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What is neutralisation? Give examples from daily life.

Class 7 ScienceNCERT chapter: Exploring Substances: Acidic, Basic, and Neutralइसे हिंदी में पढ़ें

Exam-ready answer

Neutralisation is the reaction between an acid and a base in which they cancel each other's effect and form a salt and water; heat is also given out. Acid + Base → Salt + Water (+ heat). Examples: antacids for acidity, baking soda on an ant bite, lime for acidic soil and treating acidic factory waste.

Explanation

Take a little dilute hydrochloric acid in a test tube and add two or three drops of phenolphthalein; the solution stays colourless. Now add dilute sodium hydroxide, a base, drop by drop while gently shaking. At one point the solution turns pink, showing that the acid has been used up and the solution has just turned basic. Touching the test tube shows that it has become warm.

This change is neutralisation. The acid and the base react to form a new substance called a salt, along with water. In the activity above, hydrochloric acid and sodium hydroxide form sodium chloride, which is common salt, and water. The warmth of the test tube shows that heat is evolved during neutralisation. The indicator helps us see the exact point at which the acid is neutralised.

Neutralisation is used every day. Too much acid in the stomach causes indigestion; an antacid such as milk of magnesia, which contains magnesium hydroxide, neutralises the extra acid. An ant's sting injects formic acid, so moist baking soda or calamine solution (zinc carbonate) is rubbed on the skin. Farmers add quicklime or slaked lime to acidic soil, and acidic factory wastes are neutralised before they are released into water bodies.

Points to remember

  1. Neutralisation: the reaction between an acid and a base that forms a salt and water.
  2. Word equation: Acid + Base → Salt + Water, with heat evolved.
  3. Example: hydrochloric acid + sodium hydroxide → sodium chloride + water.
  4. Antacids such as milk of magnesia (magnesium hydroxide) neutralise excess acid in the stomach.
  5. Ant sting (formic acid) is treated with baking soda or calamine (zinc carbonate); acidic soil is treated with quicklime or slaked lime.

Exam tip

Always write the word equation Acid + Base → Salt + Water and state that heat is evolved; then give two daily-life examples, naming the acid and the base involved in each.

Questions students also ask

Why does the test tube become warm during neutralisation?
Neutralisation gives out heat as the acid and the base react, so the mixture and the test tube become warm. This release of heat is one sign that a chemical reaction has taken place and the substances have not just mixed.
Why is baking soda applied on an ant bite?
An ant's sting injects formic acid into the skin, which causes pain and a burning feeling. Baking soda is basic, so it neutralises the acid and reduces the effect. If the swelling or pain is severe, a doctor should be consulted.
Why is factory waste neutralised before it is released?
Wastes from many factories contain acids. If they are released directly into rivers and lakes, they can harm fish and other living things. Adding basic substances neutralises these wastes, so the water becomes less harmful to aquatic life.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.