CBSE Class 7 Science Acids, Bases and Neutral Substances Notes Set 01

Why We Classify Substances

Many things we eat, drink and use at home can be placed in three groups: acidic, basic and neutral. Tasting unknown substances is unsafe, so scientists use indicators to test them.

Acids

Acids taste sour, turn blue litmus red and release hydrogen ions (H⁺) when dissolved in water. Natural or organic acids are found in food. Examples are citric acid in lemon and orange, acetic acid in vinegar, lactic acid in curd and tartaric acid in tamarind. Mineral or inorganic acids such as sulphuric acid, hydrochloric acid and nitric acid are made in industry and are strong and corrosive.

Bases and Alkalis

Bases taste bitter, feel soapy to touch, turn red litmus blue and release hydroxide ions (OH⁻) in water. A base that dissolves in water is called an alkali. Sodium hydroxide, potassium hydroxide and calcium hydroxide (lime water) are alkalis. Baking soda, soap and detergent solutions are basic.

Never taste or touch strong acids and bases. Strong ones can burn the skin.

Neutral Substances

A neutral substance is neither acidic nor basic and does not change the colour of litmus. Pure water, sugar solution and common salt solution are neutral.

Indicators

An indicator is a substance that shows by a colour change whether a solution is acidic, basic or neutral.

1. Litmus: acid turns blue litmus red; base turns red litmus blue; neutral solutions change neither.

2. Turmeric: stays yellow in acids and neutral solutions and turns red in a base.

3. China rose (gudhal) extract: turns dark pink or magenta in acids and green in bases.

4. Natural indicators such as red cabbage juice show a range of colours with different solutions.

Everyday Solutions

Acidic: vinegar, lemon juice, curd, aerated drinks. Basic: soap, detergent, shampoo, baking soda solution, lime water, milk of magnesia. Neutral: tap water, sugar solution, common salt solution.

Neutralisation

When an acid and a base are mixed in the right amounts they cancel each other's effect and form salt and water. This reaction is called neutralisation. It has many uses. Antacid tablets, which are bases, neutralise excess acid in the stomach. Ant and bee stings contain formic acid, and calamine solution or baking soda soothes it. Farmers add lime to acidic soil. Toothpaste, which is basic, neutralises acids formed by bacteria in the mouth.

Acid Rain

Gases such as sulphur dioxide and nitrogen oxides released by vehicles and factories dissolve in rainwater and make it acidic. Acid rain damages buildings, crops and aquatic life.

Worked Examples

Example 1. A solution turns red litmus blue. What is its nature? It is basic, as only bases turn red litmus blue.

Example 2. Why is baking soda applied on a bee sting? A bee sting contains an acid. The baking soda is a base and neutralises it.

Example 3. A solution does not change the colour of either litmus paper. Can we say it is neutral? It is neutral to litmus, so it is neither acidic nor basic.

Quick Check Questions