Natural Acids and Indicators
~6 min read
- Natural acids: Lemon — citric. Vinegar — acetic. Milk — lactic. Apples — malic. Grapes — tartaric. Spinach — oxalic. Ant — formic.
- Indicators: Substances that change colour with pH. Natural: turmeric (red in base), litmus (red/blue), red cabbage (multi-colour). Synthetic: phenolphthalein, methyl orange.
- Universal indicator: Mixture showing different colour for each pH from 1 to 14.
Many household substances are mild acids; many natural plant extracts indicate pH. NDA tests these everyday-chemistry facts.
Common Natural Acids
| Source | Acid | Formula |
|---|---|---|
| Lemon, orange (citrus) | Citric acid | C₆H₈O₇ |
| Vinegar | Acetic acid | CH₃COOH |
| Sour milk, yogurt | Lactic acid | C₃H₆O₃ |
| Apples | Malic acid | C₄H₆O₅ |
| Grapes, tamarind | Tartaric acid | C₄H₆O₆ |
| Spinach, rhubarb | Oxalic acid | C₂H₂O₄ |
| Ant sting, nettle | Formic acid | HCOOH |
| Stomach | Hydrochloric acid | HCl |
| Tea | Tannic acid | (complex) |
| Vitamin C (citrus, amla) | Ascorbic acid | C₆H₈O₆ |
Acid-Base Indicators
| Indicator | Acidic colour | Basic colour |
|---|---|---|
| Litmus (from lichens) | Red | Blue |
| Phenolphthalein (synthetic) | Colourless | Pink |
| Methyl orange (synthetic) | Red | Yellow |
| Bromothymol blue | Yellow | Blue |
| Turmeric (haldi) | Yellow | Reddish-brown |
| Red cabbage juice | Red/pink (acidic) | Yellow-green (basic) |
| China rose (gudhal) | Pink/magenta | Green |
Universal Indicator
- A mixture of several indicators that gives a different colour for each pH value across the 1-14 range.
- Allows estimation of pH without a pH meter.
- Approximate colour scale:
- pH 1-3 (strong acid): red
- pH 4-6 (weak acid): orange-yellow
- pH 7 (neutral): green
- pH 8-10 (weak base): blue
- pH 11-14 (strong base): violet/purple
NDA PYQ Examples
Q: Vinegar contains:
(a) Citric acid (b) Acetic acid (c) Lactic acid (d) Hydrochloric acid
Answer: (b) Acetic acid (CH₃COOH).
Q: Ant sting contains:
(a) Citric acid (b) Formic acid (c) Malic acid (d) Acetic acid
Answer: (b) Formic acid (HCOOH).
Q: Turmeric in a basic solution turns:
(a) Yellow (b) Red (c) Green (d) Blue
Answer: (b) Reddish-brown — used as natural indicator.
Q: Stomach acid is mainly:
(a) Sulphuric (b) Hydrochloric (c) Acetic (d) Lactic
Answer: (b) Hydrochloric acid (HCl).
Drill Natural Acids and Indicators for NDA
NDA-pattern items on Natural Acids and Indicators with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does turmeric stain become red when soap is rubbed on it?
Soap is basic. Turmeric (curcumin) is yellow in neutral/acidic and red in basic environments. Applying soap to a turmeric stain converts it to red — and the bright red stain is more visible than the original yellow! Add lemon (acid) to revert.
Why is unripe fruit sour?
Unripe fruits contain higher concentrations of organic acids (malic, citric, tartaric). As fruits ripen, these acids are converted to sugars, raising sweetness and reducing sourness. Tamarind stays sour because its tartaric acid concentration remains high.
How does an ant sting hurt?
Ants inject formic acid (HCOOH) when they sting. This burns and irritates the skin. Antidote: rub a paste of baking soda (sodium bicarbonate) — base neutralises the formic acid.
What is litmus made of?
Natural dyes extracted from lichens (symbiotic algae-fungi organisms). Used since Middle Ages as an acid-base indicator. Litmus papers (red and blue) are dipped in respective dye solutions.
Why does red cabbage juice work as an indicator?
Red cabbage contains anthocyanin pigments whose colour depends on pH — red in strong acid, pink in weak acid, purple in neutral, blue in weak base, green-yellow in strong base. A natural universal indicator.