Question

What is meant by demineralized water and how it can be obtained?

Answer 1

Hint: Demineralized water is the same as deionized water.Water contains a large variety of dissolved minerals.

Complete step by step answer:
Water contains a large variety of dissolved minerals. Sometimes it is necessary to obtain water in which no mineral is present.
demineralization water is devoid of all soluble mineral salts.
A demineralization water can be obtained by removing minerals from water, For this, ion exchange resins are used. First water is passed through cation exchange resin. Then water is passed through anion exchange resin.

When water is passed through cation exchange resin, cations, such as sodium, magnesium and calcium (from water to be demineralization) are exchanged with protons (from cation exchange resin). In this step, water becomes free from cations such as sodium, magnesium and calcium.
\[{\rm{2 RH}}\left( s \right){\rm{ + }}{{\rm{M}}^{2 + }}\left( {aq} \right){\rm{ \backslash rightleftharpoons M}}{{\rm{R}}_2}\left( s \right){\rm{ + 2 }}{{\rm{H}}^ + }\left( {aq} \right){\rm{ }}...{\rm{ }}...\left( 1 \right){\rm{ }}\]

In the next step, water is passed through anion exchange resin, anions, such as chloride, bicarbonate and sulfate (from water to be demineralization) are exchanged with hydroxide (from anion exchange resin). In this step, water becomes free from anions such as chloride, bicarbonate and sulfate.

\[{\rm{ RNH}}_3^ + {\rm{O}}{{\rm{H}}^ - }\left( s \right){\rm{ + }}{{\rm{X}}^ + }\left( {aq} \right){\rm{ \leftrightharpoons RNH}}_3^ + {{\rm{X}}^ - }\left( s \right){\rm{ + O}}{{\rm{H}}^ - }\left( {aq} \right){\rm{ }}...{\rm{ }}...\left( 2 \right){\rm{ }}\]

Thus, in the above two processes, water is free from all soluble minerals. This water is demineralization water.

Protons from the first step combine with hydroxide ions from the second step to form water.
\[{{\rm{H}}^ + }\left( {aq} \right){\rm{ + O}}{{\rm{H}}^ - }\left( {aq} \right){\rm{ \leftrightharpoons }}{{\rm{H}}_2}{\rm{O}}\left( l \right)\]

Note:
For demineralization, water should be passed through both cation exchange resin and anion exchange resin.