Acids and Bases

Acids and bases are popular chemicals which interact with each other resulting in the formation of salt and water. The word acid comes from a Latin word ‘acere’ which means ‘sour’. Acidic substances are usually identified by their sour taste. An acid is basically a molecule which can donate an H+ ion and can remain energetically favourable after a loss of H+. Acids are known to turn blue litmus red.

Bases, on the other hand, are characterized by a bitter taste and a slippery texture. A base that can be dissolved in water is referred to as an alkali. When these substances chemically react with acids, they yield salts. Bases are known to turn red litmus blue.

What are Acids and Bases?

Three different theories have been put forth in order to define acids and bases. These theories include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases. A brief description of each of these theories is provided in this subsection.  Acids and bases can be defined via three different theories.

  • The Arrhenius theory of acids and bases states that “an acid generates H+ ions in a solution whereas a base produces an OH ion in its solution”.

  • The Bronsted-Lowry theory defines “an acid as a proton donor and a base as a proton acceptor”.

  • Finally, the Lewis definition of acids and bases describes “acids as electron pair acceptors and bases as electron pair donors”.

In order to find the numeric value of the level of acidity or basicity of a substance, the pH scale (wherein pH stands for ‘potential of hydrogen’) can be used. The pH scale is the most common and trusted way to measure how acidic or basic a substance is. A pH scale measure can vary from 0 to 14, where 0 is the most acidic and 14 is the most basic a substance can be.

Another way to check if a substance is acidic or basic is to use litmus paper. There are two types of litmus paper available that can be used to identify acids and bases – red litmus paper and blue litmus paper. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions.

Properties of Acids

  • Acids are corrosive in nature.
  • They are good conductors of electricity.
  • Their pH values are always less than 7.
  • When reacted with metals, these substances produce hydrogen gas.
  • Acids are sour-tasting substances.
  • Examples: Sulfuric acid [H2SO4], Hydrochloric acid [HCl], Acetic acid [CH3COOH].

Properties of Bases

  • They are found to have a soapy texture when touched.
  • These substances release hydroxide ions (OH ions) when dissolved in water.
  • In their aqueous solutions, bases act as good conductors of electricity.
  • The pH values corresponding to bases are always greater than 7.
  • Bases are bitter-tasting substances which have the ability to turn red litmus paper blue.
  • Examples: Sodium hydroxide [NaOH], milk of magnesia [Mg(OH)2], calcium hydroxide [Ca(OH)2].

Neutral Substances

  • These substances do not display any acidic or basic characteristics.
  • Their pH values approximate to 7.
  • Neutral substances have no effect on red or blue litmus paper.
  • The pH of pure water is exactly 7.
  • Examples: Water, Common salt (NaCl)

Arrhenius Concept of Acids and Bases

  • The Swedish scientist Svante August Arrhenius defined acids as substances that increase the H+ ion concentration of water when dissolved in it.
  • These protons go on to form hydronium ions (H3O+) by combining with water molecules.
  • Similarly, the Arrhenius definition of a base states that bases are the substances that, when dissolved in water, increase the concentration of OH ions in it.
  • One of the merits of this theory is that it successfully explains the reaction between acids and bases that yield salts and water.
  • An important limitation of the Arrhenius definitions of acids and bases is that it fails to explain how substances lacking hydroxide ions form basic solutions when dissolved in water, such as NO2 and F.

Bronsted-Lowry Theory of Acids and Bases

  • The Bronsted-Lowry theory defines an acid as a donor of protons.
  • A base is defined as a proton acceptor (or H+ ion acceptor) by this theory.
  • Bronsted acids undergo dissociation to yield protons and therefore increase the concentration of H+ ions in the solution.
  • On the other hand, Bronsted bases accept protons from water (the solvent) to yield hydroxide ions.
  • An advantage of the Bronsted-Lowry definition of acids and bases is its ability to explain the acidic or basic nature of ionic species.
  • An important limitation of this theory is that it fails to explain how compounds lacking hydrogen exhibit acidic properties, such as BF3 and AlCl3.

Lewis Concept of Acids and Bases

  • The Lewis definition of an acid states that it is a species that has a vacant orbital and therefore has the ability to accept an electron pair.
  • A Lewis base is a species that holds a lone pair of electrons and can, therefore, act as an electron pair donor.
  • This theory does not involve the hydrogen atom in its definition of acids and bases.
  • Lewis acids are electrophilic in nature whereas Lewis Bases possess nucleophilic qualities.
  • Examples of Lewis acids: Cu2+, BF3, and Fe3+. Examples of Lewis bases: F, NH3, and C2H4 (ethylene).
  • A Lewis acid accepts an electron pair from a Lewis base, forming a coordinate covalent bond in the process. The resulting compound is referred to as a Lewis adduct.
  • A notable advantage of this concept is that many compounds can be defined as acids or bases by it. However, it offers little insight into the strength of these acids and bases.
  • One of the disadvantages of this theory is that it fails to explain the acid-base reactions that do not involve the formation of a coordinate covalent bond.

Uses of Acids and Bases

The various uses of acids and bases are listed in this subsection.

Uses of Acids

  • Vinegar, a diluted solution of acetic acid, has various household applications. It is primarily used as a food preservative.
  • Citric acid is an integral part of lemon juice and orange juice. It can also be used in the preservation of food.
  • Sulfuric acid is widely used in batteries. The batteries used to start the engines of automobiles commonly contain this acid.
  • The industrial production of explosives, dyes, paints, and fertilizers involve the use of sulfuric acid and nitric acid.
  • Phosphoric acid is a key ingredient in many soft drinks.

Uses of Bases

  • The manufacturing of soap and paper involve the use of sodium hydroxide. NaOH is also used in the manufacture of rayon.
  • Ca(OH)2, also known as slaked lime or calcium hydroxide, is used to manufacture bleaching powder.
  • Dry mixes used in painting or decoration are made with the help of calcium hydroxide.
  • Magnesium hydroxide, also known as milk of magnesia, is commonly used as a laxative. It also reduces any excess acidity in the human stomach and is, therefore, used as an antacid.
  • Ammonium hydroxide is a very important reagent used in laboratories.
  • Any excess acidity in soils can be neutralized by employing slaked lime.