Acidic, Basic, and Neutral

1. Sour, Bitter and the Idea of Acids and Bases

Many substances around us have a characteristic taste. Lemon juice, tamarind, raw mango and curd taste sour. Baking soda solution and soap taste bitter and feel soapy. These tastes are clues to a deeper chemical nature.

Important safety rule: we must never taste or touch laboratory chemicals to test them, because many acids and bases are very harmful (corrosive) and can burn the skin. We use safe substances called indicators instead.

• Common acidic substances: lemon juice (citric acid), vinegar (acetic acid), curd (lactic acid), tamarind, raw mango, tomato, ant sting (formic acid).
• Common basic substances: baking soda, washing soda, soap, lime water (calcium hydroxide), milk of magnesia, window cleaner.

2. Indicators

Since we cannot taste or touch chemicals safely, scientists use indicators. An indicator is a substance that changes its colour (or sometimes smell) in acidic and basic solutions, telling us the nature of the substance.

2.1 Litmus — the most common indicator

Litmus is a natural dye obtained from a plant-like organism called lichen. It is available as a solution and as red and blue litmus paper.

• In an acid: blue litmus turns red.
• In a base: red litmus turns blue.
• In a neutral substance: litmus does not change colour.

3. Natural Indicators

Many natural materials act as indicators. They are easy to make at home and are completely safe.

3.1 Turmeric

Turmeric is yellow. It stays yellow in acidic and neutral substances, but turns red/brown in a basic substance. This is why a curry stain (turmeric) on cloth turns reddish-brown when soap (a base) is rubbed on it, and goes back to yellow when washed with water.

3.2 China rose (Hibiscus)

The solution made by soaking China rose (hibiscus) petals in warm water is light pink. It turns dark pink (magenta) in an acid and green in a base.

3.3 Red cabbage

Red/purple cabbage extract is a colourful natural indicator. It turns reddish/pink in acids and greenish/yellow in bases, with purple in neutral solutions.

4. Neutral Substances

Some substances are neither acidic nor basic — they are neutral. They do not change the colour of litmus or other indicators.

Distilled water is the most common neutral substance. Many neutral substances are formed when an acid and a base react and cancel each other out, as we will see in neutralisation.

5. The Universal Indicator and the Strength of Acids and Bases

Litmus can tell us whether a substance is acidic or basic, but not how strong it is. The universal indicator is a mixture of several indicators that shows a whole range of colours, giving an idea of the strength of an acid or base.

• Strong acid → red/orange.
• Weak acid → yellow.
• Neutral → green.
• Weak base → blue.
• Strong base → dark blue/violet.

6. Neutralisation

When an acid is mixed with a base in the right amounts, they react and cancel each other's effect. The acidic nature of the acid and the basic nature of the base are both destroyed. This reaction is called neutralisation.

For example, when dilute hydrochloric acid reacts with sodium hydroxide (a base), the products are common salt (sodium chloride) and water. The resulting solution does not change the colour of litmus — it has become neutral.

7. Neutralisation in Everyday Life

Neutralisation is used in many practical situations to control unwanted acidity or basicity.

7.1 Indigestion and acidity

Our stomach contains hydrochloric acid that helps digest food. Eating too much spicy or oily food can produce excess acid, causing the burning feeling of acidity. Antacids such as milk of magnesia (a mild base) neutralise the extra acid and give relief.

7.2 Ant and bee stings

An ant sting injects formic acid, causing pain and itching. Rubbing a mild base such as baking soda (or calamine, which contains zinc carbonate) neutralises the acid and reduces the pain. A wasp sting is basic, so it is soothed with a mild acid like vinegar.

7.3 Soil treatment

Plants grow best in soil that is neither too acidic nor too basic. If the soil is too acidic, farmers add bases such as quicklime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is too basic (alkaline), they add organic matter (compost) or other acidic materials to neutralise it.

7.4 Factory wastes

Wastes from many factories contain acids. If released directly into rivers, they harm aquatic life. The acidic waste is first neutralised by adding basic substances before being released, to protect the water and the living things in it.

8. Acid Rain (a brief idea)

When gases such as the oxides of sulphur and nitrogen released by burning fuels mix with rainwater, they make the rain acidic. This is called acid rain. Acid rain damages buildings (especially marble monuments), harms plants, and makes river water acidic, harming fish. It is an example of why controlling acids in the environment is important.

The famous example often quoted is that acid rain slowly damages the marble of monuments because marble (calcium carbonate) reacts with acids.