Acids and Bases: What you need to know first

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Updated on: Educator Review By: Michelle Connolly

Acids and bases discovery

People have known for thousands of years that vinegar, lemon juice, and many other foods have an acidic taste. However, it wasn’t until they discovered acids and bases in chemistry that they discovered why these things taste sour. The term acid actually comes from the Latin term “Acere”, which means acid, while there are many slightly different definitions of acids and bases that exist.

The discovery of acids and bases in chemistry began at the beginning of the seventeenth century. They were classified and differentiated more closely to primitive methods than to the newly known chemical methods and equations.

What are Acids and Bases?

Acid: Any substance that contains hydrogen and can donate a proton (hydrogen ion) to another substance.

Base: any substance capable of receiving a hydrogen ion from acid during a chemical reaction.

Definition of Acids and Bases:

There are several ways to identify acids and bases. While these definitions do not contradict each other, they differ in how comprehensive they are. The most common definitions for acids and bases are:

  • Arrhenius
  • Brønsted-Lowry, 
  • and Lewis. 

Antoine Lavoisier, Humphrey Davy, and Justus Liebig also made observations regarding acids and bases but did not formally define them.

The concept of acids and bases, according to Arrhenius:

Arrhenius’ theory of acids and bases dates back to 1884 based on his observation that salts such as sodium chloride separate into what he called ions when placed in water.

Arrhenius acid is any substance that increases the H+ concentration in an aqueous solution.

An Arrhenius base is any substance that increases the concentration of OH- in an aqueous solution.

These protons go on to form hydronium ions (H 3 O + ) by fusing with water molecules.

This theory has the advantage of successfully explaining the reaction between acids and bases that produce salts and water.

One significant limitation of Arrhenius’ definitions of acids and bases is that it does not explain how substances that do not contain hydroxide ions form basic solutions when dissolved in water, such as NO − and F −.

The concept of acids and bases, according to Brønsted-Lowry:

The acid is the proton donor.

The base is the proton acceptor.

Bronze acids undergo dissociation to produce protons and thus increase the concentration of H+ ions in the solution.

Bronsted bases, on the other hand, accept protons from water (the solvent) to produce hydroxide ions.

One advantage of Brønsted-Lowry’s definition of acids and bases is that it can explain the acidic or basic nature of ionic species.

One significant limitation of this theory is that it fails to explain how compounds that lack hydrogen exhibit acidic properties, such as BF 3 and AlCl 3.

The concept of acids and bases according to Lewis:

Acid Any substance that can accept one or more pairs of electrons.

Base Any substance that can donate one or more pairs of electrons.

This theory does not include the hydrogen atom in its definition of acids and bases.

Lewis acids are electrophilic in nature, while Lewis bases have nucleophilic properties.

Examples of Lewis acids: Cu 2+, BF 3 and Fe 3+. Examples of Lewis bases: F –, NH 3, C 2 H 4 (ethylene).

Resulting in the formation of a covalent bond. The resultant compound is known as a Lewis conjugate.

One significant advantage of this concept is that many compounds can be classified as acids or bases. However, it provides little insight into the strength of these acids and bases.

This theory has the disadvantage of failing to explain acid-base reactions that do not involve the formation of a coordinate covalent bond.

Acid and Bases :properties and Characteristics

Acids and bases belong to two classes of chemical compounds that exhibit completely opposite properties. The acids have a predominantly sour taste; they give litmus paper a red colour, and in contact with almost all metals, they react by releasing hydrogen in a gaseous state; the bases have a bitter taste, giving the litmus a blue colour and are slimy to the touch. By mixing aqueous solutions of an acid and a base, a reaction called neutralization develops, which has the characteristic of proceeding rapidly, producing salt and water. For example, if we make hydrochloric acid and sodium hydroxide react together, we will have a typical neutralization reaction:

HCl + NaOH ⇄ H2O + NaCl

Hydrochloric acid + sodium hydroxide ⇄ water + sodium chloride.

we can summarize their characteristics in simple points as the following:

  • Good Conductors of Electricity.
  • PH values are always less than 7.
  • When they interact with metals, these materials produce hydrogen gas.
  • Sour-tasting items.
  • Examples: hydrochloric acid [HCl], sulfuric acid , and acetic acid [CH 3 COOH].
  • These substances release hydroxide ions (OH – ions) when they are dissolved in water.
  • Bases in their aqueous solutions act as good conductors of electricity.
  • Bases have pH values that are always greater than 7.
  • Bases are bitter-tasting substances that can change the colour of litmus paper from red to blue.
  • Examples: milk of magnesia [Mg (OH) 2 ], calcium hydroxide [Ca (OH) 2 ] and sodium hydroxide [NaOH].

How Neutral Materials are different from Acids ?

  • These materials do not show any acidic or basic properties.
  • The pH values for these substances are close to 7.
  • Neutral materials have no effect on red or blue litmus paper.
  • Pure water has a pH of exactly 7.
  • Examples: water, salt (NaCl).

What are acidic substances?

Examples of acidic substances are vinegar, muriatic acid and lemon juice. Acids are generally divided into strong acids and weak acids. An index of the strength of an acid, a function of its nature and concentration, is the pH.

What are inorganic acids?

Inorganic acids include hydracids: which are composed of hydrogen and halogens; sulphur, selenium, nitrogen or cyanide ion (CN -); oxoacids: which are composed of hydrogen, a nonmetal, and oxygen. They are formed by reacting an anhydride with water.

Strong and Weak Acids and Bases

Acids are generally divided into strong acids and weak acids.

The ability of acids and bases to dissociate or penetrate their ions in water determines their strength. A strong acid or strong base dissociates completely (for example, hydrochloric acid or sodium hydroxide), while a weak acid or base dissociates only partially (for example, acetic acid). The acid dissociation constant and the base dissociation constant refers to the relative strength of an acid or a base. The acid dissociation constant K a is the equilibrium constant for acid-base dissociation:

HA + H 2 O ⇆ A — + H 3 O +

Where HA is the acid and A – is the conjugate base.

[K a = [A– ] / [HA]

This is used to calculate pKa, the logarithmic constant: PKA = – log10Ka. The higher the pK value, the lower the acid’s dissociation and the weaker the acid. The pKa of strong acids is less than -2.

Uses of Acids and Bases

A large number of bases and acids are used in many areas of life, and here are the most important uses of acids as follows:

  • Vinegar is a dilute acetic acid solution that has various household applications. Primarily used as a food preservative.
  • Citric acid which is an integral part of lemon juice and orange juice. It can also be used to preserve food.
  • Sulfuric acid is widely used in battery manufacturing.
  • This acid is commonly found in batteries used to start automobile engines.
  • The industrial production and manufacturing of explosives, paints, dyes, and fertilizers involve the use of sulfuric acid and nitric acid.
  • Phosphoric acid is a major ingredient in many soft drinks.
  • Nitric acid and sulfuric acid are used in the manufacture of dyes, explosives, fertilizers and paints.
  • Phosphoric acid is one of the basic elements found in soft drinks.
  • Hydrochloric acid is used in several areas, the most important of which are the steel industry and cleaning.

Bases are involved in many industries, so it is easy to find them in many examples of materials that we use indoors constantly, including:

  • The chlorine we use to make clothes is the basic equivalent of hydrochloric acid.
  • Tea and coffee contain caffeine, which is a base substance, and tea leaves contain a high concentration of it, as well as purine, which is a nitrogenous base.
  • Chocolate contains coffee, in addition to containing theobromine, which is a nitrogenous base which gives the distinctive mood of chocolate.
  • Soap, which consists of fatty acids and sodium hydroxide, is found in every home and cannot be dispensed with.
  • Deodorants are aluminium hydroxide, an active substance used to remove sweat and germicide and help get rid of germs that feed on fats and proteins in sweat.
  • Antacids are weak bases and active bicarbonate compounds, and milk of magnesia is a laxative and also an antacid.
  • Bleach, which is sodium hydroxide, is one of the strongest bases out there and the primary ingredient for drain cleaners.

Glossary of basic concepts

The following are the most important concepts related to acids and bases:


Any chemical compound that can form positive hydrogen ions (H+) when it is dissolved in water, such as hydrochloric acid.


Any chemical compound that can form negative hydroxide ions (OH-) when it is dissolved in water, such as sodium hydroxide.

Neutralization reaction

A chemical reaction takes place between equal amounts of an acid and a base to form water and salt, such as the reaction of hydrochloric acid with sodium hydroxide.

Strong acids

A strong acid is an acid that ionizes completely in an aqueous solution, dissociating into a positive hydrogen ion and another negative ion, such as nitric acid.

Weak acids

A weak acid is an acid that does not completely dissociate in solution, as it dissociates only in small proportions and remains present in the form of unseparated molecules, and an example is hydrogen sulfide acid.

Strong bases

A strong base is a base that completely dissociates in an aqueous solution, forming the negative hydroxide ion and another positive ion, such as lithium hydroxide.

Weak base

A weak base is a base that does not completely dissociate or ionize in an aqueous solution, but only in small proportions, such as ammonia.


An indicator to measure the acidity or basicity of different solutions. This scale ranges from 0 to 14, where values ranging from 0 to 6 indicate acidic substances, while values from 8 to 14 indicate basic substances and 7 indicates that the solution is neutral.

Acid solutions

A solution is formed when an acid dissociates into a positive hydrogen ion and a negative hydrogen ion.

Base solutions

A solution is formed when a base dissociates into a negative hydroxide ion and another positive ion.


They are very weak acids or bases that change colour according to the change in pH, that is, according to the acidity or alkalinity of the solution in which they are placed.

Acid ionization constant

A quantitative measure that expresses the strength of an acid in a solution.

Base ionization constant

A measure of the strength of a base in a solution.

Acid-Base reaction

A chemical reaction takes place between an acid and a base.

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