What Is a Salt?

Introduction

In everyday language, "salt" means the white crystals on the dinner table — sodium chloride. To a chemist, sodium chloride is just one member of a huge family. Copper sulfate, calcium carbonate, potassium nitrate, ammonium chloride and magnesium sulfate are all salts too. They have different colours, solubilities and uses, but they share one feature: each can be thought of as an acid in which the hydrogen has been replaced. This page sets out that definition and what it tells you about any salt.

Core explanation

The definition. A salt is the compound formed when the hydrogen ions of an acid are replaced by metal ions or ammonium ions. For example:

- In hydrochloric acid, HCl, replacing H with sodium gives sodium chloride, NaCl. - In sulfuric acid, H₂SO₄, replacing both H atoms with copper gives copper(II) sulfate, CuSO₄. - In nitric acid, HNO₃, replacing H with ammonium gives ammonium nitrate, NH₄NO₃.

Two parts from two parents. Every salt has a positive ion (cation) and a negative ion (anion).

- The anion comes from the acid : chloride (Cl⁻) from hydrochloric acid, sulfate (SO₄²⁻) from sulfuric acid, nitrate (NO₃⁻) from nitric acid. - The cation comes from the metal, base or carbonate that reacted with the acid: Na⁺, K⁺, Cu²⁺, Mg²⁺, Ca²⁺, Zn²⁺, or the ammonium ion NH₄⁺.

So "copper sulfate" tells you at once that it could be made from a copper compound and sulfuric acid.

How salts form. The reactions from earlier pages all produce salts:

- acid + metal → salt + hydrogen - acid + metal oxide or hydroxide → salt + water - acid + carbonate → salt + water + carbon dioxide - acid + ammonia → ammonium salt

In each case the H⁺ ions of the acid are removed (as H₂, H₂O or via carbonic acid) and their place is taken by the positive ions from the other reactant.

Salts are ionic. Solid salts are made of ions arranged in a giant ionic lattice , held together by strong attractions between opposite charges. This is why salts are usually crystalline solids with high melting points, and why they conduct electricity when molten or dissolved, when their ions are free to move. Overall, a salt has no charge: the positive and negative charges balance.

Salts are not always neutral or harmless. Some salt solutions are slightly acidic or alkaline, and some salts are toxic. The word "salt" describes how the compound is built, not whether it is safe to eat.

Partial replacement. An acid with two replaceable hydrogens, such as sulfuric acid, can have just one replaced, giving salts such as sodium hydrogensulfate, NaHSO₄. These are still salts because at least some hydrogen has been replaced.

Step-by-step reasoning

To identify the parent acid and base of a salt:

1. Split the name into its two parts, for example "potassium" and "nitrate". 2. The negative ion names the acid: nitrate → nitric acid. 3. The positive ion names the metal or base: potassium → potassium hydroxide, oxide or carbonate.

Visual explanation

Imagine a model of HCl as a hydrogen ball attached to a chlorine ball. Pull off the hydrogen and snap a sodium ball into its place: you now have NaCl. In the solid, picture a chessboard-like cube of alternating Na⁺ and Cl⁻ ions extending in all directions.

Real-world analogy

Think of an acid as a coat hanger with a hydrogen "coat" on it. Making a salt is swapping that coat for a metal one. The hanger — the acid's anion — stays the same, which is why every salt from sulfuric acid is a sulfate.

Real-world example

Plaster of Paris and gypsum are forms of calcium sulfate; chalk and limestone are calcium carbonate; Epsom salts are magnesium sulfate; many fertilisers are ammonium or potassium salts. Salts are everywhere, from building materials to medicines.

Why?

Why are salts electrically neutral? The number of positive and negative ions in a salt is always such that their charges cancel. For example, calcium chloride has two Cl⁻ ions for each Ca²⁺ ion, giving CaCl₂.

Common misconception

"Salt means sodium chloride." Sodium chloride is common salt, but chemically any ionic compound formed from an acid by replacing its hydrogen is a salt, including coloured and insoluble compounds.

Worked example

Question: Name the salt formed when zinc oxide reacts with sulfuric acid, and identify which part comes from the acid.

Reasoning: The metal is zinc, giving Zn²⁺; the acid is sulfuric acid, giving SO₄²⁻.

Answer: Zinc sulfate, ZnSO₄; the sulfate ion comes from the acid.

Quick check

1. Which acid would be used to make potassium chloride? Answer: Hydrochloric acid.

Exam focus

Learn the definition of a salt, including "hydrogen replaced by a metal or ammonium ion". Be able to name the acid and base needed for a given salt, and state the general equations that produce salts.

Advanced insight

The Brønsted–Lowry view defines neutralisation as proton transfer, so a salt is the ionic product left when the acid's protons have been transferred to a base. Salts need not come from an acid in practice — sodium chloride forms directly from sodium and chlorine — but the definition still classifies it by the acid it corresponds to.

Summary

A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal ion or ammonium ion. The anion comes from the acid and the cation from the metal or base. Salts form in reactions of acids with metals, oxides, hydroxides, carbonates and ammonia. They form ionic lattices and are electrically neutral overall.

Practice questions

1. Define the term salt. Answer: An ionic compound formed when the hydrogen of an acid is replaced by a metal ion or an ammonium ion. 2. Name the acid and a suitable base used to make magnesium nitrate. Answer: Nitric acid and magnesium oxide (or magnesium hydroxide or carbonate). 3. Which part of copper(II) chloride comes from the acid? Answer: The chloride ions, Cl⁻, from hydrochloric acid. 4. Explain why solid salts have high melting points. Answer: They are giant ionic lattices with strong electrostatic attractions between many oppositely charged ions, which need a lot of energy to overcome.