How to Prepare Salts

Copper doesn't react with dilute sulfuric acid, so how does anyone make copper(II) sulfate, the brilliant blue crystals you may have grown yourself in a school lab? The trick is to start from a different, insoluble copper compound instead. This guide covers how salts get their names, the solubility rules that decide which route to take, the four reactions that all produce a salt, and the exact excess-solid method used to turn a reaction mixture into pure, dry crystals.

What Are Salts, and Where Do Their Names Come From?

Every acid contains hydrogen. When that hydrogen is replaced by a metal (or by an ammonium ion, NH₄⁺), the compound formed is a salt. Crucially, a salt's name always comes from its parent acid, never from how it was made.

Sulfuric acid is the "parent acid" of all the sulfates -- even copper(II) sulfate, which can never be made directly from copper metal and dilute sulfuric acid (they simply don't react). There is always another route to the same salt, as you'll see below.

The Solubility Rules You Need

Before planning how to make a salt, you need to know whether it dissolves. Sodium, potassium and ammonium compounds are all soluble, with no exceptions -- this single fact underpins most of the rest of the table below.

Acids and Bases as Proton Transfer

A more general definition than "a source of H⁺ ions" covers reactions beyond aqueous solution: the Brønsted-Lowry theory defines an acid as a proton (hydrogen ion) donor, and a base as a proton acceptor. When hydrochloric acid dissolves in water, HCl donates a proton to a water molecule: H₂O(l) + HCl(aq) → H₃O⁺(aq) + Cl^-(aq) -- so HCl is the acid, and water is the base. The H₃O⁺ ion formed here is called the hydroxonium ion, usually written more simply as H⁺(aq). This framework is also why ammonia counts as a base even though it contains no hydroxide ions of its own: it accepts a proton from an acid, NH₃(aq) + HCl(aq) → NH₄Cl(aq).

Four Reactions, Four Routes to a Salt

Whichever route you use, every single one of these reactions is a neutralisation reaction, and every one produces a salt.

The Excess Solid Method, Step by Step

For any soluble salt except a sodium, potassium or ammonium one, the standard preparation is the excess solid method, using a moderately reactive metal, metal oxide, metal hydroxide or carbonate.

• Measure a fixed volume of dilute acid into a beaker, warming gently if using a metal oxide.

• Add the insoluble solid a spatula at a time, stirring well after each addition, until no more will react -- some solid is deliberately left over, undissolved.

• This leftover solid proves that every trace of acid has been neutralised -- this is exactly why the solid must be in excess, not just "a lot".

• Filter the mixture to remove the excess unreacted solid; the filtrate that passes through is a pure solution of the salt.

For copper(II) sulfate specifically: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l), adding black copper(II) oxide to hot dilute sulfuric acid until a visible excess remains, then filtering off that excess to leave a clear blue filtrate.

Filtration, Evaporation and Crystallisation Explained

Three separate techniques finish the job. Filtration removes the excess insoluble solid (the residue) from the salt solution (the filtrate). Evaporation then boils off some of the water to concentrate the solution close to saturation -- tested by dipping a glass rod in the solution: if crystals form on the rod as it cools, the solution is ready. Crystallisation is the slow-cooling stage: as the hot saturated solution cools, the salt becomes less soluble and solid crystals grow out of it.

It would seem easier to just boil the whole solution to dryness -- but this is a genuine exam trap. Evaporating copper(II) sulfate solution fully to dryness gives a white powder of anhydrous copper(II) sulfate, not blue crystals, because it drives off the water of crystallisation chemically bound into the crystal: CuSO₄(aq) + 5H₂O(l) → CuSO₄·5H₂O(s). A salt containing water of crystallisation is hydrated; one without it is anhydrous.

Choosing the Right Reactants for a Target Salt

Given a target salt's name, work backwards: the second part of the name (chloride, nitrate, sulfate...) tells you which acid to use, and the metal part tells you which insoluble compound of that metal to start from -- its oxide, hydroxide, carbonate, or (for a moderately reactive metal) the metal itself. One exception is worth memorising: copper never reacts with dilute acids (it sits below hydrogen in the reactivity series), so copper metal is never a valid choice -- always start from copper(II) oxide or copper(II) carbonate instead.

Key Takeaways

• A salt's name always comes from its parent acid: hydrochloric → chlorides, nitric → nitrates, sulfuric → sulfates.

• Sodium, potassium and ammonium compounds are always soluble; learn the exceptions among chlorides, sulfates, carbonates and hydroxides.

• An acid is a proton donor; a base is a proton acceptor (Brønsted-Lowry theory).

• Metal + acid, oxide + acid, hydroxide + acid and carbonate + acid all give a salt -- every one is a neutralisation reaction.

• Excess solid method: react acid with excess insoluble solid, filter, evaporate to concentrate, then cool slowly to crystallise.

Want more practice? Download the Science A Plus Edu Acids, Bases and Salt Preparations Revision Notebook, with 25 pages of notes, diagrams and revision activities, and read our companion guide to Acids and Alkalis for the pH and indicator chemistry that sits behind every neutralisation reaction here.