Products of Neutralisation: Salt and Water

Introduction

When an acid is neutralised by a base, the acidic and basic properties disappear, but the particles do not vanish. They rearrange into new substances. Every neutralisation between an acid and an alkali produces the same pair of products: a salt and water . Table salt is only one member of a huge family of salts, many of which are used as fertilisers, food additives and medicines. This page introduces the general word equation and explains what chemists mean by "salt".

Core explanation

The general word equation. For an acid reacting with a metal hydroxide or metal oxide:

acid + base → salt + water

For example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

The water forms when hydrogen ions from the acid join hydroxide ions (or oxide ions) from the base. The salt is made from what is left over: the metal from the base and the negative ion from the acid.

What is a salt? In everyday language, "salt" means sodium chloride. To a chemist, a salt is any ionic compound formed when the hydrogen of an acid is replaced by a metal ion (or an ammonium ion). The first part of a salt's name comes from the metal in the base; the second part comes from the acid. Some examples:

Salt Formula Made from --- --- --- Sodium chloride NaCl hydrochloric acid + sodium hydroxide Potassium nitrate KNO₃ nitric acid + potassium hydroxide Copper(II) sulfate CuSO₄ sulfuric acid + copper(II) oxide Calcium chloride CaCl₂ hydrochloric acid + calcium hydroxide Ammonium nitrate NH₄NO₃ nitric acid + ammonia

Where does the salt go? Most salts formed in school neutralisations are soluble, so the salt stays dissolved in the water. The neutral solution looks just like water, but it contains salt ions. If some of the water is evaporated by gentle heating and the solution is left to cool, the salt comes out as crystals . Evaporating a sample of neutralised hydrochloric acid and sodium hydroxide leaves white crystals of sodium chloride.

Why the solution must be neutral. If the salt is to be pure, there should be no left-over acid or alkali in the solution. That is why the neutral point is found carefully, often by first using an indicator and then repeating the neutralisation without it so the salt is not coloured by the dye.

A special case: ammonia. Ammonia solution neutralises acids to form ammonium salts. The overall equation is written simply as acid + ammonia → ammonium salt, with no separate water shown, because the ammonia takes the hydrogen ion directly. Carbonates, by contrast, give three products: salt, water and carbon dioxide.

Step-by-step reasoning

To predict the products of a neutralisation:

1. Identify the acid and the base. 2. Write "salt + water" as the products. 3. Take the metal from the base as the first word of the salt name. 4. Take the ending from the acid as the second word. 5. Check whether the base is a carbonate or ammonia, which change the products.

Visual explanation

Picture a set of building blocks. The acid is a hydrogen block joined to a chloride block; the base is a sodium block joined to a hydroxide block. In the reaction, the hydrogen block snaps onto the hydroxide block to make water. The sodium and chloride blocks are left and join to make the salt.

Real-world analogy

Neutralisation is like two couples swapping dance partners. Hydrogen was dancing with chloride and sodium was dancing with hydroxide. After the swap, hydrogen dances with hydroxide (making water) and sodium dances with chloride (making salt). Nobody leaves the dance; they just change partners.

Real-world example

Farmers use potassium nitrate and ammonium nitrate as fertilisers. Both are salts made industrially by neutralisation: nitric acid with a potassium compound, or nitric acid with ammonia. The salts supply plants with the potassium and nitrogen they need for growth.

Why?

Why is water always one of the products when an acid reacts with an alkali? Every acid supplies H⁺ ions and every alkali supplies OH⁻ ions. These two ions combine to make H₂O whatever the acid and alkali are. The other ions change from reaction to reaction, which is why the salt differs but the water does not.

Common misconception

"The salt made in neutralisation is always table salt." Only hydrochloric acid and sodium hydroxide make sodium chloride. Sulfuric acid makes sulfates, nitric acid makes nitrates, and the metal depends on the base used, so there are many different salts.

Worked example

Question: Write the word equation for the reaction between nitric acid and potassium hydroxide, and name the salt formed.

Reasoning: Acid + base → salt + water. The metal comes from the base (potassium) and the ending comes from nitric acid (nitrate).

Answer: nitric acid + potassium hydroxide → potassium nitrate + water. The salt is potassium nitrate.

Quick check

1. What are the two products when an acid reacts with an alkali? Answer: A salt and water.

Exam focus

Learn the word equation acid + base → salt + water and be able to apply it to specific examples. Define a salt as the compound formed when the hydrogen of an acid is replaced by a metal. Remember that carbonates also produce carbon dioxide.

Advanced insight

A salt is held together by ionic bonds between positive metal ions and negative ions from the acid. In solution these ions are free to move, which is why a salt solution conducts electricity even though it is neutral. When the water evaporates, the ions pack together in a regular lattice, giving each salt its own characteristic crystal shape.

Summary

Every neutralisation of an acid by an alkali or metal oxide produces a salt and water: acid + base → salt + water. Water forms from H⁺ and OH⁻ ions. A salt is an ionic compound in which the hydrogen of an acid has been replaced by a metal or ammonium ion. Soluble salts stay in solution and can be obtained as crystals by evaporating some of the water.

Practice questions

1. Complete the word equation: hydrochloric acid + sodium hydroxide → + . Answer: sodium chloride + water. 2. Define a salt. Answer: A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal ion or an ammonium ion. 3. How can solid salt be obtained from a neutral salt solution? Answer: By evaporating some of the water with gentle heating and leaving the solution to cool so that crystals form. 4. Name the salt formed when sulfuric acid reacts with copper(II) oxide. Answer: Copper(II) sulfate. 5. Which extra product is formed when an acid reacts with a carbonate? Answer: Carbon dioxide.