A Level Chemistry Revision: Physical Chemistry – Acids And Bases

by Lucy Bell-Young

Acids and bases are fundamental in the study of chemistry. The reactions between the two are fast and exothermic, meaning they release heat, which makes them measurable and observable. Even without precise instruments, you can easily estimate whether a reaction has reached its saturation point.

Acids and bases are polar opposites of each other, but also complementary. Therefore, their reactions are also known as neutralisation reactions (more on those below). These substances are part of a continuum that can be measured in various ways, such as using the pH scale and computing for the dissociation constants (pKa and pKb).

Many industrial processes, such as the manufacture of medicines, involve the use of acids and bases in synthesising a wide range of inorganic and organic chemicals. Similarly, many materials, such as polymer fibres, would not be possible to manufacture without acids and bases.

How to Identify Acids and Bases

According to the Brønsted-Lowry theory of acids and bases, a base is a proton acceptor while an acid is a proton donor. This principle is made obvious in acid-base reactions, where the ‘proton’ is actually a hydrogen atom or ion. This makes acids and bases pretty easy to identify based on their chemical formulas.

The chemical formula for acids typically have H (hydrogen) at the left end of the formula. Here are some examples of strong acids and their respective chemical formulas:

  • Perchloric acid: HClO4
  • Hydrochloric acid: HCl
  • Nitric acid: HNO3
  • Sulphuric acid: H2SO4

Meanwhile, the chemical formulas for bases have -OH (hydroxyl) at the end of the formula. It’s important to note, however, that alcohols and other chemicals also have hydroxyl groups. The main difference is to which functional group the hydroxyl is attached. For example, alcohols have alkyl R groups or chains. Here are some examples of bases:

  • Potassium hydroxide: KOH
  • Sodium hydroxide: NaOH
  • Barium hydroxide: Ba(OH)2
  • Calcium hydroxide: Ca(OH)2

As you can see from the above examples, the H- and -OH generalisations are mainly applicable to inorganic acids and bases. But this changes when it comes to organic acids and bases. 

The most common of the organic acids are called carboxylic acids, and their acidity is due to their carboxyl group (–COOH). Sulphonic acids are another common category of organic acids. They contain the group –SO2OH. While alcohols also have an –OH group, they’re considered very weak acids.

Chemically speaking, organic acids still donate protons or hydrogen ions. The illustration below highlights the acidic hydrogens found in carboxylic acids and sulphonic acids:

Acidic hydrogens found in carboxylic acids and sulphonic acids

Similarly, organic bases still function as proton acceptors, though they’re a bit more complex than inorganic bases. An example of an organic base is pyridine, a basic heterocyclic organic compound. It’s a highly flammable liquid that’s soluble in water, and is mainly used as a pesticide. It has the following molecular structure: Molecular structure of pyridine

What is a Neutralisation Reaction?

When an acid and base react, they form a salt, which can either be organic or inorganic depending on the reactants. An acid-base reaction is commonly called a neutralisation reaction because the pH level of the final product and the solution is neutral (provided that the reaction is complete and no acid or base is left in the solution). In many reactions, however, wherein the concentrations are unknown, some acid or base could still be present.

Generally, acid-base reactions involve the exchange or transfer of hydrogen ions. In an aqueous acidic solution, the hydrogen ions usually do not exist as H+ ions. Instead, they exist as hydronium ions (H3O+). In most cases, the reaction involves an acid and a base.

However, while this type of reaction generally involves the transfer of hydrogen ions, some exceptions exist. For instance, analogous chemical behaviour of aluminium chloride (AlCl3) and the silver ion Ag+ is similar to acid-base reactions, but without the transfer of hydrogen ions.

In some cases, the reaction can be between a neutral chemical species and an acid, such as the reaction between water and acetic acid. It can also be between electrically charged ions, such as ammonium and hydroxide or carbonate.

Here are some categories of neutralisation reactions:

  • Metal hydroxides

The reaction between a base and an acid is always exothermic, i.e. it releases heat. Sometimes, it can even be explosive. The reaction between metal hydroxides and acids produces salt and water. The generalised chemical reaction can be written as:

metal hydroxide + acid → a salt + water

The most common example of this neutralisation reaction is the reaction between sodium hydroxide and hydrochloric acid. This produces water and sodium chloride, a.k.a. table salt:

NaOH + HCl → H2O + NaCl

  • Metal oxides

Metal oxides act as base substitutes when reacting with acids. The reactants form salt and water as byproducts. The generalised chemical reaction is shown below:

metal oxide + acid → a salt + water

The type of salt produced is dependent on the chemical constituents of the metal oxide and the acid. 

  • Metal carbonates

Carbonate compounds contain carbonate ions (CO2/3), which are mostly insoluble in water. These compounds are neutralised by acids, forming byproducts of salt, water, and carbon dioxide gas. The generalised chemical equation can be written as:

metal carbonate + acid → a salt + water + carbon dioxide

An example of this type of reaction is the reaction between copper carbonate and nitric acid, which produces copper nitrate, water, and carbon dioxide, as shown below:

CuCO3 + 2HNO3 → Cu(NO3)2 + H2O + CO2

Illustration of acid-base reaction

What Are the Applications of Neutralisation Reactions?

Neutralisation reactions occur in nature – even some physiological or metabolic processes involve neutralisation reactions. But they also have several industrial and agricultural applications. For example, farmers have used lime or calcium oxide as a means of neutralising acidic soils for centuries.

Medically, we use antacids that have basic compounds, such as magnesium hydroxide, as an active ingredient to treat hyperacidity. You can also treat bee stings with baking powder, which contains the basic compound sodium hydrogen carbonate that can neutralise the acidic bee sting.


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