What Is a Base in Chemistry?

by Kate Onissiphorou

In simple terms, a basic or an alkaline solution has a pH value of above 7.

As you probably recall from your secondary school chemistry lessons, a base is a proton acceptor. This means it attracts the hydrogen of an acid during a neutralisation reaction. That means, according to the Brønsted-Lowry definition, a base has a negatively charged ion that accepts a positively charged hydrogen ion or proton.

Basic substances have a wide range of applications and are commonly used in the chemical manufacture of detergents and the synthesis of pharmaceutical products. Alkaline solutions are also particularly useful in making long-lasting disposable batteries.

What is the Definition of a Base in Chemistry?

A base substance is typically a solution that has a pH level of more than 7. Bases react with acids to form salts during a neutralisation (or double replacement) reaction.

In chemistry, there are three definitions of base substances: Arrhenius bases, Brønsted bases, and Lewis bases. All of these definitions agree that a base reacts with an acid to neutralise each other.

  • Arrhenius bases – It was in 1884 that the Swedish scientist, Svante Arrhenius, proposed the idea that when a base substance is dissolved in water, it forms negatively charged hydroxide ions (OH). These ions react with the positively charged hydrogen ions (H+) to form water.
  • Brønsted-Lowry acid-bases – In 1923, the Brønsted-Lowry acid-base theory was developed to generalise the definitions of acids and bases. It classifies acids as proton donors and bases as proton acceptors. This theory is necessary to explain bases that do not have hydroxide ions such as ammonia. Arrhenius bases are a subcategory of the Brønsted-Lowry acid-base theory.
  • Lewis bases – In the Lewis theory of acids and bases, a base is defined as an electron-pair donor that is capable of sharing electrons with an electron acceptor (known as a Lewis acid). It’s a more generalised description because a Lewis acid does not necessarily have a proton ion, such as in the case of boron trifluoride.

What Does a Base Do in Chemistry?

A base is very chemically reactive due to its charge imbalance. Depending on the type, a base has excess negatively charged ions when it’s dissolved in water. These excess ions tend to attract positively charged ions, particularly hydrogen ions from acids.

Many bases contain alkali and alkaline earth metals, such as sodium, potassium, magnesium and calcium, which easily form salts with halogens and other negatively charged components of acids. 

Consider the reaction between copper hydroxide and sulphuric acid:

Cu(OH)2 + H2SO4 → CuSO4 + 2H2O

In this reaction, two molecules of water and one molecule of copper sulphate are produced for every molecule of copper hydroxide and sulphuric acid. Under ideal conditions when the reaction is perfectly balanced, the products will have neutral pH levels. 

Aside from the defining chemical characteristics, here are some of the other general properties of bases:

  • Strong bases are highly caustic and can easily dissolve organic matter, particularly fats and oils
  • They react violently with acidic substances, producing high temperatures
  • The positive and negative ions dissociate when dissolved in water
  • Bases change the colours of indicators – for example, they can turn red litmus paper blue and change methyl orange to yellow
  • Bases have a bitter taste, whereas acids have a distinctly sour taste

How to Identify a Base in Chemistry

There are several ways of empirically identifying a base. You can use qualitative indicators, such as litmus paper, or you can use a pH scale. You can also identify a base by its bitter taste…but this is not recommended! Some base or alkaline solutions have a slippery feel.

A scientist investigating the acidity or alkalinity of a chemical using litmus paper.

You can easily determine if a substance is a base if you know its chemical formula. You may also write a balanced chemical equation between two substances to determine whether the parent chemicals have lost or gained protons (hydrogen ions).

Illustration of an acid/base chemical reaction to determine the pH level of a substance using litmus paper.

Types of Bases in Chemistry

Bases can be classified into different types according to their strength and composition. A base can either be strong or weak. It can also be organic or inorganic. 

  1. Strong bases

There’s a common notion that the strength or weakness of a base or acid is measured by its pH level. However, this is only partially true. 

The concentration of a base in a solution largely affects the pH level. Therefore, the most accurate measure of a base’s strength is its dissociation, or ionisation constant.

If a base completely dissociates into its ions when dissolved in water, it’s considered to be a strong base. This constant also determines how easily a base can remove a proton from a compound, particularly a weak acid. The constant is the ratio between the concentration of ions and the non-dissociated molecules of the base in terms of moles. The formula can be written as:

The equation for basicity constant KbSome examples of strong bases include:

  • Lithium hydroxide – LiOH
  • Sodium hydroxide – NaOH
  • Magnesium hydroxide – Mg(OH)2
  • Calcium hydroxide – Ca(OH)2.
  1. Weak bases

Conversely, weak bases do not totally dissociate in a water solution, hence protonation is always incomplete. Many organic bases are classified as weak. Here are some examples of weak bases:

  • Alanine – C3H7NO2
  • Ammonia – NH3
  • Ammonium hydroxide – NH4OH
  • Trimethylamine – N(CH3)3.

Why are Acids and Bases Important in Chemistry?

Acids and bases are important in chemistry as they serve both an analytic and a synthetic purpose. 

Many analytical methods in chemistry, including titration, depend on the reactions of acids and bases. Acids or bases also serve as precursors for the synthesis of industrial and commercial products, such as polymer fabrics.

A-Level Chemistry Acids and Bases

If you’re planning to take an A-Level exam in chemistry, acids and bases is one of the key topics you’ll need to master. You must be familiar with how to balance neutralisation reactions and classify various types of acids and bases. You should also know the various analytical methods and practical laboratory techniques in analysing acids and bases.


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).