What is an Acid in Chemistry?

by Kate Onissiphorou

Acids are solutions that are both corrosive and reactive, especially with metals. They react with bases to form salts in neutralisation reactions. 

Acids can be organic or inorganic and either strong or weak. They often occur naturally. For example, hydrochloric acid is found in the stomachs of mammals, including humans. Acids are also commonly produced by plants, such as the citric acid contained in citrus fruits like lemons and oranges.

In laboratory settings, acids are used as reagents and as analytical solutions, particularly in titrations. They also play an important role in many industrial processes, such as petroleum refining, fertiliser production, and pharmaceutical manufacturing.

Sulphuric acid, for example, acts as a basic raw material and reagent in various processes. Consequently, it’s used in a wide range of sectors including the automobile, pharmaceutical, steel manufacturing, and fabric industries.

What is the Definition of Acid in Chemistry?

An acid has a pH level of less than 7 and it reacts with bases in neutralisation. Similar to the definition of bases, the definition of acid and the concepts related to it are categorised into three main theories: the Arrhenius definition, the Brønsted–Lowry definition, and the Lewis definition.

Acids with different pHs

Arrhenius definition 

Named after the Swedish scientist, Svante Arrhenius, this definition is based on the observation that when acid is dissolved in water, it increases the concentration of hydrogen ions. That means an Arrhenius acid is any substance that increases hydrogen ions (H+) when dissolved in water. 

It’s also a substance that produces other types of hydrogen ions in water, for example, hydronium ions (H3O+) or other forms, such as H5O2+, H9O4+.

Brønsted–Lowry definition 

According to this theory, an acid can only act as an acid when a base is present. Conversely, a base can only act as a base in the presence of an acid. This thesis defines an acid as a proton donor to a base, which serves as the proton acceptor. The Arrhenius theory is insufficient in describing many organic reactions that do not involve hydronium ions, such as the dynamic equilibrium reactions between acetic acid and ammonia, as shown below.


Lewis definition

In 1923, Gilbert N. Lewis proposed an improvement on the previous theories regarding acids and bases. The Lewis definition does not involve hydrogen ions or proton transfers. Instead, it states that an acid is a substance that readily accepts a pair of electrons from another substance in a solution.

How Acids Work

Referring to the three definitions of acids and bases, an acid forms hydrogen or hydronium ions in water, donates a proton to a base, and accepts a pair of electrons. An acid is chemically reactive and, as a result, can corrode metals and produce exothermic reactions with bases.

When an acid is dissolved in water, it dissociates into positively charged hydrogen or cations and negatively charged anions. The dissociation increases the number of hydrogen or hydronium ions in the water, which reduces the pH level of the solution. If a metal, base or other reactive substances are present, they may react with the acid by either donating electron pairs or accepting protons.

Different Types of Acids and Their Uses

Acids are common in our daily lives and can be found in everything ranging from the physiological or biological functions in our body to the products we consume. 

Our stomach is an acidic environment due to the presence of hydrochloric acid, one of the main components in the digestive juices that help to break down food. Likewise, our liver produces bile acids that digest fats in food. 

We also drink orange juice and use lemon juice in cooking, both of which contain citric acid. We season our food with acetic acid. We use plastic products that are produced with the help of acids. These are all examples of different types of acids in everyday life.

Some common examples of acids and their uses are outlined below.

Citric acid 

Citric acid is an organic acid that occurs naturally. It’s found in citrus fruits such as lemons and oranges. It’s used as a food flavouring and food preservative.

Ascorbic acid 

This is another naturally occurring organic acid commonly found in citric fruits. Otherwise known as vitamin C, it’s a powerful antioxidant that’s also used to treat scurvy and bone marrow diseases.

Acetic acid

Acetic acid is an organic acid that’s often used to flavour food. It’s also used as a food preservative, such as in pickling.

Bottles of acetic acid

Sulphuric acid 

Sulphuric acid is an inorganic acid that has a variety of applications. It’s used as an electrolyte in lead batteries and is a key component in industrial processes like fertiliser manufacturing.

Boric acid 

Boric acid also has several industrial applications and is commonly used in the manufacture of glass, paper and leather.

Strong vs Weak Acids

The strength or weakness of acid in terms of its pH level mainly depends on the concentration of the acid solution. The higher the concentration, the stronger the acid. However, other factors, such as the temperature of the solution, also affect the pH level and strength of an acid. Hence, the pH is not a particularly good measure of an acid’s strength.

A more objective measure of acid strength is to look at the dissociation constant, which is the ratio between the ions and the amount of acid in a solution. Strong acids dissociate more completely in an aqueous solution compared to weak acids. The constant can be expressed in this formula:

Acid dissociation constants

Some examples of strong acids include:

  • Hydrochloric acid HCl
  • Nitric acid HNO3
  • Perchloric acid HClO4.

Examples of weak acids include:

  • Sulphurous acid H2SO3
  • Phosphoric acid H3PO4
  • Nitrous acid HNO2

A Level Chemistry Acids and Bases

If you’re revising for your GCSE or A level chemistry exams, acids and bases is a key topic that you must master. You’ll need to be familiar with the different types of acids and understand the difference between weak and strong acids. You should also know how to balance the chemical equations of acid-base neutralisation reactions.


The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).