Practical Procedure: Producing a Pure, Dry Sample of a Salt

What this practical is trying to make

A salt can be made by reacting an acid with a suitable metal oxide or carbonate. For example, dilute sulfuric acid reacts with copper(II) oxide to make copper(II) sulfate solution and water:

$\mathrm{CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O}$

The copper(II) sulfate is dissolved at first. To obtain a pure, dry sample, you need to use up the acid, remove any unreacted solid, then separate the salt from the water as crystals and dry them. OCR gives production of a pure, dry salt as a suitable activity for Practical Activity Group 7; the particular salt and method can vary.

Reacting and monitoring the mixture

One workable route uses a beaker of measured dilute sulfuric acid, copper(II) oxide, a spatula and a stirring rod. Wearing eye protection, warm the acid gently using suitable heating apparatus. Add the black oxide in small portions and stir after each addition. Small additions keep the mixing controlled and let you observe the oxide reacting as the blue copper(II) sulfate solution forms.

Continue until a little oxide remains even after stirring: this shows that the insoluble oxide is in excess, so the acid has been used up. Indicator paper can be used to check that the solution is approximately neutral before separation. Record what you observe, including the colour change and the pH result. If an insoluble carbonate is used instead, bubbles of carbon dioxide are another sign of reaction; add it carefully because it can fizz.

From solution to dry crystals

Let the hot mixture cool enough to handle safely. Pour it through filter paper in a funnel. Unreacted oxide stays on the paper, while the dissolved copper(II) sulfate passes through as the clear blue filtrate. Pour that filtrate into an evaporating basin and heat it gently to remove some water. Stop before it boils dry: the aim is a concentrated solution from which crystals can form, not a dry crust in a hot basin.

Leave the concentrated solution to cool and crystallise. Collect the crystals, then allow them to dry, for example on filter paper or a paper towel. The final sample is dry rather than simply a wet mass of crystals and remaining solution.

Measuring, heating and working safely

Accurate records make the method understandable and repeatable. Record the volume of acid in cm³ using suitable volume-measuring apparatus, and use a balance if you need the mass of a dry product. A thermometer records temperature in °C; a stopwatch records time in seconds or minutes. Gas volume can be measured in cm³ with suitable apparatus when a gas-producing reaction is being investigated. OCR includes mass, time, temperature and volumes of liquids and gases among the practical techniques to be covered, but one salt preparation need not use every measurement.

Use the heating device appropriate to the step and the activity: practical work may involve a Bunsen burner and a water bath or electric heater. Heat gently, keep flammable materials clear, and take care with hot glassware and solutions. Handle acids, solids and any gas-producing mixture under controlled conditions rather than adding large amounts of reagent at once. Record the apparatus and procedure in a clear scientific diagram where helpful.

A soluble alkali needs a different approach from an insoluble oxide: excess alkali cannot simply be filtered away. An acid–alkali titration can first establish the volumes that neutralise each other using an appropriate indicator. Repeating with those volumes without indicator gives a salt solution that is not coloured by the indicator; it can then be concentrated and crystallised.