What is the correct sequence of the following salts in ascending order according to their presence in the sea water? 1. Calcium Carbonate 2. Calcium Sulphate 3. Magnesium Chloride Select the answer using the code given below:

Correct — C, 1, 2, 3. Sea water carries about 35 grams of dissolved salt in every kilogram, and that salt is very unevenly distributed among the compounds. Common salt, sodium chloride, dominates at roughly three-quarters of the total. Magnesium chloride is the clear second, at about a tenth. Calcium sulphate, the gypsum-forming salt, is smaller again at a few per cent, and calcium carbonate is a minor constituent measured in tenths of a per cent — which is unsurprising, since marine organisms are constantly withdrawing carbonate to build shells and coral skeletons. Ascending means smallest first, so the order runs calcium carbonate, then calcium sulphate, then magnesium chloride: 1, 2, 3. You can rebuild this from the ion table rather than memorising it — magnesium is present at about three times the concentration of calcium, and the carbonate-bicarbonate pool is smaller than either.

• (a)3, 1, 2 — Puts magnesium chloride first, which reverses the largest term in the list — it is the most abundant of the three, not the least.
• (b)1, 3, 2 — Gets calcium carbonate right as the smallest but then swaps the other two. Calcium sulphate is present in smaller quantity than magnesium chloride, not larger.
• (d)2, 1, 3 — Places calcium sulphate below calcium carbonate. Sea water holds far more calcium than bicarbonate, and most of that calcium pairs with the abundant sulphate ion, so the sulphate salt is several times the carbonate.

Salinity is the total content of dissolved salts in sea water, expressed as grams of salt per kilogram of water and written in parts per thousand. The open ocean averages about 35 parts per thousand, and a water body needs at least 24.7 to count as a sea in this scheme. What is remarkable is not the amount but the constancy of the proportions — whatever the salinity of a sample, the ratio of chloride to sodium to sulphate to magnesium is very nearly the same everywhere, because the ocean mixes far faster than any of these ions is added or removed.

The safe way to attack a sequence question on sea-salt composition is to think in ions, which are what oceanographers actually measure, and only then pair them into compounds. Chloride and sodium tower over everything else, so sodium chloride is the bulk of the salt. Next comes sulphate, then magnesium; magnesium pairs mostly with chloride, which is why magnesium chloride is the runner-up compound. Calcium is roughly a third of magnesium, and the carbonate and bicarbonate pool is smaller still, which puts the two calcium salts at the bottom, with the carbonate last because biology is constantly taking it out of solution.

• Salinity is expressed as grams of salt dissolved in 1,000 grams of sea water, and the open ocean normally runs between 33 and 37 parts per thousand.
• Per kilogram of sea water the major ions are chlorine about 18.97 g, sodium about 10.47 g, sulphate about 2.65 g, magnesium about 1.28 g, calcium about 0.41 g, potassium about 0.38 g and bicarbonate about 0.14 g.
• In compound terms sodium chloride is roughly three-quarters of the dissolved salt, magnesium chloride about a tenth, calcium sulphate a few per cent and calcium carbonate only a fraction of one per cent.
• The relative proportions of the major ions stay nearly constant across the oceans even where total salinity varies.
• Enclosed water bodies reach far higher figures — Lake Van in Turkey about 330, the Dead Sea about 238 and the Great Salt Lake about 220 parts per thousand.

• Reading 'ascending' as 'descending' — the commonest way this question is lost.
• Assuming calcium is more abundant than magnesium in sea water; it is roughly a third as abundant.
• Treating the carbonate content as large because coral and shells are made of it — the carbonate pool in solution is small precisely because organisms keep removing it.

As an ascending or descending sequence of the constituents of sea water, or as a single-answer question on which salt is most abundant after sodium chloride.

• practice — not a real PYQ

  After sodium chloride, which one among the following salts is present in the largest proportion in sea water?

  • (a)Calcium carbonate
  • (b)Magnesium chloride
  • (c)Potassium sulphate
  • (d)Magnesium bromide

  Answer(b) Magnesium chloride — magnesium is the third most abundant ion after chloride and sodium, and it pairs mostly with chloride.

• practice — not a real PYQ

  The salinity of the open ocean normally lies closest to which one of the following values?

  • (a)3.5 parts per thousand
  • (b)35 parts per thousand
  • (c)150 parts per thousand
  • (d)238 parts per thousand

  Answer(b) 35 parts per thousand — the normal open-ocean range is about 33 to 37; 238 is the Dead Sea, which is not open ocean.