Question

What is meant by demineralised water? (a) Water free from cations and anions. (b) Water free from minerals dissolved in it. (c) Water free from impurities. (d) Water free from \(\mathrm{Na}^{+}\)and \(\mathrm{K}^{+}\)ions.

Short answer

Demineralised water refers to water that is free from cations and anions, represented by option (a).

Step-by-step solution

- Understanding the concept of demineralised water

Demineralised water refers to water that has had most or all of its mineral and salt ions removed, such as calcium, magnesium, sodium, chloride, sulfate, and bicarbonate. This can be achieved through processes like distillation, deionization, or reverse osmosis.

- Analyzing the options given

Considering the definition from Step 1, we have to identify which option best describes water with removed mineral and salt ions. Option (a) mentions freedom from cations and anions, reflecting the ionic composition of minerals in water. Option (b) also looks similar, but it's broader since not all minerals in water are necessarily in ionic form. Option (c) is too general as impurities can include organic matter, bacteria, and more, not just minerals. Option (d) specifies only two types of ions, making it incorrect as demineralisation involves removing a broad spectrum of ions, not just sodium and potassium.

- Selecting the most accurate option

Based on the analysis in Step 2, the term 'demineralised water' most accurately pertains to water from which a wide variety of both cations and anions have been removed. Although option (b) is technically correct, option (a) specifically lists both cations and anions, which aligns more closely with the demineralisation process. Therefore, option (a) is the best choice.

Key concepts

Deionization

Deionization is a water purification method that removes ions, which are charged particles, from water. This is achieved by passing water through a medium that contains electrically charged resins. Cations (positive ions) from the water are exchanged with hydrogen ions, and anions (negative ions) are exchanged with hydroxide ions. When hydrogen and hydroxide ions combine, they form pure water (H2O), leaving the unwanted ions behind in the resin.

The process of deionization is especially critical in environments where highly pure water is required, such as in laboratories or the medical field. It is important to note that while deionization effectively removes salt and mineral ions, it may not eliminate all forms of impurities, such as organic compounds or microorganisms.

Advantages and Disadvantages of Deionization:

• Advantages: Effective at removing ions, causing a significant reduction in water conductivity, which is desirable in certain technical applications.
• Disadvantages: Does not remove non-ionic contaminants and requires regular resin replacement or regeneration to maintain efficacy.

Reverse Osmosis

Reverse osmosis (RO) is another water treatment process that forces water through a semi-permeable membrane. The membrane's small pores allow water molecules to pass through, but block larger molecules, such as dissolved salts (ions), metals, and organic compounds. This is essentially opposite to the natural osmosis process where solvent moves from a low concentration to a high concentration.

RO is widely used in both industrial and domestic settings for producing pure water. It is one of the most effective technologies for reducing a broad range of contaminant levels including sediment, chlorine, and volatile organic compounds (VOCs).

Application Examples of Reverse Osmosis:

• Desalination of seawater or brackish water for drinking water.
• Concentration of foods and pharmaceuticals.
• Water purification in aquariums and water recycling systems.

Distillation Process

Distillation is the process of purifying water through heating it to create steam and then cooling that steam to return it to a liquid state, thereby leaving behind most of the solid contaminants. It operates on the principle that different substances have different boiling points, so by carefully controlling the temperature, one can selectively boil off and condense the purest part of the solution (the solvent, which is water, in this case).

While distillation is a fairly simple and ancient technique, it requires a significant amount of energy to heat the water. Therefore, it is less commonly used for large-scale water purification than methods like reverse osmosis or deionization. Distillation is more suited for small-scale applications where exceptionally high purity is required or where other methods are not practical.

Key Uses of Distilled Water:

• In autoclaves and sterilization processes in medical settings.
• For lead-acid batteries and laboratory use where mineral-free water is crucial.
• In food and beverage production where specific mineral content is required for taste or health reasons.