MPSC 2026 Prelims Paper-I · Question 66 of 100

Correct — option (2), '(b), (c) and (d)'. Acids are sorted into two families by where they come from and how they are built. Mineral acids, also called inorganic acids, are prepared from mineral sources and have no carboxyl group; the three classical members are sulphuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl), and phosphoric acid (H3PO4) is usually added to the list. Organic acids come from plant and animal material and carry the carboxyl group -COOH: acetic acid in vinegar, citric acid in lemon and orange, lactic acid in sour milk, formic acid in the sting of an ant or nettle, tartaric acid in tamarind and grapes, malic acid in apple. Now take the four statements in turn. Statement (a), oxalic acid, has the formula (COOH)2 — two carboxyl groups joined together, the simplest dicarboxylic acid. It occurs naturally in tomato, spinach and wood sorrel, the plant Oxalis from which it takes its name, and it is an organic acid, not a mineral one. Statements (b), (c) and (d) are the three classical mineral acids. So exactly three of the four qualify, and the option that names those three is option (2). The disciplined method here is to classify each statement independently before looking at the option list at all — sort first, then read off the option that matches the sort. Working the other way round, by testing options against half-remembered statements, is how a candidate ends up choosing an option that happens to contain the one acid he is sure about.

• (1)Only (a) — This is the exact inversion of the truth: it names the one organic acid in the list and rejects the three mineral acids. It is the answer a candidate reaches by misreading the stem as 'identify the acid that is not a mineral acid', or by half-remembering that oxalic acid is somehow the odd one out and then attaching that memory to the wrong question. Oxalic acid is indeed the odd one out — which is why it is the one statement that must be excluded, not the one that must be selected. Whenever a question asks you to identify members of a category and one item is conspicuously different, check which way the stem is pointing before you mark.
• (3)Only (d) — Hydrochloric acid is certainly a mineral acid, so this option is not wrong about what it contains — it is wrong about what it leaves out. Sulphuric acid and nitric acid are mineral acids on exactly the same footing, and no defensible definition admits HCl while excluding H2SO4 and HNO3. An option of the form 'Only (x)' in a question whose statement list contains three obvious members of the category is asking you to doubt yourself; the cure is to have classified all four before you look at the options, so that a partial option can be discarded on sight.
• (4)Only (b) and (c) — Sulphuric and nitric acid are correctly included, but hydrochloric acid is dropped for no reason that survives inspection. HCl is a mineral acid by every criterion in use: it is made from inorganic starting materials, it carries no carboxyl group, and it is one of the three strong acids of the school laboratory. It also occurs in the human stomach, where the gastric glands secrete it to give the stomach its acidic medium — a fact that sometimes misleads candidates into filing it as 'biological' and therefore organic. Origin in a living body is not the test; the presence of the carboxyl group and the inorganic constitution of the molecule are.

The mineral-versus-organic split is a classification by constitution and source rather than by strength. A mineral acid is built entirely from inorganic components; an organic acid is built on a carbon skeleton carrying one or more carboxyl (-COOH) groups, and is characteristically obtained from plant or animal material. Strength is a separate axis altogether and must not be substituted for the classification: hydrochloric, sulphuric and nitric acid are strong, but carbonic acid and boric acid are weak and are still mineral acids, while among organic acids some are weak and a few are very strong. There is also one honest exception to the shortcut 'contains carbon means organic' — carbonic acid, H2CO3, contains a carbon atom and is nonetheless classed as an inorganic acid, because it is simply carbon dioxide dissolved in water and carries no carboxyl group of the organic kind.

Each of the three mineral acids here is worth knowing for its own sake, because general-studies papers ask about their uses as often as about their classification. Sulphuric acid is manufactured in the greatest quantity of any industrial chemical and is often called the king of chemicals; it is used in fertiliser manufacture, in petroleum refining, in metal processing and in lead storage batteries, and concentrated sulphuric acid is a powerful dehydrating agent. Nitric acid is central to the making of explosives and nitrogenous fertilisers; mixed with concentrated hydrochloric acid in a ratio of one part nitric to three parts hydrochloric it forms aqua regia, the mixture that will dissolve gold and platinum, which neither acid attacks alone. Hydrochloric acid is used in cleaning metal surfaces and in the chemical industry, and it is also secreted by the gastric glands to give the human stomach its strongly acidic medium. Oxalic acid, the organic member of the set, is best known outside the laboratory through its calcium salt, calcium oxalate, which is the main chemical constituent of most human kidney stones.

• Mineral (inorganic) acids: sulphuric acid H2SO4, nitric acid HNO3, hydrochloric acid HCl and phosphoric acid H3PO4 are the standard members. They are prepared from inorganic sources and carry no carboxyl group.
• Organic acids carry the carboxyl group -COOH and come from plant or animal material: acetic acid in vinegar, citric acid in citrus fruit, lactic acid in sour milk and curd, formic acid in ant and nettle stings, tartaric acid in tamarind and grapes, malic acid in apple.
• Oxalic acid is (COOH)2, the simplest dicarboxylic acid, found in tomato, spinach and wood sorrel (Oxalis, which gives it its name). It is an organic acid. Its calcium salt, calcium oxalate, is the chief constituent of most human kidney stones.
• Strength is a separate axis from the mineral/organic classification. Carbonic acid and boric acid are weak yet inorganic; carbonic acid also shows that 'contains carbon' is not by itself a test for an organic acid.
• Aqua regia is a mixture of concentrated hydrochloric and concentrated nitric acid in a 3:1 ratio by volume and dissolves gold and platinum, which neither acid dissolves on its own. Sulphuric acid, the most heavily manufactured industrial chemical, is used in fertilisers, refining, metal processing and lead storage batteries.

• Marking option (1) because oxalic acid stands out from the other three. It does stand out — as the one that must be excluded. Read the direction of the stem before letting an odd-one-out instinct decide.
• Using strength as the test for 'mineral'. Carbonic acid and boric acid are weak mineral acids, so a weak acid is not automatically organic.
• Filing hydrochloric acid as biological because the stomach secretes it. Where an acid is found says nothing about how it is classified; the carboxyl group and the constitution of the molecule do.

State-service papers ask this topic in two shapes. The first is classification, as here — a mixed list of acids with the mineral or the organic members to be picked out, sometimes with a weak inorganic acid such as carbonic or boric slipped in to punish anyone using strength as a proxy. The second is the source pairing, where an acid must be matched to the fruit, plant or animal it occurs in: citric with citrus, tartaric with tamarind, lactic with curd, formic with ants, oxalic with tomato and spinach, malic with apple. A third, rarer shape asks about a use — aqua regia, the acid in a lead storage battery, the acid of the stomach. All three are covered by one table of acid, formula, class, source and use, and none of them by remembering only that acids taste sour and turn litmus red.

• practice — not a real PYQ

  Which one of the following acids is present in tomato and spinach, and forms the salt that is the chief constituent of most human kidney stones?

  • (a)Tartaric acid
  • (b)Oxalic acid
  • (c)Lactic acid
  • (d)Formic acid

  Answer(b) Oxalic acid — an organic dicarboxylic acid, (COOH)2, found in tomato, spinach and wood sorrel. Its calcium salt, calcium oxalate, is the main chemical constituent of most kidney stones. Tartaric acid occurs in tamarind and grapes, lactic acid in sour milk and curd, and formic acid in the sting of ants and nettles.

• practice — not a real PYQ

  Aqua regia, which is able to dissolve gold and platinum, is a mixture of concentrated :

  • (a)Sulphuric acid and nitric acid
  • (b)Hydrochloric acid and nitric acid
  • (c)Hydrochloric acid and sulphuric acid
  • (d)Nitric acid and phosphoric acid

  Answer(b) Hydrochloric acid and nitric acid — in a ratio of three parts concentrated HCl to one part concentrated HNO3 by volume. Neither acid dissolves gold on its own; the mixture does, which is why the name means 'royal water'. All the acids named in the options are mineral acids.