Acids and Bases

For thousands of years, people have known that vinegar, lemon juice, Amla, tamarind and many other food items taste sour. However, only a few hundred years ago it was proposed that these things taste sour because they contain acids.

The term acid comes from Latin term accre which means sour. It was first used in the seventeenth century by Robert Boyle to label substances as acids and bases according to the following characteristics:

Acids

  1. taste sour
  2. are corrosive to metals
  3. change blue litmus red
  4. become less acidic on mixing with bases

Bases

  1. taste bitter
  2. feel slippery or soapy
  3. change red litmus blue
  4. become less basic on mixing with acids

While Robert Boyle was successful in characterising acids and bases he could not explain their behaviour on the basis of their chemical structure. This was accomplished by Swedish scientist Svante Arrhenius in the late nineteenth century. He proposed that on dissolving in water, many compounds dissociate and form ions and their properties are mainly the properties of the ions they form.

Governed by this, he identified the ions furnished by acids and bases responsible for their characteristic behaviour and gave their definitions.

Acids

An acid is a substance which furnishes hydrogen ions (H+) when dissolved in water. For example, in its aqueous solution hydrochloric HCl (aq) dissociates as:

HCl (aq) → H+(aq) + Cl–(aq)

Some examples of acids are:

  1. Hydrochloric acid (HCl) in gastric juice
  2. Carbonic acid (H2CO3) in soft drinks
  3. Ascorbic acid (vitamin C) in lemon and many fruits
  4. Citric acid in oranges and lemons
  5. Acetic acid in vinegar
  6. Tannic acid in tea
  7. Nitric acid (HNO3) used in laboratories
  8. Sulphuric acid (H2SO4) used in laboratories

Bases

A base is a substance which furnishes hydroxide ions (OH–) when dissolved in water. For example, sodium hydroxide NaOH (aq), in its aqueous solutions, dissociates as:

NaOH (aq) → Na+(aq) + OH–(aq)

The term alkali is often used for water soluble bases. Some examples of bases are:

  1. Sodium hydroxide (NaOH) or caustic soda used in washing soaps.
  2. Potassium hydroxide (KOH) or potash used in bathing soaps.
  3. Calcium hydroxide (Ca(OH)2) or lime water used in white wash.
  4. Magnesium hydroxide (Mg(OH)2) or milk of magnesia used to control acidity.
  5. Ammonium hydroxide (NH4OH) used in hair dyes.

Indicators

The spot of turmeric or gravy on cloth becomes red when soap is applied on it. Turmeric has acted as an indicator of base present in soap. There are many substances that show one colour in an acidic medium and another colour in a basic medium. Such substances are called acid-base indicators.

Litmus is a natural dye found in certain lichens. It was the earliest indicator to be used. It shows red colour in acidic solutions and blue colour in basic solutions.

Phenolphthalein and methyl orange are some other indicators.