Detection of Elements: Lassaigne's Test

Preparation of Sodium Fusion Extract (Lassaigne’s Extract)

Materials Required

Real Lab Procedure

  • Take a small piece of dry sodium  in a fusion tube.
  • Heat the tube slightly on a Bunsen burner so that the sodium melts to a shining globule.
  • Add a pinch of the organic compound.
  • Heat it slowly to start with so that the compound reacts with sodium metal.
  • Now heat the tube strongly till it becomes red hot.
  • Plunge the red hot tube into a china dish containing distilled water.
  • Crush the contents with a glass rod and heat to boiling point.
  • Stop heating and remove the insoluble matter by filtration.
  • The filtrate is called Lassaigne's Extract.

Detection of Nitrogen

Materials Required

Real Lab Procedure 

Experiment Observation Inference
To a small portion of the Lassaigne’s extract, add 2 ml of freshly prepared ferrous sulphate solution and heat. Now add 2-3 drops of ferric chloride solution and acidify with conc. hydrochloric acid . Appearance of a prussian blue coloration Presence of Nitrogen in the compound.

Detection of Sulphur

Materials Required

Real Lab Procedure 

Test Experiment Observation Inference
Sodium nitroprusside Test To a small portion of Lassaigne's extract, add a few drops of sodium nitroprusside solution. Appearance of a purple colouration. Presence of sulphur in the compound.
Lead acetate Test Acidify a small portion of Lassaigne's extract with a few drops of acetic acid and add a few drops of lead acetate solution. A black precipitate forms. Presence of sulphur in the compound.

Detection of Halogens

Materials Required

 Real Lab Procedure 

Test Experiment Observation Inference
Silver nitrate Test To a small portion of Lassaigne's extract add 1 ml of conc. nitric acid and boil for some time. Cool the contents and to it add silver nitrate solution. A white precipitate soluble in ammonium hydroxide forms. Presence of chlorine in the  compound.
Pale yellow precipitate sparingly soluble in ammonium hydroxide forms. Presence of bromine in  compound.
Yellow precipitate insoluble in ammonium hydroxide forms. Presence of iodine in  compound.
Carbon disulphide Test Acidify a small portion of Lassaigne's extracts with dil. HCl and add a few drops of carbon disulphide. Now add freshly prepared chlorine water and shake vigorously. Appearance of orange colour in the carbon disulphide layer. Presence of bromine in  compound.
Appearance of violet colour in the carbon disulphide layer. Presence of iodine in  compound.

Simulator Procedure (as performed through the Online Labs)

Detection of Nitrogen

  • You can select the unknown element from the ‘Select the unknown element’ drop down list.
  • Select the identification test from the ‘Select the identification test’ drop down list.
  • To pour ferrous sulphate (FeSO4) solution into the Lassaigne’s extract, drag the test tube towards it.
  • Click on the knob of the burner to turn it on.
  • Drag the test tube over the Bunsen burner to heat it.
  • Drag the test tube to place it back.
  • To pour conc. HCl into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To pour ferric chloride (FeCl3) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • Click on the ‘Reset’ button to redo the experiment.

Detection of Sulphur

  • You can select the unknown element from the ‘Select the unknown element’ drop down list.
  • You can select the identification test from the ‘Select the identification test’ drop down list.
  • You can select the identification method from the ‘Select the method’ drop down list.

a) Sodium nitroprusside test:

  • Lassaigne’s extract is taken in a test tube.
  • Drag the dropper towards the test tube to pour the sodium nitroprusside solution into it.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • Click on the ‘Reset’ button to redo the experiment.

b) Lead acetate test:

  • To pour acetic acid into the test tube containing Lassaigne’s extract, drag the test tube containing acetic acid towards it.
  • To pour lead acetate solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • You can redo the experiment by clicking on the ‘Reset’ button.

Detection of Chlorine

Silver nitrate test

  • You can select the unknown element from the ‘Select the unknown element’ drop down list.
  • You can select the identification test from the ‘Select the identification test’ drop down list.
  • To pour conc. HNO3 into the test tube containing Lassaigne’s extract, drag the test tube towards it.
  • Click on the knob of the burner to turn it on.
  • Drag the test tube over the Bunsen burner to heat it.
  • Once done, drag the test tube to place it back.
  • To pour silver nitrate (AgNO3) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To pour ammonium hydroxide (NH4OH) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • You can click on the ‘Reset’ button to redo the experiment.

Detection of Bromine and Iodine

  • You can select the unknown element from ‘Select the unknown element’ drop down list.
  • You can select the identification test from ‘Select the identification test’ drop down list.
  • You can select the identification method from ‘Select the method’ drop down list.

a) Silver nitrate test:

  • To pour conc. HNO3 into the test tube containing Lassaigne’s extract, drag the test tube towards it.
  • Click on the knob of the burner to turn it on.
  • Drag the test tube over the Bunsen burner to heat it.
  • Drag the test tube to place it back.
  • To pour silver nitrate (AgNO3) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To pour ammonium hydroxide (NH4OH) solution into the test tube containing Lassaigne’s extract, drag the dropper towards the test tube.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • You can click on the ‘Reset’ button to redo the experiment.

b) Carbon disulphide test:

  • To pour dil. HCl into the test tube containing Lassaigne’s extract , drag the dropper towards the test tube.
  • To pour Carbon disulphide into the test tube containing Lassaigne’s extract , drag the dropper towards the test tube.
  • To pour chlorine water into the test tube containing Lassaigne’s extract, drag the test tube towards it.
  • Drag the test tube to take it from the test tube stand and to shake it well.
  • To see the inference, click on the inference icon.
  • You can choose the correct answer from ‘Select the element’ drop down list.
  • You can click on the ‘Reset’ button to redo the experiment.

Note: Click on the ‘HELP’ button to see the instructions.

Precautions:

  • Sodium metal is highly reactive. When exposed to air, it reacts even with the moisture present in the atmosphere. It also reacts with the sweat of hands. Hence do not hold it with hands. Always use forceps while handling sodium.
  • Before using sodium, press it within the folds of the filter paper to remove kerosene oil.
  • Use dry ignition tubes for sodium fusion. Sodium reacts with water violently.
  • Put the unused sodium piece back in the bottle containing kerosene oil. Do not throw it into the sink.
  • Repeat the process of sodium fusion with three ignition tubes. This is to ensure that the fusion has taken place.
  • After immersing the red hot ignition tube in water, break it with a glass rod gently. Boil the contents for 2-3 minutes so as to extract the soluble sodium salts in water. 

 

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