ScienceAcids, bases and salts
Sour and bitter substances, indicators such as litmus and turmeric, and neutralisation.
Lemon juice tastes sour. Soap feels slippery and tastes bitter. Common salt is neither. Chemists sort many substances into three groups on the basis of such properties: acids, bases and salts.
Acids
Substances that taste sour contain acids. The word acid comes from the Latin word for sour. Acids found in food are weak and safe:
- Lemon and orange: citric acid
- Curd: lactic acid
- Vinegar: acetic acid
- Tamarind and grapes: tartaric acid
- Amla: ascorbic acid, which is vitamin C
Acids used in laboratories and factories, such as hydrochloric acid, sulphuric acid and nitric acid, are strong. They can burn the skin and must never be touched or tasted. Our stomach makes a little hydrochloric acid to help digest food.
Bases
Substances that taste bitter and feel soapy contain bases. Baking soda, soap, lime water (calcium hydroxide) and milk of magnesia (magnesium hydroxide) are common ones. Bases that dissolve in water are called alkalis. Strong bases such as sodium hydroxide (caustic soda) are as dangerous as strong acids.
Indicators
We must never taste or touch an unknown chemical. Instead we use an indicator, a substance that shows one colour in an acid and another in a base.
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | Turns red | Stays blue |
| Red litmus | Stays red | Turns blue |
| Turmeric | Stays yellow | Turns red-brown |
| China rose (gudhal) extract | Turns dark pink | Turns green |
| Phenolphthalein | Colourless | Turns pink |
Litmus is obtained from lichens. A solution that changes neither red nor blue litmus is neutral; pure water is an example. Have you seen a yellow curry stain on a white shirt turn red when washed with soap? Turmeric is the indicator and soap is the base.
Neutralisation
When an acid and a base are mixed in the right amounts, they cancel each other. The product is a salt and water, and some heat is given out. This reaction is called neutralisation.
Acid + Base → Salt + Water
For example, hydrochloric acid and sodium hydroxide give sodium chloride, which is common salt, and water. "Salt" in chemistry means any substance formed this way, not only table salt.
Neutralisation in daily life
- Indigestion: too much acid in the stomach causes burning. An antacid such as milk of magnesia, a mild base, neutralises it.
- Ant bite: an ant injects formic acid. Rubbing moist baking soda on the spot brings relief.
- Soil: if soil is too acidic, farmers add lime; if it is too basic, they add compost, which releases acids.
- Factory waste: acidic waste is neutralised with bases before it is let into rivers.
Key terms
- Acid
- a sour substance that turns blue litmus red
- Base
- a bitter, soapy substance that turns red litmus blue
- Indicator
- a substance that changes colour in acids and bases
- Neutralisation
- the reaction of an acid with a base to form salt and water
- Salt
- a substance formed when an acid reacts with a base
Try it yourself
Make turmeric paper: rub a paste of haldi and water on a strip of white paper and let it dry. Put a drop of soap solution on it, then a drop of lemon juice on a fresh strip. Which one turns red? Test other safe kitchen liquids too.
Quick questions
Answer in your head or on paper first, then tap to check.
Which acid is present in lemon?
Citric acid.
What colour change does an acid cause in blue litmus?
It turns blue litmus red.
What is neutralisation?
The reaction between an acid and a base that gives a salt and water.
Why does baking soda soothe an ant bite?
The bite contains formic acid, and baking soda, a base, neutralises it.
This is a simple explanation to help you understand. For examinations, read the same chapter in your prescribed textbook. Last checked: 11 November 2026.