NDA (II) 2017 General Ability Test · Question 120 of 150

Correct — D, It is an oxidizing agent. Very nearly every use of potassium permanganate depends on its power to oxidise, and water treatment is no exception. In a water works it oxidises dissolved iron and manganese into insoluble oxides that a filter can then trap, destroys hydrogen sulphide and with it the rotten-egg smell, and breaks down the organic matter responsible for bad taste and odour. The same oxidising action attacks microbial cells, so any germ-killing it does is a consequence of the property named in this option rather than a separate mechanism.

• (a)It kills germs — This is the most tempting option because it is not simply false — but it names an effect, while the stem asks why. The killing follows from the oxidation, and in any case the compound is a poor disinfectant beside chlorine or ultraviolet treatment, so this is not the reason it is used.
• (b)It dissolves the impurities — It does the opposite of dissolving them. Oxidising dissolved iron and manganese turns them into solids that settle or are filtered out, which is how they leave the water.
• (c)It is a reducing agent — The chemistry is inverted. Permanganate is a classic oxidising agent, with manganese in its highest oxidation state; it accepts electrons rather than donating them.

In permanganate the manganese is in its highest oxidation state, so the ion has nowhere to go but down, and it takes electrons from whatever it meets. That is what an oxidising agent does, and it is why potassium permanganate serves so many roles — a laboratory titrant, a bleach, a first-aid antiseptic wash and a water-treatment chemical. In water works it is dosed ahead of filtration, precisely so that the solids it creates can be removed downstream.

The question rewards a candidate who distinguishes property from purpose. Two of the options describe outcomes of treatment and only one names a chemical property, which is the shape the key follows. A note on how practice has moved since 2017 is worth carrying: potassium permanganate was historically used to disinfect drinking water, and it is still handy for iron and manganese removal, but current guidance discourages its use as a field disinfectant because the dose is hard to judge. Chlorination, which works through hypochlorous acid, and ultraviolet treatment, which damages microbial DNA, are the standard methods today. A pink tinge in the water is a visible sign that permanganate has been added, and historically served as a rough dosing indicator.

• Almost all applications of potassium permanganate exploit its oxidising properties.
• In water treatment it oxidises dissolved iron and manganese to insoluble oxides, which are then removed by filtration.
• It destroys hydrogen sulphide, removing the rotten-egg odour, and oxidises taste and odour causing organic matter.
• It is rated a poor disinfectant compared with modern alternatives, and present-day guidance discourages field use because correct dosing is difficult.
• Chlorination disinfects through hypochlorous acid formed when chlorine dissolves in water, and ultraviolet treatment works by inactivating microorganisms.

• Selecting the outcome rather than the property when a question asks why.
• Calling permanganate a reducing agent because it takes part in so many reactions.
• Assuming a chemical that colours the water is dissolving the impurities.

NDA asks for the property behind a named use, or which listed chemical is used for a stated water-treatment job.

• practice — not a real PYQ

  In potassium permanganate, the manganese is in

  • (a)its lowest oxidation state, so the compound is a reducing agent
  • (b)its highest oxidation state, so the compound is an oxidising agent
  • (c)the zero oxidation state, so the compound is inert
  • (d)a negative oxidation state

  Answer(b) its highest oxidation state, so the compound is an oxidising agent — it can only accept electrons.

• practice — not a real PYQ

  The disinfecting action of chlorine added to drinking water is mainly due to the formation of

  • (a)hydrochloric acid
  • (b)hypochlorous acid
  • (c)hydrogen gas
  • (d)sodium chloride

  Answer(b) hypochlorous acid — the weak acid formed when chlorine dissolves, which oxidises and kills pathogens.