CDS Chemistry · Acids, Bases and Salts
Acids in Food and the Body
Which acid is in which fruit, food, sting or body fluid, and what common acids and antacids are used for.
Why this matters
Eleven CDS questions, every one a recall of source and acid. The same pairs come back: methanoic acid in nettle and ant stings (three times), hydrochloric acid in the stomach (twice), oxalic acid in tomato (twice).
Concept 1 of 2: Acids in fruits, foods and household products
Most acids in food are weak organic acids, and each has a signature source. Learn the pairs as a list: citrus has citric acid, tomato has oxalic, tamarind has tartaric, curd has lactic, vinegar is acetic.
Definition
The pairs CDS asks:
- Citric acid: lemon, orange and other citrus fruits; also a food preservative.
- Ascorbic acid is vitamin C (amla, citrus); lack of it causes scurvy.
- Oxalic acid: tomato, spinach.
- Tartaric acid: tamarind, grapes. (Oxalic acid is not the acid of tamarind.)
- Lactic acid: curd and sour milk.
- Acetic acid: vinegar, which is a 5–8% solution of acetic acid in water; used in pickles.
- Boric acid is a mild antiseptic. Magnesium hydroxide (milk of magnesia) is a base used as an antacid.
| Source or use | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Vitamin C (amla, citrus) | Ascorbic acidQ CDS 2019 (II): vitamin C is ascorbic acid. |
| Tomato | Oxalic acidQ Asked in CDS 2021 (I) and again in a 2026 (II) match-the-list. |
| Tamarind, grapes | Tartaric acidQ CDS 2023 (II): 'oxalic acid is found in tamarind paste' was the incorrect statement. |
| Curd, sour milk | Lactic acid |
| Vinegar (5–8% in water), pickles | Acetic acidQ |
| Mild antiseptic | Boric acidQ CDS 2020 (I) matched boric acid to antiseptic, citric acid to preservative, Mg(OH)₂ to antacid, acetic acid to pickle. |
Practice this conceptself-check · 5 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q68Moderate
Example 1 · Acids, Bases and Salts · Acids in Food and the Body
Match List I with List II and select the correct answer using the code given below the Lists :
Code :
| List I (Source) | List II (Acid present) |
|---|---|
| A. Tomato | 1. Acetic acid |
| B. Sour milk | 2. Methanoic acid |
| C. Ant sting | 3. Oxalic Acid |
| D. Vinegar | 4. Lactic acid |
Tomato is oxalic, tamarind is tartaric
The two are swapped often in the options. Oxalic acid is in tomato and spinach; tartaric acid is in tamarind and grapes.
Milk of magnesia is a base, not an acid
In a list of 'compound and use', magnesium hydroxide is the antacid: it is a mild base that neutralises excess stomach acid.
Concept 2 of 2: Acids in stings and in the stomach
Two body facts carry most of these questions. The sting of an ant, bee or nettle injects methanoic (formic) acid. The stomach makes hydrochloric acid, a mineral acid, to digest food and kill germs.
Definition
The facts:
- Methanoic acid (formic acid, HCOOH) is injected by ant and bee stings and by the stinging hairs of nettle leaves. Its name comes from the Latin formica, ant.
- A sting is soothed by a mild base: baking soda, or the leaf of the dock plant that grows beside nettle.
- The stomach secretes hydrochloric acid (HCl). It is a mineral acid; lactic, uric and methanoic acids are organic.
- Stomach HCl kills microbes and activates the enzyme pepsin. Too much of it causes acidity, treated with an antacid.
- Lactic acid builds up in muscles during hard exercise.
| Source | Acid | Kind |
|---|---|---|
| Ant or bee sting | Methanoic (formic) acid | OrganicQ |
| Nettle leaf hairs | Methanoic (formic) acid | OrganicQ Asked twice: CDS 2021 (II) and 2022 (II). |
| Stomach (gastric juice) | Hydrochloric acid | MineralQ Asked twice: CDS 2020 (I) and 2022 (I). HCl is the only mineral acid among the usual options. |
| Tired muscles | Lactic acid | Organic |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q11Easy
Example 2 · Acids, Bases and Salts · Acids in Food and the Body
Stinging hair of nettle leaves inject fluid in the human body causing burning pain. The fluid is
The stomach acid is hydrochloric, not sulphuric
Gastric juice contains hydrochloric acid. Sulphuric, nitric and formic acids are distractors; lactic and uric acids are organic, not mineral.
Nettle and ant share one acid
Both inject methanoic (formic) acid. Citric, tartaric and acetic acids are food acids and do not sting.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Acids in fruits, foods and household products7 rows
| Source or use | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Vitamin C (amla, citrus) | Ascorbic acidQ CDS 2019 (II): vitamin C is ascorbic acid. |
| Tomato | Oxalic acidQ Asked in CDS 2021 (I) and again in a 2026 (II) match-the-list. |
| Tamarind, grapes | Tartaric acidQ CDS 2023 (II): 'oxalic acid is found in tamarind paste' was the incorrect statement. |
| Curd, sour milk | Lactic acid |
| Vinegar (5–8% in water), pickles | Acetic acidQ |
| Mild antiseptic | Boric acidQ CDS 2020 (I) matched boric acid to antiseptic, citric acid to preservative, Mg(OH)₂ to antacid, acetic acid to pickle. |
Acids in stings and in the stomach4 rows
| Source | Acid | Kind |
|---|---|---|
| Ant or bee sting | Methanoic (formic) acid | OrganicQ |
| Nettle leaf hairs | Methanoic (formic) acid | OrganicQ Asked twice: CDS 2021 (II) and 2022 (II). |
| Stomach (gastric juice) | Hydrochloric acid | MineralQ Asked twice: CDS 2020 (I) and 2022 (I). HCl is the only mineral acid among the usual options. |
| Tired muscles | Lactic acid | Organic |
Watch out for (4)
- Tomato is oxalic, tamarind is tartaric→ Acids in fruits, foods and household products
- Milk of magnesia is a base, not an acid→ Acids in fruits, foods and household products
- The stomach acid is hydrochloric, not sulphuric→ Acids in stings and in the stomach
- Nettle and ant share one acid→ Acids in stings and in the stomach
Test yourself on Acids, Bases and Salts
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.