Soluble salts

In this topic, you'll learn:

What soluble salts are and how to name them based on their reactants.
Why we react acids with an excess of an insoluble solid to make them.
The exact step-by-step required practical method for making pure, dry salt crystals.

What is a soluble salt?

When an acid reacts with a base, a metal, or a carbonate, a salt is formed. If that salt dissolves easily in water, we call it a soluble salt.

Definition

Soluble salt

A salt that dissolves in water to form an aqueous solution. In chemical equations, it is given the state symbol (aq).

The name of the salt always comes from the two reactants you used:

The first part of the name comes from the metal in the solid (e.g., copper, magnesium, zinc).
The second part of the name comes from the acid:
- Hydrochloric acid produces chlorides.
- Sulfuric acid produces sulfates.
- Nitric acid produces nitrates.

For example, reacting copper oxide with sulfuric acid makes copper sulfate.

The core method: Acid + Insoluble solid

To make a pure sample of a soluble salt, we need to react an acid with a solid containing the metal we want. This solid is usually a metal oxide, metal hydroxide, or metal carbonate.

The biggest challenge when making a salt is ensuring the final product is pure. If you don't add enough solid, you will have leftover unreacted acid mixed in with your salt solution. To fix this, we always add the solid in excess.

Key Idea

The magic of excess

By adding the insoluble solid until no more reacts (an excess), you guarantee that all of the acid has been neutralised. Because the leftover solid is insoluble, it won't dissolve into your new salt solution, making it very easy to remove later.

Required Practical: Making pure, dry crystals

You need to know the step-by-step method for producing a pure, dry sample of a soluble salt. The classic example is making copper(II) sulfate crystals from copper(II) oxide (a black powder) and dilute sulfuric acid.

Here is the exact sequence of steps you must follow.

1. Heating the acid

Measure a set volume of dilute acid into a beaker. Gently heat the acid using a Bunsen burner until it is almost boiling. We heat the acid to speed up the rate of reaction.

Common Mistake

Safety first

Do not boil the acid vigorously, as dangerous, corrosive acid fumes could escape into the room.

2. Adding the solid

Remove the beaker from the heat. Use a spatula to add a small amount of the insoluble solid (e.g., copper(II) oxide) to the acid and stir with a glass rod. The black powder will react and disappear, and the solution will start turning blue as copper(II) sulfate forms.

Keep adding the solid, half a spatula at a time, until the powder stops disappearing and simply sinks to the bottom. This means the solid is in excess and all the acid has been neutralised.

3. Filtration

Next, we need to remove the leftover, unreacted solid to leave just the pure salt and water. We do this by pouring the mixture through filter paper into a conical flask.

Filtration setup

The excess solid is caught in the filter paper. The clear blue liquid that passes through is our pure filtrate — a solution of copper(II) sulfate and water.

4. Crystallisation

To get solid crystals, we need to remove the water from our salt solution. We pour the filtrate into an evaporating basin.

Instead of heating the basin directly over a roaring Bunsen flame, we place it on top of a beaker of boiling water to act as a water bath.

Crystallisation setup

Heat the setup until about half of the water has evaporated. This leaves a highly concentrated (saturated) solution. Turn off the heat and leave the basin in a warm place for a few days so the remaining water evaporates slowly, allowing large crystals to form.

Finally, gently pat the crystals dry using a paper towel or fresh filter paper.

Common Mistake

Evaporating to dryness

Never heat the evaporating basin until all the water is gone (evaporating to dryness). If you do this, the sudden intense heat can cause the crystals to thermally decompose (break down chemically) into a completely different powder, ruining your salt. Always leave some water to evaporate naturally.

Applying the method

In the exam, you might be given the name of a completely different salt and asked to describe how to make it. You have to work backwards to figure out which chemicals to use.

Example

Designing a preparation method

Describe how to make a pure, dry sample of magnesium chloride crystals from an insoluble metal oxide.

Identify the acid: The salt is a chloride, so the acid used must be hydrochloric acid.
Identify the solid: The salt contains magnesium, and the question specifies an insoluble metal oxide, so the solid must be magnesium oxide.
State the core reaction: Heating dilute hydrochloric acid and adding an excess of magnesium oxide.
State the purification steps: Filter the mixture to remove the excess magnesium oxide.
State the crystallisation steps: Heat the filtrate in a water bath to evaporate some of the water, leave it to cool and crystallise, then pat the crystals dry.

Tip

Reacting metals directly

You can sometimes use a pure metal instead of a metal oxide (e.g., using magnesium ribbon instead of magnesium oxide). However, this only works for metals that are moderately reactive (like magnesium, zinc, or iron). Very unreactive metals like copper will not react with dilute acids at all, so you must use copper oxide or copper carbonate instead.

Exam technique

In the exam

If an exam question asks for a "pure, dry sample", you will lose marks if you forget the final step: patting the crystals dry with filter paper.
Always explicitly state that you add the solid "in excess" or "until no more reacts". Examiners specifically look for this phrase.
Be careful with your equipment names. You filter into a conical flask (not a beaker), and you evaporate water in an evaporating basin (not a crucible or a bowl).
If a question asks you to write a balanced symbol equation for this required practical, remember the state symbols. For example:

\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \to \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}

Self review

Check yourself

Which acid would you use to make a nitrate salt?
Why is it important to add the insoluble solid until it is in excess?
Why do we use a water bath to heat the evaporating basin instead of a direct Bunsen flame?
What physical technique is used to separate the unreacted solid from the salt solution?