Preparing a soluble salt

This practical experiment develops skills in measuring chemical substances, filtration, evaporation and crystallisation to produce hydrated copper(II) sulfate crystals.  

Chapter titles: 00:10 Introduction; 01:30 Base + acid reaction; 03:36 Filtration; 05:13 Evaporation; 06:27 Crystalisation.

Teacher notes

Full teacher notes are available in the supporting resources booklet, including ideas for how to use this video and the resources as part of your teaching. 

Notes on running the practical experiment

Technician notes including the equipment list and safety notes are provided, as well as integrated instructions for learners.

Making salts is a worthwhile activity that can help learners to explore the concept of neutralisation. It also deepens learners’ understanding and develops their practical skills when the reason for doing each practical step are made absolutely clear, eg why they are adding the solid reagent in excess.

Using this method, you should be able to make the copper(II) sulfate crystals from start to finish within a 60 minute lesson. 

The video follows the method recommended by CLEAPSS for this practical (PP027), which differs slightly from versions that learners may see in other resources or in examination questions.

The specific concentration of sulfuric acid recommended, should produce an almost saturated solution of copper(II) sulfate, hence minimal heating is needed to evaporate enough water.

This practical provides the opportunity to use real data to calculate the percentage yield of copper(II) sulfate crystals – see the follow-up worksheet provided. The worksheet includes the data required to do this calculation, however, if you are carrying out this practical in class, all you need to do is to remember to weigh the dry copper sulfate crystals at the end of the experiment.

You may consider running an additional practical activity Finding the formula of hydrated copper(II) sulfate. The follow-up worksheet offers structured questions to support this. If you are not carrying out the additional experiment, you can show this short video from the Just Add Water lecture series by Dr Peter Wothers to set the scene.

Key terms

Learners will need to have a clear understanding of the following scientific terminology:

acid – substance producing more hydrogen ions than hydroxide ions when dissolved in water.

base – any substance that reacts with an acid to form a salt and water only.

crystallisation – the process used to produce solid crystals from a concentrated salt solution.

evaporation – the process by which substance in the liquid state go into the gaseous state.

filtrate – the liquid that passes through the filter during filtration.

filtration – the process by which solid particles are removed from a liquid by a filter. The particles in the liquid state pass through the filter whilst those in the solid state do not.

insoluble – a substance is insoluble if it does not dissolve in a solvent.

limiting reactant – the reactant that is first to be completely used up, therefore limiting the amount of product formed and stopping the reaction.

neutralisation – a reaction between an acid and a base that forms a salt and water.

residue – solid stopped by the filter during filtration.

salt – a group of ionic compounds formed from the neutralisation reaction between an acid and a base.

soluble – a substance is soluble when it dissolves in a solvent, eg water.

Prior knowledge

Before launching into neutralisation concepts, it is essential students have a basic understanding of what happens when ionic substances are added to water.

Learners should be able to recall:

• Particles can be atoms, molecules or ions.
• An ion is a positively or negatively charged particle.
• An atom or a molecule can lose or gain electron(s) to form an ion.
• When an atom/molecule gains negatively charged electron(s), a negative ion is formed. When an atom/molecule loses negatively charged electron(s), a positive ion is formed.
• A solution is formed when a solute (salt) is dissolved in a solvent (water).
• Acids release hydrogen ions (H⁺) in solution, and alkalis hydroxide ions (OH^-).
• The pH scale is used to measure the acidity or alkalinity of a substance.
• A neutral substance has a pH of 7.

Learners should be confident writing word and symbol equations.

There are some questions included in the resources which ask learners to balance symbol equations. Depending on where making a soluble salt comes in your scheme of work your learners may not have come across this yet. Adapt the questions to make them relevant to the stage and level that you are at. Some of the challenge tasks require learners to use and apply their knowledge from other topics.

Common misconceptions

There is often confusion around the definitions of base and alkali. Alkalis such as the metal hydroxides are soluble in water and therefore bases. Insoluble bases such as copper oxide are not alkaline.

Some learners hold the notion that a base somehow halts the acidic behaviour of an acid. They need to appreciate neutralisation is the chemical reaction of an acid and a base. This can be addressed by considering the particle nature of solutions of acids and alkalis and the underlying reaction as hydrogen ions react with hydroxide ions.

Writing simplified ionic equations that highlight the ions taking part in the reaction can also help students to focus on the underlying reaction

Diagnostic questions are a great way to explore learners’ reasoning behind their answers. The Best Evidence Science Teaching resources provide a great starting point to explore their ideas about acids, alkalis and neutralisation. Students are given a question and multiple plausible explanations for an observation. They then choose and justify which explanation they agree with. You can also provide students with thought experiments and ask them to provide their own explanations. Find the BEST resources, topic four. Read more about diagnostic questioning.