To prepare a salt, first ask whether it dissolves in water, and then ask what you are starting from. Solubility decides the method: an insoluble starting solid allows an excess and a filter, a soluble alkali needs a titration, and an insoluble salt is made by precipitation.
This lesson teaches the decision and the reasoning, not a lab recipe. Any real preparation follows your teacher’s instructions and the school’s safety rules.
It follows distinguishing neutralisation from dilution and belongs to acids, bases and salts.
What are the three principles?
| Principle | Use when | Key idea |
|---|---|---|
| Acid + insoluble solid (metal, metal oxide or metal carbonate) | The salt is soluble | Add solid until some remains, so all the acid is used. Filter off the excess, then crystallise. |
| Acid + alkali (titration) | The salt is soluble and the base is a soluble alkali | The exact amount must be found, because neither reactant can be filtered off. |
| Precipitation | The salt is insoluble | Mix two soluble solutions that hold the ions. Filter, wash and dry the solid. |
The salt takes its name from the acid (hydrochloric gives chlorides, sulfuric gives sulfates, nitric gives nitrates) and from the metal in the base.
Some metals are not suitable for the first method. Copper does not react with dilute acids, and very reactive metals react too violently to be used safely in a school preparation. A metal oxide or carbonate is often the better starting solid.
A decision sequence
- Is the target salt soluble in water? If not, use precipitation.
- If it is soluble, what is the base? Is it an insoluble solid (oxide, carbonate, or a suitable metal)?
- If yes, use the excess-solid method with filtration and crystallisation.
- If the base is a soluble alkali (for example NaOH or KOH), use titration.
Worked example
Choose a method for each salt and write the equation. The salts are chosen for teaching.
(a) Copper(II) sulfate. Copper(II) sulfate is soluble. Copper(II) oxide is an insoluble base, so use the excess-solid method with sulfuric acid.
CuO + H₂SO₄ → CuSO₄ + H₂O
Atom check: Cu 1 and 1, S 1 and 1, H 2 and 2, O 1 + 4 = 5 on the left and 4 + 1 = 5 on the right. Balanced.
(b) Sodium chloride. It is soluble and sodium hydroxide is a soluble alkali. Use titration.
HCl + NaOH → NaCl + H₂O
(c) Barium sulfate. It is insoluble. Use precipitation from two soluble compounds that contain its ions.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Atom check: Ba 1 and 1, Cl 2 and 2, Na 2 and 2, S 1 and 1, O 4 and 4. Balanced. Soluble barium compounds are toxic, so this is a classroom decision example only.
The mistake to watch for
Mistaken answer: “To make sodium chloride, add excess sodium hydroxide to hydrochloric acid, then filter off the extra.”
The student copied the excess-solid method without checking solubility.
Sodium hydroxide dissolves, so there is nothing to filter. An excess would stay in the solution and contaminate the salt. The correction is to ask “can I filter the extra reactant out?” If not, the exact amounts must be found by titration.
Check yourself
1. Name a suitable principle for potassium nitrate from potassium hydroxide and nitric acid.
Show answer
Potassium hydroxide is a soluble alkali, so use titration to find the exact volumes, then crystallise the solution.
2. A student plans to make copper(II) chloride by adding copper metal to dilute hydrochloric acid. Explain the problem and suggest a better starting solid.
Show answer
Copper does not react with dilute hydrochloric acid, so no salt forms. Use copper(II) oxide or copper(II) carbonate instead, for example CuO + 2HCl → CuCl₂ + H₂O.
3. Silver chloride is insoluble. Name the principle and complete the equation: AgNO₃(aq) + NaCl(aq) → ?
Show answer
Use precipitation. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Each atom appears once on both sides.
Where this leads next
The next lesson reads a titration from results, in explaining an endpoint from supplied observations. Use the equation balance reasoning trainer to practise the equations above, and the acids, bases and salts practice set for mixed questions.
If you are unsure why a method works rather than simply which one to pick, our teachers can walk through it in online one-to-one Chemistry tuition.