SSC
Which acid, linked with the formation of kidney stones, is found in spinach?
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Answer: Option D — Oxalic acid
Detailed Solution
Spinach contains a large amount of oxalic acid, an organic acid that is also found in tomatoes, rhubarb, beetroot leaves, tea, nuts and some other plants.
Oxalic acid combines with calcium in the body to form calcium oxalate, a salt that is insoluble in water. When too much calcium oxalate builds up in the kidneys, it can crystallise and form kidney stones. In fact, calcium oxalate is the most common component of kidney stones in humans. This is why doctors sometimes advise people who are prone to kidney stones to limit foods that are very rich in oxalates, such as spinach. Oxalic acid in food also reduces the amount of calcium the body can absorb from that food. Cooking spinach can reduce its oxalic acid content.
Oxalic acid is the simplest dicarboxylic acid (its formula is C2H2O4, or (COOH)2). In industry, it is used to remove rust and ink stains, to bleach wood and in leather tanning. It is poisonous in large amounts.
Acids found in common foods and natural substances include:
- Orange and lemon: citric acid.
- Apple: malic acid.
- Grapes and tamarind: tartaric acid.
- Curd: lactic acid.
- Vinegar: acetic acid.
- Ant sting: formic acid.
- Acids turn blue litmus red, have a sour taste and a pH of less than 7.
Why not the other options?
- ✗Citric acid is found in oranges and lemons.
- ✗Malic acid is found in apples.
- ✗Formic acid is found in ant stings and nettles.
Quick tip
Remember: oxalic acid = spinach and tomato; calcium oxalate = the most common kidney stone.
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Acids, Bases & Salts — Preparation
This question tests your understanding of Acids, Bases & Salts, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers acids, bases and salts, their properties, indicators, the pH scale, neutralisation, the acids found in natural substances, and important salts such as common salt, baking soda, washing soda, bleaching powder and plaster of Paris. Questions often ask which acid is found in which substance (citric acid in lemons, lactic acid in curd, formic acid in ant stings and tartaric acid in tamarind), the pH range of acids and bases, and the uses of everyday salts. The pH of pure water is 7, and a pH below 7 means a solution is acidic. Practise the acids, bases and salts questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.