Experiment: Preparation of a soluble salt

Preparing a soluble salt is a standard Form 4 practical that brings together neutralisation, filtration and crystallisation. The method used here, reacting an acid with an excess of an insoluble reactant, works when the salt is soluble but the base, metal or carbonate is not. This guide sets out the method for a salt such as copper(II) sulfate, the observations to expect, and the science process skills the practical papers reward.

Aim

To prepare pure, dry crystals of a soluble salt, for example copper(II) sulfate, by reacting dilute sulfuric acid with an excess of copper(II) oxide.

Apparatus and materials

• Beaker and glass stirring rod
• Bunsen burner, tripod and wire gauze
• Filter funnel and filter paper
• Evaporating dish
• Spatula
• Wash bottle of distilled water
• Dilute sulfuric acid
• Copper(II) oxide powder (the insoluble base, used in excess)

Procedure

1. Measure a fixed volume of dilute sulfuric acid into a beaker and warm it gently with the Bunsen burner; do not boil it.
2. Add a spatula of copper(II) oxide to the warm acid and stir with the glass rod.
3. Continue adding copper(II) oxide a little at a time, stirring after each addition, until some solid remains undissolved even after stirring. This shows the acid has all reacted and the base is in excess.
4. Filter the mixture to remove the excess copper(II) oxide, collecting the blue filtrate in a clean beaker.
5. Pour the filtrate into an evaporating dish and heat it gently to evaporate some of the water, until a hot saturated solution forms (test by dipping in a glass rod; crystals form on the cool rod).
6. Stop heating and leave the solution to cool slowly so that crystals form.
7. Filter off the crystals, wash them with a little cold distilled water, and dry them by pressing gently between sheets of filter paper.

Expected observations

The black copper(II) oxide dissolves in the warm acid to give a blue solution, and the blue colour deepens as more salt forms. When the acid is used up, further copper(II) oxide no longer dissolves and black solid stays on the bottom, the sign that the base is in excess. On filtering, the black excess stays on the filter paper and a clear blue filtrate passes through. On cooling the concentrated filtrate, blue crystals of the hydrated salt form.

Inference and conclusion

An acid reacts with a metal oxide (an insoluble base) to give a salt and water, this is neutralisation. Adding the base in excess ensures that all the acid reacts, so the filtrate contains only the salt and water and no leftover acid. Filtration removes the unreacted excess base, and crystallisation from the saturated solution gives pure crystals of the hydrated salt. The conclusion is that a soluble salt can be prepared pure by the excess-insoluble-reactant method, provided the excess is removed before crystallising.

Science process skills (Paper 3 style)

Operational definition of “excess”. Define the point of excess operationally: the base is in excess when added solid no longer dissolves on stirring and remains at the bottom of the beaker.

Identifying variables. If the investigation compared how much salt formed with different volumes of acid, the manipulated variable is the volume of acid, the responding variable is the mass of dry crystals obtained, and the controlled variables are the concentration of the acid and the identity of the base.

Classifying steps. Classify each step by its purpose: reaction (adding base to acid), separation (filtering off the excess), and purification by crystallisation (evaporating, cooling and drying).

Communicating. Write a conclusion that names the salt formed and states, with a reason, why the crystals are pure, the excess base was filtered off and only the salt crystallises from the filtrate.

Safety precautions

• Wear safety goggles, because dilute sulfuric acid is corrosive and hot solution can spit.
• Warm the acid gently and do not let it boil, so that hot acid does not spray out of the beaker.
• Point the mouth of the beaker and evaporating dish away from people while heating, so nobody is hit by spitting liquid.
• Let hot apparatus cool before handling, or hold it with tongs, to avoid burns.

Common errors

• Not adding the base in excess. If too little base is added, unreacted acid stays in the filtrate and the crystals are impure; keep adding until solid no longer dissolves.
• Filtering before the reaction is complete. Filter only once excess solid is clearly present, or acid will remain in the filtrate.
• Evaporating to dryness. Heating the filtrate until it is completely dry drives off the water of crystallisation and can decompose the salt; stop at a hot saturated solution and let it cool.
• Washing the crystals with too much water. The salt is soluble, so washing with a lot of water dissolves the crystals; use only a little cold distilled water.
• Drying with strong heat. Drying the crystals with a flame can change the hydrated salt; press them between filter paper instead.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can justify every step, why excess, why filter, why crystallise rather than evaporate to dryness, because Paper 2 and Paper 3 both reward reasons, not just steps. Salt preparation is a recurring theme across SPM Chemistry, and this excess-base route is the model answer for soluble salts of metals like copper, so securing it here also prepares the contrast with the precipitation method used for insoluble salts, and with the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).