NDA (II) 2017 General Ability Test · Question 139 of 150

Correct — A, 6.5 to 8.5. This is the band used by the World Health Organization and by the European Union drinking-water standards, and it is also the acceptable limit laid down in the Indian standard IS 10500:2012, where pH is one of the few parameters allowed no relaxation at all. The range is set on both sides for practical reasons rather than for taste alone. Water more acidic than about 6.5 attacks metal pipes and fittings and can dissolve lead and copper into the supply; water more alkaline than about 8.5 tastes soapy or bitter, encourages scale, and makes chlorine disinfection less effective. A band straddling neutrality on both sides is therefore what a standard specifies.

• (b)5·0 to 6·5 — Entirely on the acidic side and below neutral. Water in this range is corrosive to the distribution system and would leach metals from pipes; no drinking-water standard sets its desirable band here.
• (c)6·5 to 7·0 — It has the right lower bound but stops at neutrality. Mildly alkaline water up to about 8.5 is perfectly acceptable, and hard groundwater in much of India sits above pH 7, so a ceiling of 7.0 would fail supplies that are entirely safe.
• (d)7·0 to 8·5 — The mirror error — the correct upper bound but a floor set at neutral. Slightly acidic water down to 6.5 remains within the standard, so this range is too narrow at the lower end.

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is, with 7 as neutral at ordinary temperature. It is a logarithmic scale, so a fall of one unit means a tenfold rise in hydrogen-ion concentration. For drinking water, pH is classed as an operational and aesthetic parameter rather than a direct health parameter: the value itself rarely makes water unsafe, but it governs corrosion of the pipe network, the solubility of metals, the formation of scale and the efficiency of chlorination.

Numerical-range items reward two checks. First, the answer will almost always straddle the natural or ideal value rather than sit to one side of it, so a band centred on neutral is more likely than one entirely above or below it. Second, look for the option whose bounds appear in a real standard — 6.5 and 8.5 are the numbers that recur across the WHO guidelines, the European Union standard and IS 10500. The three wrong options here are built by shifting one bound at a time, which is the usual way such distractors are made.

• The acceptable pH range for drinking water is 6.5 to 8.5 under the WHO guidelines, the EU standard and Indian standard IS 10500:2012.
• In IS 10500:2012 pH is one of the parameters for which no relaxation is permitted.
• pH is a logarithmic scale — one unit corresponds to a tenfold change in hydrogen-ion concentration.
• Water below the lower bound corrodes pipes and can dissolve lead and copper into supply.
• Water above the upper bound tastes soapy, forms scale and reduces the effectiveness of chlorine disinfection.

• Assuming drinking water must be exactly neutral at pH 7.
• Reading pH as a measure of purity; it says nothing about bacteria or dissolved solids.
• Forgetting that the scale is logarithmic, so small differences in pH are large differences in acidity.

Water quality appears as a numerical-range item like this one, as a parameter-to-limit match, or as a question on which contaminant causes which disease.

• practice — not a real PYQ

  A fall of one unit on the pH scale corresponds to a change in hydrogen-ion concentration by a factor of

  • (a)2
  • (b)5
  • (c)10
  • (d)100

  Answer(c) 10 — the scale is logarithmic, so each unit is a tenfold change.

• practice — not a real PYQ

  Drinking water with a pH well below the acceptable lower limit is undesirable mainly because it

  • (a)kills all bacteria in the supply
  • (b)corrodes pipes and can dissolve metals into the water
  • (c)increases the fluoride content
  • (d)raises the total dissolved solids

  Answer(b) corrodes pipes and can dissolve metals into the water — the chief practical objection to acidic supply.