Testing for Unknowns

Flame Test: Lithium (Li)

Red

Flame Test: Sodium (Na)

Yellow Orange

Flame Test: Potassium (K)

Lilac/Pink

Flame Test: Rubidium (Rb)

Red/Violet

Flame Test: Caesium (Cs)

Blue/Violet

Flame Test: Calcium (Ca)

Orange Red

Flame Test: Strontium (Sr)

Brick Red

Flame Test: Barium (Ba)

Pale Green

Flame Test: Copper (Cu)

Blue/Green

Flame Test: Lead

White/Grey

Test for Halide Ions

Silver Nitrate Test

• Test done in solution. If starting from solid, sample must first be dissolved in pure water.

• Add dilute nitric acid to the samples. This is to remove/react away any interfering ions (carbonate or hydroxide ions) that may give a false positive result/precipitation.

• Add silver nitrate solution to the samples and observe the precipitate.

Silver Nitrate Test: Fluoride Ions

no precipitate

Silver Nitrate Test: Chloride Ions

white precipitate

Silver Nitrate Test: Bromide Ions

cream precipitate

Silver Nitrate Test: Iodide Ions

yellow precipitate

Test for Sulphate Ions

• Test must be done in solution. If starting from solid, it must first be dissolved in pure water.

• Acidify the sample with dilute hydrochloric acid.

• Add barium chloride (BaCl₂).

• If a sulphate is present, a white precipitate will form (insoluble barium sulphate).

Test for Carbonate Ions

• Add a dilute acid to the compound (usually dilute hydrochloric acid).

• If carbonate ions are present, they will react with the acid and produce carbon dioxide, which will be observed as bubbles/effervescence.

• To test the gas being produced, collect a sample and test with limewater. Carbon dioxide turns limewater cloudy.

Testing Alkenes

• Shake an alkene with bromine water, or bubble a gaseous alkene through bromine water.

• Alkenes decolourise bromine water, so the solution will become colourless (from brown/orange).

Testing Aldehydes

To test for aldehydes, you can use Fehling’s solution, Tollens’ Reagent or Benedict’s solution.

Fehling’s Solution

blue solution → brick red precipitate (copper (I) oxide)

Tollens’ Reagent

no visible change → silver mirror (coating of silver on surface of test tube, or black precipitate)

Benedict’s Reagent

blue solution

→ green → yellow (low conc. aldehyde)

→ orange → red ppt. (high conc. aldehyde)

Fehling’s Solution

blue solution → brick red precipitate (copper (I) oxide)

Tollens’ Reagent

no visible change → silver mirror (coating of silver on surface of test tube, or black precipitate)

Benedict’s Solution

blue solution

→ green → yellow (low conc. aldehyde)

→ orange → red ppt. (high conc. aldehyde)