UPPSC 2024 Prelims GS Paper-I · Question 66 of 150

Correct — D, NaHCO₃. Baking soda is the common name of sodium hydrogencarbonate, also written sodium bicarbonate: one sodium ion, Na⁺, paired with one hydrogencarbonate ion, HCO₃⁻. The formula can be read straight out of the chemical name, and that is the safest way to answer this item under time pressure. 'Sodium' fixes the metal as Na, which eliminates the two potassium options at once. 'Bi-carbonate', or in modern nomenclature 'hydrogencarbonate', fixes the anion as HCO₃⁻ — carbonate with one hydrogen attached and therefore a single negative charge — which eliminates the plain carbonates whose anion is CO₃²⁻ with no hydrogen. Only NaHCO₃ satisfies both halves of the name. Industrially it comes from the Solvay or ammonia-soda process, in which brine saturated with ammonia is treated with carbon dioxide: NaCl + NH₃ + CO₂ + H₂O gives NaHCO₃ + NH₄Cl, and the sparingly soluble bicarbonate separates out first. Chemically, baking soda dissolves to give a mildly alkaline solution and releases carbon dioxide when heated or treated with an acid: 2NaHCO₃ on heating gives Na₂CO₃ + H₂O + CO₂. That one reaction explains every household use it has. The carbon dioxide bubbles trapped in dough make a cake rise; the mild alkali neutralises excess hydrochloric acid in the stomach, which is why it works as an antacid; and the gas released smothers a flame, which is why it is the active ingredient in soda-acid and dry-chemical fire extinguishers.

• (a)KHCO₃ — Potassium hydrogencarbonate. The anion is right — HCO₃⁻, the bicarbonate — but the metal is potassium, not sodium, and baking soda is by name a sodium salt. KHCO₃ is a genuine and useful compound in its own right: it serves as a leavening agent and antacid where sodium intake has to be limited, and it is the powder in Purple-K dry-chemical fire extinguishers. It simply is not baking soda. This option is designed to catch a candidate who remembers 'bicarbonate' but not which metal.
• (b)Na₂CO₃ — Sodium carbonate — and this is the classic trap of the question, because it is the other, more famous 'soda'. Anhydrous Na₂CO₃ is soda ash, and its decahydrate, Na₂CO₃·10H₂O, is washing soda, used for cleaning, for softening hard water and in the manufacture of glass, soap and paper. It is far more strongly alkaline than baking soda and is not edible. The metal is correct here; the anion is not, because the carbonate ion CO₃²⁻ carries no hydrogen. UPSC set precisely this trap in 2005 with a statement claiming that anhydrous sodium carbonate is commonly known as baking soda, which was false.
• (c)K₂CO₃ — Potassium carbonate, known historically as potash or pearl ash, used in glass-making, in soft soaps and as a mild drying agent. It is wrong on both counts at once — wrong metal (potassium instead of sodium) and wrong anion (carbonate instead of hydrogencarbonate) — which makes it the option furthest from baking soda and the easiest of the three to eliminate.

A small family of sodium and potassium salts supplies most of the everyday chemistry that state PCS papers test, and the four options here are the whole family. Sodium hydrogencarbonate, NaHCO₃, is baking soda; sodium carbonate decahydrate, Na₂CO₃·10H₂O, is washing soda, with anhydrous Na₂CO₃ known as soda ash; sodium hydroxide, NaOH, is caustic soda; and the potassium analogues K₂CO₃ and KHCO₃ are potash and potassium bicarbonate. All the sodium compounds trace back to common salt through two great industrial routes: the Solvay process, which makes sodium bicarbonate and hence sodium carbonate from brine, ammonia and carbon dioxide, and the chlor-alkali process, which electrolyses brine to give sodium hydroxide, chlorine and hydrogen.

The examinable skill here is translating between a common name and a formula, and the reliable method is to decompose the name rather than to recall the formula as a picture. The metal prefix — sodium or potassium — fixes Na or K. The anion word decides the rest: 'carbonate' is CO₃²⁻ with no hydrogen and a double negative charge, so it needs two sodiums (Na₂CO₃); 'bicarbonate' or 'hydrogencarbonate' is HCO₃⁻ with one hydrogen and a single negative charge, so it needs only one (NaHCO₃). The prefix 'bi-' is a historical survival meaning that this salt has twice as much carbonate per sodium as the normal carbonate does, not that there are two of anything. Because the difference between the right answer and the classic distractor is a single hydrogen atom, candidates who have learned the two 'sodas' only as words rather than as formulas lose this mark; candidates who can also say what each one is used for never do.

• Baking soda is sodium hydrogencarbonate (sodium bicarbonate), NaHCO₃; washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, and anhydrous sodium carbonate is soda ash; caustic soda is sodium hydroxide, NaOH.
• Solvay (ammonia-soda) process: NaCl + NH₃ + CO₂ + H₂O gives NaHCO₃ + NH₄Cl. Heating the bicarbonate gives sodium carbonate — 2NaHCO₃ gives Na₂CO₃ + H₂O + CO₂ — which is how one soda is turned into the other.
• The uses of baking soda all follow from carbon dioxide release and mild alkalinity: baking powder is baking soda mixed with an edible acid such as tartaric acid; as an antacid it neutralises excess hydrochloric acid in the stomach; and it supplies the CO₂ in soda-acid and dry-chemical fire extinguishers.
• The potassium analogues are different compounds: K₂CO₃ is potash or pearl ash, used in glass and soft soap, and KHCO₃ is potassium bicarbonate, used as a low-sodium leavening agent and in Purple-K fire extinguishers.
• The naming rule that settles this question: 'hydrogencarbonate' or 'bicarbonate' means HCO₃⁻ — one hydrogen, one negative charge, so one sodium; 'carbonate' means CO₃²⁻ — no hydrogen, two negative charges, so two sodiums.

• Confusing Na₂CO₃ with NaHCO₃. The entire difference is a single hydrogen atom, but one is washing soda for cleaning and the other is edible baking soda.
• Assuming that the word 'soda' always means the same compound. Baking soda (NaHCO₃), washing soda (Na₂CO₃·10H₂O), soda ash (anhydrous Na₂CO₃) and caustic soda (NaOH) are four distinct substances.
• Being caught by the potassium look-alikes. KHCO₃ and K₂CO₃ are real compounds with real uses, so they cannot be dismissed as nonsense options — check the metal first, then the anion.

UPPSC asks the formula or the common name outright, or puts it into a 'which pair is NOT correctly matched' item on substances and their uses. UPSC prefers to bury it — in a matching list of common and chemical names, as in 1998, or in a multi-statement item that deliberately mixes baking soda with washing soda and with fire extinguishers, as in 2005.

• practice — not a real PYQ

  The chemical name of washing soda is :

  • (a)Sodium hydrogencarbonate
  • (b)Sodium carbonate decahydrate
  • (c)Sodium hydroxide
  • (d)Calcium oxychloride

  Answer(b) Sodium carbonate decahydrate, Na₂CO₃·10H₂O — washing soda. Sodium hydrogencarbonate (NaHCO₃) is baking soda, sodium hydroxide (NaOH) is caustic soda and calcium oxychloride is bleaching powder.

• practice — not a real PYQ

  Baking soda is used in soda-acid fire extinguishers mainly because it :

  • (a)absorbs the oxygen present in air
  • (b)releases carbon dioxide on reacting with the acid
  • (c)forms a strong alkali that burns out the fuel
  • (d)lowers the temperature by evaporating rapidly

  Answer(b) releases carbon dioxide on reacting with the acid — the CO₂ produced is heavier than air, forms a blanket over the burning material and cuts off its oxygen supply. The same gas release makes baking soda a leavening agent in baking powder.