NDA (I) 2018 General Ability Test · Question 66 of 150

Correct — A, NaCl. Brine is the chemist's name for a concentrated solution of common salt, sodium chloride, in water. It is what sea water becomes as it is concentrated in a salt pan, and it is the starting material of the chlor-alkali industry — passing electricity through brine gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide in the solution, and from those three products come bleaching powder, hydrochloric acid, washing soda and baking soda. All four options in this item are sodium compounds, and the one that dissolves in water to make brine is the chloride.

• (b)NaOH — Sodium hydroxide is caustic soda, and it is a product of the electrolysis of brine rather than brine itself. A solution of it is strongly alkaline and would burn the skin; brine merely tastes salty.
• (c)NaHCO₃ — Sodium hydrogen carbonate is baking soda. It is made from brine by the Solvay process, in which brine saturated with ammonia is treated with carbon dioxide, so it too is downstream of brine and not the same thing.
• (d)Na₂CO₃ — Sodium carbonate is washing soda, obtained by heating the baking soda made in the Solvay process. Like the other two, it comes out of the brine route rather than defining it.

Common salt, sodium chloride, is obtained by evaporating sea water in shallow salt pans and by mining rock salt deposits left by ancient seas. Its concentrated solution is called brine, and brine is the single most important raw material in the sodium branch of the chemical industry. The electrolysis of brine, known as the chlor-alkali process because of the two main products, yields sodium hydroxide, chlorine and hydrogen; the Solvay process converts brine into sodium hydrogen carbonate and then into sodium carbonate.

Notice how the question is built. All four options are sodium compounds and three of them are made from the first, so a candidate who knows only that 'brine is salty' still has to decide which sodium compound the salt is. The way to lock this family down is to draw the family tree once — brine at the top, then caustic soda, chlorine and hydrogen from electrolysis, then bleaching powder from chlorine and slaked lime, hydrochloric acid from hydrogen and chlorine, and baking soda and washing soda from the Solvay route. Once that tree is in your head, several NDA chemistry items in every paper answer themselves, including the item in a later paper that asks which compound common salt is NOT the raw material for.

• Brine is a concentrated aqueous solution of sodium chloride, NaCl.
• Electrolysis of brine gives chlorine, hydrogen and sodium hydroxide — the chlor-alkali process.
• Common salt is obtained by evaporating sea water in salt pans and by mining rock salt.
• Baking soda and washing soda are made from brine by the Solvay process.
• Plaster of Paris, by contrast, comes from gypsum and has nothing to do with common salt.

• Confusing brine with caustic soda solution, which is a product of brine rather than brine itself.
• Mixing up baking soda and washing soda, which differ by one sodium and by ten waters of crystallisation.
• Assuming every sodium compound in the syllabus comes from salt — plaster of Paris does not.

NDA asks either for the identity of brine or for the products of its electrolysis, and sometimes as an odd-one-out among compounds made from common salt.

• practice — not a real PYQ

  Electrolysis of brine yields chlorine at the anode and which gas at the cathode?

  • (a)Oxygen
  • (b)Hydrogen
  • (c)Carbon dioxide
  • (d)Nitrogen

  Answer(b) Hydrogen — the chlor-alkali process gives chlorine, hydrogen and sodium hydroxide.

• practice — not a real PYQ

  The chemical name of common salt is

  • (a)sodium carbonate
  • (b)sodium hydroxide
  • (c)sodium chloride
  • (d)sodium bicarbonate

  Answer(c) sodium chloride — its concentrated solution in water is called brine.