Making Soluble Salts from Acids Revision Notes | GCSE AQA Higher Chemistry

1. Why Make Salts from Acids?

You sprinkle salt on your chips, but that's just one type of salt. In chemistry, salts are everywhere — from the magnesium sulphate in bath salts to the calcium chloride that melts ice on roads.

Making pure salts from acids is like following a recipe, but with precise chemistry. You need the right ingredients, the right method, and you need to know when to stop. Get it wrong and you'll have impure crystals or no crystals at all.

2. The Four Routes to Soluble Salts

There are four main ways to make soluble salts from acids. Each uses a different type of solid reactant, but the principle is the same — the acid reacts with the solid to form a salt plus other products.

The reaction is: Metal + Acid → Salt + Hydrogen. For example, zinc reacts with hydrochloric acid to make zinc chloride: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

The reaction is: Metal Oxide + Acid → Salt + Water. Copper oxide with sulfuric acid gives copper sulphate: $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$

The reaction is: Metal Carbonate + Acid → Salt + Water + Carbon Dioxide. Calcium carbonate with nitric acid produces calcium nitrate: $CaCO_3 + 2HNO_3 \rightarrow Ca(NO_3)_2 + H_2O + CO_2$

3. The Standard Method - Getting Pure Salt Crystals

Here's the key insight: you can't measure exactly when all the acid has reacted. So instead, you add excess solid until no more dissolves. This guarantees all the acid is used up.

Worked example: Making Copper Sulfate Crystals

Prepare pure copper sulphate crystals from copper oxide and sulfuric acid.

1. Warm dilute sulfuric acid in a beaker using a Bunsen burner — Heating speeds up the reaction but don't boil the acid
2. Add copper oxide powder slowly, stirring after each addition — Keep adding until no more dissolves - this ensures all acid is neutralised
3. Use filter paper and funnel to remove unreacted copper oxide — The filtrate contains dissolved copper sulphate, the residue is excess solid
4. Heat the filtrate gently until crystals start to form around the edges — Don't evaporate to dryness or crystals will be too small
5. Leave to cool slowly, then filter off the crystals and wash with distilled water — Slow cooling gives larger, purer crystals

Answer: Blue copper sulphate crystals, CuSO₄·5H₂O

The key scientific idea happens in step 2. When you see no more solid dissolving, even with stirring, you know all the acid has reacted. The excess solid acts as a visual indicator that the reaction is complete.

4. Why This Method Works - The Science Behind It

Think about what happens at the molecular level. Acid molecules react with the solid surface. As the reaction continues, there are fewer and fewer acid molecules left to react.

When you add excess solid, you reach a point where there are no acid molecules left — only salt dissolved in water, plus unreacted solid. The solid stops dissolving because there's nothing left for it to react with.

Filtering removes the unreacted solid, leaving you with a pure salt solution. No contamination, no leftover acid — perfect for making crystals.

5. Choosing Your Reactants

Not every combination works. You need to match the right acid with the right solid to get the salt you want. The metal part of the salt comes from the solid, the non-metal part comes from the acid.

Hydrochloric acid makes chlorides. Sulfuric acid makes sulphates. Nitric acid makes nitrates. Want copper chloride? Use copper oxide (or copper carbonate) with hydrochloric acid.

There's one crucial limitation: metals below hydrogen in the reactivity series (like copper) won't react with acids. That's why we use copper oxide, not copper metal, to make copper salts.

6. Common Mistakes and Exam Technique

The biggest mistake is trying to add exactly the right amount of solid. Students calculate the exact mass needed, add it, then wonder why their salt is contaminated with leftover acid.

Another trap: forgetting to wash the crystals. Even after filtering, crystals can have impurities on their surface. A quick rinse with distilled water removes these without dissolving the crystals significantly.

In exam questions, look for command words carefully. 'Describe the method' wants the steps in order. 'Explain why excess solid is used' wants the scientific reasoning — that you can't tell when neutralisation is complete.