Organic & Inorganic Practical Techniques (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about organic and inorganic practical techniques.

These four practicals are split into two halves: inorganic techniques for identifying substances by test-tube reactions, and organic techniques for isolating and identifying organic products.

This video covers two halves. The inorganic half covers the reactions of Group 2 elements, and test-tube reactions used to identify anions and cations. The organic half covers distillation, used to isolate a product from a reaction, and test-tube reactions used to identify organic functional groups.

We'll take the inorganic half first — Group 2 reactions, then identifying anions and cations — and then move to the organic half: distillation, then testing for functional groups.

Group 2 metals react with oxygen to form the metal oxide, MO — strontium and barium also form the peroxide, MO2. They react with water to form the metal hydroxide and hydrogen gas, though beryllium does not react with water. With dilute hydrochloric acid or dilute sulfuric acid, they form the metal salt and hydrogen gas. Strontium sulfate and barium sulfate are insoluble, which distinguishes them from the more soluble sulfates further up the group.

Group 2 ions can be identified by the pattern of precipitates formed with sodium hydroxide, which varies down the group. Ammonium ions are identified by warming with sodium hydroxide and holding damp red litmus paper near the mouth of the tube — the paper turns blue in the presence of ammonia gas. Halide ions are identified by adding silver nitrate solution, which gives a white, cream, or pale yellow precipitate for chloride, bromide, and iodide respectively.

When testing an unknown sample for these ions, the order matters: carbonate is tested first, then sulfate, then halide, since carbonate would otherwise interfere with both later tests.

That's the inorganic half covered. Now for the organic half — distillation, then identifying organic functional groups.

Distillation is used to isolate a product from a reaction mixture. An electric heating mantle heats the mixture, since the temperature can be controlled and the chemicals involved are often flammable. As the desired product forms, it evaporates and passes through a condenser, where it cools and condenses back into a liquid — the distillate — which is collected in a receiving vessel. Only the distillate collected within around 2 degrees Celsius of the product's boiling point should be kept. Once collected, the product is purified: washing with sodium hydrogen carbonate solution removes acidic impurities, and solid calcium chloride is then added to remove any remaining water.

Alkenes are identified using bromine water, which decolourises from orange to colourless when shaken with an unsaturated compound. Primary and secondary alcohols are identified using acidified potassium dichromate, which changes from orange to green — tertiary alcohols give no colour change. Aldehydes are identified using Fehling's solution, which forms a brick-red precipitate, or Tollens' reagent, which forms a silver mirror; both give a negative result with ketones. Carboxylic acids are identified by their reaction with a carbonate solution, which produces effervescence as carbon dioxide gas is released.

In the inorganic half: always add dilute nitric acid first when testing an unknown sample, to clear any carbonates before testing for sulfate or halide ions. In the organic half: write observations, not deductions — write 'effervescence', not 'carbon dioxide is given off', and for a negative test write 'no visible change', not 'nothing happens'.

The inorganic half covered the reactions of Group 2 elements and the test-tube reactions used to identify anions and cations. The organic half covered distillation, used to isolate a product from a reaction, and the test-tube reactions used to identify organic functional groups. Together, these four practicals are the core practical skills examined across A Level Chemistry.