Five sodium salts sit before you, can you use your laboratory knowledge to identify them?
Sodium is a common element, and compounds like sodium chloride are common in your home.
Materials per group
1 g of each of the following:
- Sodium metabisulfite (NaHSO3) (harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas)
- Sodium sulfate (Na2SO4.10H2O) (no significant hazard)
- Sodium peroxodisulfate (Na2S2O8) (Skin/respiratory sensitiser, skin/eye/respiratory irritant, harmful if swallowed)
- Sodium sulfite (Na2SO3.7H2O) (harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas)
- Sodium thiosulfate (Na2S2O3.5H2O) (No significant hazard)
- Hydrochloric acid, 2 mol dm–3 (No significant hazard)
- Acidified potassium dichromate(VI) solution (1% solution in 1 M sulphuric acid) (Carcinogen, mutagen, reproductive toxin, harmful if inhaled, skin/eye irritant)
- Potassium iodide solution, 0.5 mol dm–3 (No significant hazard)
- Iron(III) chloride solution, 0.5 mol dm–3 (Corrosive to skin/eyes, harmful if swallowed, hazardous to the aquatic environment)
- Iodine in aqueous potassium iodide, 0.2 mol dm–3 (No significant hazard but will stain skin and clothing)
- Silver nitrate solution, 0.1 mol dm–3 (Skin/eye irritant)
- Sodium hydroxide solution, 2 mol dm–3 (corrosive to skin and eyes)
- 3% hydrogen peroxide solution (No significant hazard)
- deionised water
- litmus paper
- strips of filter paper
- protective gloves
Equipment per group
- Test tubes x 10
- Test tube rack
- Test tube holders
- Bunsen burner
- Wood splints
- Wash bottle of deionised water
- Dropping pipettes
- Safety goggles
Health, safety and technical notes
- Read our standard health and safety guidance here.
- Wear eye protection.
- Wear clothing protection if desired.
- This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
- Some of the tests produce sulphur dioxide (corrosive, toxic - asthmatics are especially sensitive, so it should only be handled in a fume cupboard).
- Warming with dilute HCl produces chlorine (toxic if inhaled) this too should be carried out in a fume cupboard or on a very small scale. Do not inhale the gas.
- Silver solutions should be kept for recycling/ disposal. The other test tube reactions can be washed to waste with plenty of water.
- For more information on sodium, see CLEAPSS Hazcard HC095a.
- For more information on hydrochloric acid, see CLEAPSS Hazcard HC047a
- For more information on potassium dichromate, see CLEAPSS Hazcard HC078c
- For more information on potassium iodide, see CLEAPSS Hazcard HC047b
- For more information on iron(III) chloride, see CLEAPSS Hazcard HC055b
- For more information on silver nitrate, see CLEAPSS Hazcard HC087
- For more information on sodium hydroxide, see CLEAPSS Hazcard HC091a
Students should be encouraged to predict the reactions of the salts and then draw up a systematic plan for the experiment. The following scheme is set out in the Independent learning project for advanced chemistry. It identifies all of the sulfur oxo-anions except the metabisulfite.
|1, Warm with dilute hydrochloric acid||No reaction in cold. Bubbles of gas on warming. Choking smell. K2Cr2O7 paper turned green (SO2 produced)||No visible reaction.||Solution turned slightly cloudy. Denser when warm. Choking smell. K2Cr2O7 turned green (SO2 produced).||No reactions in cold. Bubbles of gas on warming. Choking smell. Litmus paper bleached (Cl2 produced).|
|2, Add silver nitrate solution||Initial white ppt. dissolved on shaking. With more AgNO3 a dense white ppt remained.||No reaction at first, then a faint white ppt. appeared.||Initial white ppt. dissolved on shaking. With more AgNO3 the ppt. remained and turned yellow, brown and black.||Blackish ppt. formed slowly.|
|3, Add iodine solution (in aqueous potassium iodide)||The brown colour was immediately discharged. (Iodine reduced.)||No visible reaction.||The brown colour was immediately discharged. (Iodine reduced.)||The brown colour become darker. (Iodide oxidised.)|
|4, Add potassium iodide solution||No visible reaction.||No visible reaction.||No visible reaction.||A dark brown solution was formed. (Iodide oxidised.)|
|5, Add iron (III) chloride solution and dilute acid. Warm and add sodium hydroxide solution.||A dark red-brown solution was formed, which became almost colourless when hot. Addition of alkali gave a green ppt. (Fe3+ reduced.)||A yellow solution was formed, which darkened a little on warming. Addition of alkali gave a red-brown ppt. (Fe3+ not reduced.)||A dark purple solution was formed which cleared when hot and then became cloudy. Addition of alkali gave a green ppt. (Fe3+ reduced.)||A yellow solution was formed, which darkened a little on warming. Addition of alkali gave a redbrown ppt. (Fe3+ not reduced.)|
|6, Heat a small portion of the solid salt.||Crystals turned white and gave off a steamy vapour which condensed on the upper tube (H2O). White residue turned yellow on strong heating.||A colourless liquid was rapidly formed, which boiled to give off a steamy vapour (H2O) and a white residue. No further reaction||A colourless liquid was rapidly formed, which boiled to give off a steamy vapour (H2O). The yellowish residue turned brown and gave a black viscous liquid.||The solid melted to a colourless liquid. Bubbles of gas relit a glowing splint (O).|
A procedure for distinguishing between sulfites and metabisulfites is given in Vogel as follows: Aqueous sulfite shows an alkaline reaction with litmus paper, because of hydrolysis:
SO32–(aq) + H2O(l) → HSO3–(aq) + OH–(aq)
While aqueous metabisulfite is neutral. On adding a neutral solution of dilute hydrogen peroxide to aqueous sulfite, sulfate ions are formed and the solution becomes neutral:
SO32–(aq) + H2O2(aq) → SO42–(aq) + H2O(l)
With aqueous metabisulfite hydrogen peroxide yields hydrogen ions with the same test:
HSO3–(aq) + H2O2(aq) → SO42–(aq) + H+(aq) + H2O(l)
and the solution shows a definite acid reaction.
This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.
The resources were originally published in the book In Search of More Solutions.