch in food ( and water of course ) : Adding Iodine solution which will give a blue-black colour , indicating the presence of starch
    • Detecting glucose in food ( and water of course ) : Add sugar-test ( Benedict’s solution ) food and water , and heat it , it will give an orange-red ppt. , indicating presence of glucose
    • Detecting protein in food ( and water of course ) :
      1. Add dilute sodium hydroxide2. Add drops of copper(II) sulphate solution
    • Violet colour indicates the presence of protein

 

    • Detecting fat in food :
      • 1. Smear food on paper
      • 2. Leave to dry
      • 3. Hold up to the light
    • Bright glossy area indicates the presence of fats

Test for Ethanoic acid and Ethanol : ( as in the book )

    the two substances provide a simple test reaction for each other. They both react with each other with the addition of a few drops of concentrated sulphuric acid , producing a sweet-smelling ester , the mixture is warmed gently and the fruity smell of the ester can be best detected by pouring the reaction mixture into a beaker of water. this spreads the ester and disperses the distinctive ‘pear-drop’ smell.

Chromatography :

    Individual monomers from proteins and carbohydrates can be separated by chromatography and detected using locating agents because they are colourless. if a Sample gives only one spot , therefore that sample is pure , use more than one solvent for accuracy and making sure that it is pure. For 2D ( 2 Dimensional ) Chromatography , user the paper and get the results on one dimension ( Y ) then you dry the paper from the solvent then you rotate it by 90° to get the other dimension ( X ) , and it will give us the 2D chromatography that we always see in the book

Reducing error in experiments :

    Using more accurate apparatus that has a reduced percentage of Systematic errors ( these cannot be avoided , the systematic errors in any tool in the world , No atomic computers that can do this accurately ¬¬ )
    Taking measurements carefully and multiple measurements , to achieve accuracy ( by finding the average )
    When drawing the best fit line in a graph , do not include points that seem to be out of the curve ,use a sharp and make the curve as smooth as possible , and remember to plot the independent Variable ( Temprature in a Time-of-reaction ( Y ) / Temprature ( X ) graph ) and NEVER forget to write the labels.
    SOLUBILITY OF SALTS
    all nitrates are soluble
    all potassium, sodium and ammonium salts are soluble
    all sulphates are soluble except barium sulphate, lead sulphate and calcium sulphate
    all chloride and iodide salts are soluble except silver chloride/iodide, lead chloride/iodide and mercury chloride/iodide
    all sulphides are insoluble except K, Na and ammonium salts
    all carbonates are insoluble except K,Na and NH4 salts
    all hydroxides are insoluble except K,Na and NH4 salts and magnesium and calcium hydroxide salts are partially soluble.

LEGEND : What to be tested for , Test , Alternative Test , Result , Colour and Smell States of Matter

(1) : pH measurements and other analyses are often carried out on soil samples. Soil is stirred with distilled water. The insoluble material settles out, but the solution above remains unclear because of fine suspended particles. These fine particles can be made to settle too by adding some barium sulphate to the suspension. The barium sulphate particles , themselves insoluble, physically drag the fine suspended material to the bottom of the container. The mixture can then be filtered and the solution tested in various ways for soluble ions , pH , etc.

 

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