Acids, Bases and Salts | Class 7 | Science Notes

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Acids, Bases and Salts

Acids, Bases


Acids are substances that taste sour and can be harmful.
They turn blue litmus paper to red.
Examples include orange juice, vinegar, and hydrochloric acid.
They are chemically acidic.


Bases are substances that feel slippery and taste bitter when dissolved in water.
They turn red litmus paper to blue.
Examples include soap, ammonia solution, and calcium hydroxide.
They are chemically basic.

Neutralization Reaction:

When an acid and a base react, they form salt, water, and heat.
This reaction is called neutralization.
It neutralizes the acidic and basic properties of the substances.
For example, when hydrochloric acid reacts with sodium hydroxide, it forms sodium chloride and water:
HCl + NaOH → NaCl + H2O


Salts are the products of neutralization reactions.
In the reaction between hydrochloric acid and sodium hydroxide, sodium chloride is formed:
HCl + NaOH → NaCl + H2O
Salts can be acidic, basic, or neutral.


Substances used to test if a solution is acidic or basic.
Acidic Solution does not change red litmus but turns blue litmus to red.
Basic Solution turns red litmus to blue and doesn’t change blue litmus.
Natural indicators come from plants, like China Rose and turmeric.
Litmus changes color: red for acid, blue for base.
Phenolphthalein is synthetic and turns pink in a base.
Olfactory indicators change smell, like onions or cloves.
Visual indicators change color, such as litmus and red cabbage.

Neutralization in Daily Life:

Milk of magnesia neutralizes stomach acid during indigestion.
Baking soda neutralizes ant stings caused by formic acid.
Soil is treated with acids or bases to help plants grow.
Factories neutralize acidic waste before disposal to protect the environment.
Safety Measures While Using Acids:

Pour acid into water when diluting, not the other way around.
Wear safety gloves when working with acids or bases.

Uses and Applications:

pH of Soil: Adjusting soil pH with acids or bases for better plant growth.
Food Preservation: Citric acid preserves food.
Aerated Drinks: Carbonic acid gives fizz to sodas.
Baking Powder: Tartaric acid helps cakes rise.
Cooking: Acetic acid is found in vinegar.
Manufacture of Soaps: Sodium hydroxide is used.
Manufacture of Bleaching Powder: Calcium hydroxide is used.
Fire Extinguishers: Aluminium hydroxide is used as a foaming agent.

Read More: Heat | Class 7 Notes | Science | Chapter 4

Litmus solution is indeed extracted from lichens, and its primary use is as an indicator to determine whether a solution is acidic or basic.

Distilled water is considered neutral, and you can verify this by testing it with red and blue litmus paper. If the color remains unchanged in both cases, it indicates neutrality.

Neutralization is a chemical reaction between an acid and a base, resulting in the formation of a salt and water, along with the release of heat. Your example involving sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a perfect illustration of this process.

NaOH + HCl → NaCl + H2O + Heat

Dorji can indeed use taste and litmus paper to determine the nature of the soft drinks and serve them to the customers accordingly:

  1. For the customer who wants an acidic drink:
    • Dorji can taste a few drops; if it's sour, it's likely acidic.
    • He can also use blue litmus paper. If it turns red, the drink is acidic.
  2. For the customer who wants a basic drink:
    • Dorji can taste a few drops; if it's bitter, it's likely basic.
    • He can also use red litmus paper. If it turns blue, the drink is basic.
  3. For the customer who wants a neutral drink:
    • Dorji can taste a few drops. If it has no strong taste (neither sour nor bitter), it's likely neutral.

This way, Dorji can accurately serve the drinks to his customers based on their preferences for acidity, basicity, or neutrality.

(a) Antacid tablet for acidity:

  • Antacid tablets contain bases like milk of magnesia.
  • They neutralize the excess stomach acid, providing relief from acidity.

(b) Calamine solution for ant bites:

  • Ants inject formic acid when they bite.
  • Calamine solution contains Zinc carbonate, which is basic.
  • It neutralizes the formic acid's effects, relieving discomfort.

(c) Neutralizing factory waste:

  • Factory waste is often acidic, which can harm aquatic life.
  • To protect the environment, these wastes are neutralized with a base before disposal into water bodies.

  1. Initial Test:
    • Place a drop of each liquid on the turmeric indicator.
    • The one that turns the indicator red is sodium hydroxide, indicating its basic nature.
  2. Confirmation Tests:
    • Take the remaining two liquids and add a drop of sodium hydroxide separately to each.
    • Now, place a drop of each combination on the turmeric indicator.
    • The mixture that changes the indicator to red contains a neutral solution of sugar.
    • The mixture that doesn't change the indicator contains hydrochloric acid neutralized by sodium hydroxide.

When blue litmus paper remains blue after being dipped in a solution, it indicates that the solution is either neutral or basic in nature. Neither neutral nor basic solutions will cause a change in the color of blue litmus paper.

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