The formula of baking soda is :

Question

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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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2024 Q66 — Science & Technology PYQ with Answer

State PCS 2024 Q66 — Science & Technology PYQ with Answer