Which sodium salt is which?

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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.

Equipment

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
spatula
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

Commentary

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.

Test	Na₂SO₃	Na₂SO₄	Na₂S₂O₃	Na₂S₂O₈
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:

SO₃^2–(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:

SO₃^2–(aq) + H₂O₂(aq) → SO₄^2–(aq) + H₂O(l)

With aqueous metabisulfite hydrogen peroxide yields hydrogen ions with the same test:

HSO^3–(aq) + H₂O₂(aq) → SO₄^2–(aq) + H⁺(aq) + H₂O(l)

and the solution shows a definite acid reaction.

Notes

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.