Question

What is the difference between :

(a) an alkali and a base,

(b) the chemical nature of an aqueous solution of HCI and an aqueous solution of $N{H}_3$

Answer 1

(a)

• Bases are substances that react with acids and neutralise them. They are usually metal oxides, metal hydroxides, metal carbonates or metal hydrogen carbonates. Many bases are insoluble - they do not dissolve in water. If a base does dissolve in water, we call it an alkali.
• Alkaline solution/compound is the same as a basic solution/compound. But you can specify "alkali" to refer to those bases that are soluble in water and that are salts of alkaline or alkaline earth metals too.
• A basic substance is usually one or two metal atoms bonded with an oxygen atom to make oxides.
  An alkaline substance is like a base, but it involves hydrogen in the alkali molecule, so that it forms hydroxides.

So, in general, bases are oxides and alkalis are hydroxides.

• Alkaline are those which can give OH- ions when dissolved in water but basic are those which react with acid to give water and salt so compounds containing OH- termed as Alkaline like NaOH, KOH etc on other hand CaO , $N{a}_{2}O$ are basic but not alkaline
• Any basic substance has a power of hydrogen, or pH, above 7.0, the pH of pure water. Acidic substances have a pH of below 7.0. Basic substances are basic because they are capable of accepting hydrogen ions, and hydrogen ions are what give acids their caustic properties.

(b) An aqueous solution of HCl acid contains a large amount of H+ ions along with the concerned acidic radical. An aqueous solution of HCl turns blue litmus red. HCl is a strong acid.
An aqueous solution of $N{H}_3$ contain OH- ions and the concerned basic radical in very small quantities along with a large proportion of undissociated molecules of the base. An aqueous solution of $N{H}_3$ turns red litmus blue. $N{H}_3$ is a weak base.