EPFO EO/AO 2023 General Ability Test · Question 89 of 120

Correct — A, (a) Tamarind. Tartaric acid is the organic acid that gives tamarind its sharp sourness, and tamarind pulp is one of the richest natural sources of it. Grapes are the other well-known source, and the acid takes its name from the deposit of tartar that forms in a wine cask, from which it was first obtained. The item belongs to a group of questions that all rest on one small table pairing a common natural material with the acid responsible for its taste. Each of the other three options is in that table too, opposite a different acid: tomato contains oxalic acid, orange and the other citrus fruits contain citric acid, and vinegar is a dilute solution of acetic acid. So the question is decided by which pairing the candidate has learned, and every option is a real pairing pointing to the wrong acid. Two pieces of context make tartaric acid easier to remember than a bare name. First, it is a working ingredient in every kitchen that uses baking powder: baking powder is sodium bicarbonate mixed with a solid edible acid, and tartaric acid, or its potassium salt commonly sold as cream of tartar, is the acid usually chosen. Dough moistened and heated releases carbon dioxide from the bicarbonate, and the tartaric acid is what neutralises the sodium carbonate that would otherwise be left behind and would taste bitter. Second, tartaric acid has a place in the history of chemistry out of all proportion to its size: it was working with its salts that Louis Pasteur, in 1848, separated two forms of the same compound that were mirror images of one another, the observation from which the study of optical activity and molecular handedness grew. In Indian cooking the point is practical rather than academic. Tamarind is the standard souring agent across much of the country — in sambar, in rasam, in chutneys — and where a recipe calls for tamarind, what it is really calling for is tartaric acid.

• (b)Tomato — Tomato is sour because of oxalic acid, not tartaric acid. Oxalic acid is the same compound that gives spinach and several other leafy vegetables their slightly astringent edge, and it is the reason people prone to a particular kind of kidney stone are advised to moderate those foods. Tomatoes also contain citric and malic acids in smaller quantity, but the acid the school table pairs with tomato is oxalic, and the pairing tested here is with tamarind.
• (c)Orange — Orange, like lemon, lime and the rest of the citrus family, owes its sourness to citric acid — the acid is named after the family. Citric acid is the most familiar of all the food acids and is manufactured on a large scale as a preservative and flavouring, which makes it the most likely wrong answer for a candidate who reaches for the first fruit acid that comes to mind. It is not tartaric acid, and no part of the orange is a notable source of tartaric acid.
• (d)Vinegar — Vinegar is a dilute aqueous solution of acetic acid, also called ethanoic acid, typically around five to eight per cent by volume, produced by the fermentation of alcohol. Glacial acetic acid is the pure, water-free compound, so named because it freezes into an ice-like solid a little below room temperature. Acetic acid and tartaric acid are quite different compounds — acetic acid has a single acidic group and tartaric acid has two, along with two hydroxyl groups — and vinegar contains no significant tartaric acid.

Sourness in food is almost always the taste of a weak organic acid, and a small number of these acids account for nearly every sour thing in a kitchen. The pairings worth holding are: citric acid in lemon, lime, orange and the citrus family; tartaric acid in tamarind and grapes; malic acid in apples; oxalic acid in tomato and spinach; lactic acid in curd and sour milk; acetic acid in vinegar; formic acid, also called methanoic acid, in the sting of an ant and of the nettle; and butyric acid in rancid butter. These are weak acids, which means they ionise only partly in water, and that is why they are safe to eat while a mineral acid of the same concentration would not be. Several have uses beyond flavour. Tartaric acid and its potassium salt make baking powder work by supplying the acid that reacts with sodium bicarbonate; its sodium potassium salt, Rochelle salt, is a laboratory reagent; citric acid is a preservative and a chelating agent; lactic acid preserves food by souring it, which is the principle behind curd and pickles.

Everyday chemistry of this kind is a fixture of the general science block in EO/AO papers, and it is examined by recall alone — there is nothing to derive. The habit rewarded is learning the acid table in both directions, from source to acid and from acid to source, because the paper may print either half in the stem. Note the printing convention of this booklet, which places a space before the colon at the end of the stem.

• Tartaric acid is the acid of tamarind and of grapes, and it is named from the tartar deposited in wine casks.
• Citric acid is the acid of orange, lemon, lime and the citrus family generally.
• Oxalic acid is the acid of tomato and of spinach.
• Vinegar is a dilute solution of acetic acid, also called ethanoic acid; the pure compound is glacial acetic acid.
• Malic acid occurs in apples, lactic acid in curd and sour milk, and formic acid in the sting of an ant.
• Baking powder is sodium bicarbonate together with an edible solid acid, usually tartaric acid or its potassium salt, cream of tartar.
• Tamarind is the principal souring agent in much of Indian cooking, which is tartaric acid at work.
• Louis Pasteur's separation of the mirror-image forms of a tartrate salt in 1848 began the study of optical activity.

• Choosing citric acid's source by reflex, since citrus is the fruit most associated with sourness generally.
• Pairing tomato with citric acid rather than oxalic acid.
• Assuming any sour liquid contains the same acid; vinegar's acid is acetic and quite distinct.
• Confusing tartaric acid with cream of tartar, which is its potassium salt rather than the acid itself.

The acid-and-source table appears in EO/AO papers in three shapes: name the acid in a given food, name a food containing a given acid, or match four foods against four acids. The list is short and closed, so learning it thoroughly converts an entire category of questions into free marks. Extend it with the common bases and salts, which are examined from the same table.

No directly related past PYQ was found.

• practice — not a real PYQ

  The acid present in curd is :

  • (a)Citric acid
  • (b)Lactic acid
  • (c)Tartaric acid
  • (d)Acetic acid

  Answer(b) Lactic acid

• practice — not a real PYQ

  Baking powder is a mixture of sodium bicarbonate and :

  • (a)A mild edible acid such as tartaric acid
  • (b)Sodium chloride
  • (c)Calcium carbonate
  • (d)Sodium hydroxide

  Answer(a) A mild edible acid such as tartaric acid