NDA (I) 2017 General Ability Test · Question 126 of 149

Correct — B, NaHCO₃. Baking soda is sodium hydrogencarbonate, also written as sodium bicarbonate, a white crystalline solid that is a mild base. Its usefulness in the kitchen comes from one reaction: on heating, or on meeting an acid such as the tartaric acid in baking powder or the citric acid in a lemon, it releases carbon dioxide, and the bubbles of that gas make dough rise. The same reaction explains its other uses — as an antacid, because it neutralises excess hydrochloric acid in the stomach, and as the active ingredient in soda-acid fire extinguishers, where the carbon dioxide it gives off smothers a flame.

• (a)Na₂CO₃ — Sodium carbonate is washing soda, not baking soda, and in the laboratory it is usually met as the decahydrate. It is a much stronger base than baking soda, is used to soften hard water and to make glass and soap, and is not eaten. Confusing the carbonate with the hydrogencarbonate is the trap the question is built on.
• (c)CaCO₃ — Calcium carbonate is limestone, marble and chalk, and it is also the raw material of the Solvay process. It shares the carbonate group with the answer but has calcium in place of sodium, and it is not soluble enough to work in dough.
• (d)NaOH — Sodium hydroxide is caustic soda, a strongly corrosive alkali made by the chlor-alkali process and used in soap making and in cleaning agents. It carries no carbon at all, so it can release no carbon dioxide and could never be used in food.

Baking soda is made from common salt by the Solvay process, in which brine saturated with ammonia takes up carbon dioxide and the sparingly soluble sodium hydrogencarbonate settles out. Heating that solid drives off water and carbon dioxide and leaves sodium carbonate, which is washing soda, so the two compounds are two stages of the same chemistry. Baking powder is not another name for baking soda; it is baking soda mixed with a solid edible acid, so that carbon dioxide is released as soon as the mixture is wetted rather than only when it is heated.

Read the stem as it is printed — it asks for the 'chemical name' and then offers four formulae, so what it really wants is the formula. Nothing about the chemistry changes. Sort the four by the common names attached to them, since that is how these compounds are examined: baking soda, washing soda, caustic soda and limestone. Keep the sodium compounds apart by the hydrogen — the one you eat has an H in it, the one you wash with does not. That single letter is the whole question.

• Baking soda is sodium hydrogencarbonate, NaHCO₃; washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O.
• On heating, baking soda decomposes to sodium carbonate, water and carbon dioxide, and it is the carbon dioxide that makes dough rise.
• Baking powder is baking soda mixed with a mild edible acid such as tartaric acid.
• Baking soda is made from common salt through the Solvay process and is used as an antacid and in fire extinguishers.

• Swapping the carbonate for the hydrogencarbonate; only the latter is baking soda.
• Treating baking powder and baking soda as the same thing.
• Assuming every substance called a 'soda' is a sodium carbonate — caustic soda is a hydroxide.

NDA asks for the formula behind a common chemical name, or matches a set of common names with their formulae.

• practice — not a real PYQ

  When baking soda is heated strongly, the gas evolved is

  • (a)oxygen
  • (b)hydrogen
  • (c)carbon dioxide
  • (d)ammonia

  Answer(c) carbon dioxide — sodium hydrogencarbonate breaks down into sodium carbonate, water and carbon dioxide, which is what makes a cake rise.

• practice — not a real PYQ

  Washing soda has the chemical formula

  • (a)NaHCO₃
  • (b)Na₂CO₃·10H₂O
  • (c)NaOH
  • (d)CaOCl₂

  Answer(b) Na₂CO₃·10H₂O — sodium carbonate decahydrate, used to soften hard water, is distinct from the hydrogencarbonate used in baking.