Neutralisation in chemistry is when an acid and a base react to form salt and water, which typically have a neutral pH.
Neutralisation reactions can be classified into several types depending on the respective strengths of the reactants. They may also be grouped according to the Arrhenius and Bronsted-Lowry definitions of acids and bases.
As we explain in this article, acid-base reactions have important applications in industries such as environmental protection, agriculture, and medicine. Read on to find out more about neutralisation reactions and how they work.
In this post:
What is a neutralisation reaction?
The neutralisation reactions between acids and bases almost always produce salts and water as byproducts. However, not all reactions produce solutions with a neutral pH level. This depends on two factors – the type of reactants and their respective concentrations.
There are four types of neutralisation reactions, depending on the strength of the reactants. Two of these reactions may produce either slightly basic or slightly acidic salts.
- Reactions between strong acids and strong bases
- Reactions between strong acids and weak bases
- Reactions between weak acids and strong bases
- Reactions between weak acids and weak bases
Neutralisation reactions have several applications across various industries. In the laboratory, for instance, a neutralisation reaction is commonly performed in a titration experiment to analyse the unknown concentration of either a base or an acid.
A simple stoichiometry calculation allows you to determine the concentration once the neutralisation point is reached. The neutralisation point itself is identified by using either an indicator or a pH metre.
Where are neutralisation reactions used in industry?
Neutralisation reactions are used to treat wastewater, with many water treatment plants using them as a way to control the pH of the water and reduce potential environmental damage. Some of the common chemicals that are used to neutralise wastewater include calcium carbonate, calcium oxide, magnesium hydroxide, and sodium bicarbonate. 
In the medical sector, neutralisation is commonly used to treat hyperacidity. Antacid medications, which contain sodium bicarbonate, are designed to alleviate digestive discomfort by neutralising excess stomach acid.
Neutralisation reactions have a role in the agricultural industry, too. As most crops don’t grow well in highly acidic conditions, the soil pH must be controlled. Slaked lime or limestone is therefore added to acidic soil to neutralise it. 
What’s more, many types of fertilisers are by-products of acid-base neutralisation reactions. Ammonium sulphate, for example, is a salt produced by the neutralisation reaction between ammonia and sulphuric acid.
Is neutralisation a type of chemical reaction?
Yes, neutralisation is a type of chemical reaction that occurs when a substance with a low pH (acid) and a substance with a high pH (base) are combined.
These reactions are generally exothermic because bonds are broken and heat energy is released. Neutralisation reactions are an example of double replacement reactions, which means the ionic components of the acids and bases switch partners.
What is the process of neutralisation?
Technically speaking, neutralisation is the process of making the pH of a solution neutral, or pH 7. This involves stoichiometrically balanced chemical reactions. Ideally, the acids and bases must react completely to form salt and water. 
The neutralisation process can only happen consistently in a controlled laboratory setting, such as in a titration experiment. Most of the ‘neutralisation’ reactions that occur in nature (e.g. soil pH control in agriculture) are examples of imperfect neutralisation reactions.
In some cases, the reaction may involve either a weak acid and a strong base, or vice versa. If a weak acid reacts with a strong base, the resulting salt will have basic characteristics in an aqueous solution. Conversely, if a strong acid reacts with a weak base, the salt product will display acidic characteristics in an aqueous solution. Generally, a substance only exhibits the properties of an acid or a base if it’s dissolved in water. Hence, an acid and a base can only react with each other in an aqueous solution.
According to the Arrhenius definition, an acid is a substance that increases the positively-charged hydronium or H3O+ when dissolved in water. In other words, water molecules become protonated. Normally, pure water has small amounts of both hydronium and hydroxide ions and they are in perfect balance with each other, as illustrated below. 
However, when an acidic substance is dissolved in water, the hydronium ions increase. If hydrochloric acid is dissolved in water, for instance, hydronium ions and chlorine ions are formed. 
Meanwhile, if a basic substance is dissolved in water, the hydroxide ions increase. For example, when ammonia is dissolved in water, it forms ammonium and hydroxide ions. 
Examples of neutralisation
Examples of neutralisation reactions can be summarised based on the types of acid and base interactions. Here are some examples:
Strong acid and strong base
Hydrochloric acid solution reacts with a solution of sodium hydroxide to produce table salt and water.
HCl + NaOH → NaCl + H2O
Strong base and weak acid
Sodium hydroxide solution reacts with acetic acid to form sodium acetate and water.
NaOH + CH3COOH → CH3COONa + H2O
Strong acid and weak base
The reaction between hydrochloric acid and ammonia doesn’t produce water as a final byproduct. It’s a combination reaction as opposed to a double replacement.
HCl + NH3 → NH4Cl
Weak acid and weak base
Weak acids and weak bases do not completely dissociate into ions when they’re dissolved in water. Therefore, the reaction between a weak acid and a weak base forms conjugate acids and conjugate bases that have generally reversible reactions. For example, the reaction between ammonia and formic acid produces ammonium formate, as shown here:
NH3 + HCO2H → NH4HCO2
What type of substance will neutralise potassium hydroxide solution?
Potassium hydroxide is a strong base, which means it dissociates completely into ions when dissolved in water. As a result, it can only be completely neutralised by a strong acid such as nitric acid, as follows:
HNO3 + KOH → H2O + KNO3
How to neutralise bleach
Common household bleach is a 3-6% sodium hypochlorite solution.
Classified as a weak base, it can easily be neutralised by an acidic salt such as sodium metabisulfite. At certain concentrations, bleach can produce a weak acid known as hypochlorous acid.
What can neutralise hydrochloric acid?
Hydrochloric acid is commercially sold as a cleaning agent known as muriatic acid, which is 31.45% acid by mass.
Although it can be easily neutralised by sodium hydroxide (a strong base), this isn’t recommended. It’s much safer to use sodium carbonate to neutralise hydrochloric acid on a surface like a bathroom floor.
