Acids, Bases and Salts Class 7 Science Notes - Chapter 5

Acids and Bases


  • Acids are substances that taste sour and are corrosive in nature.
  • It turns blue litmus paper to red.
  • These substances are chemically acidic in nature.E.g.:-orange juice, curd, vinegar, hydrochloric acid etc.


  • Bases are substances that, in aqueous solution, are slippery to the touch and bitter in taste.
  • It turns red litmus paper to blue.
  • These substances are chemically basic in nature. Eg:- soap, ammonium hydroxide, calcium hydroxide, etc

Neutralisation Reaction

  • When an acid and base react with each other to form a salt, water and heat then such reaction is known as neutralisation reaction.
  • In this reaction, the acidic and basic nature of the acid and base respectively are destroyed.
  • The reaction between hydrochloric acid and sodium hydroxide is a classic neutralisation reaction where sodium chloride is formed.



  • Salt is the product formed from the neutralisation reaction of acids and bases.
  • In the reaction between hydrochloric acid and sodium hydroxide the salt formed is sodium chloride.


  • Salt can be acidic, basic or neutral in nature.

Visual Indicators


Substances, which are used to test whether a substance is acidic or basic are called indicators.

Acidic Solution Basic Solution
Red litmus No change Turns blue
Blue litmus Turns red No change


Natural Indicators

  • Plants or plant parts contain useful chemicals that are used for testing an acidic or basic property of a solution are termed as natural indicators. China Rose and turmeric are examples of natural indicators.


  • Litmus is a naturally occurring purple indicator, which is extracted from lichens.
  • When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.
  • It is available in the form of a solution or as strips of paper known as litmus paper.
(A) Red litmus turns blue indicating a basic solution
(B) Blue litmus turns red indicating an acidic solution


  • Phenolphthalein is an acid-base indicator which is colourless in acid solution but turns pink to red as the solution becomes alkaline.
  • It is a synthetic indicator and is used for the neutralisation experiment.

Olfactory Indicators

  • Olfactory indicators are substances whose smell changes whether they are mixed with an acidic or a basic solution.
  • Onion, clove oil and vanilla extract are examples of such indicators.

Visual Indicators

  • Visual Indicators are substances used to show visually (as by a change in colour) the condition of a solution with respect to the presence of a particular material (as a free acid or base).
  • Common examples are litmus, red cabbage, phenolphthalein, etc


Neutralisation in Daily Life

  • During indigestion, taking milk of magnesia (magnesium hydroxide) gives us relief as it neutralizes the effect of excess acid produced inside the stomach.
  • The effect of ant sting which is caused by formic acid can be neutralized by rubbing moist baking soda (basic in nature).
  • To ensure that plants can grow well, the soil is treated with either acids or bases depending if it’s basic or acidic in nature.
  • Factory wastes, generally being acidic in nature can cause environmental damage, are treated with basic substances before discharge.

Recap of Concepts

Safety Measures While Using Acids

  • When diluting acids, pour the acid into the water, NOT water into acid as this may cause spattering of the acid.
  • Safety gloves are to be worn whenever working with acids or bases.

Uses and Applications

pH of Soil

  • Excessive use of chemical fertilisers changes the pH of the soil.
  • Plants do not grow well when the soil is either too acidic or too basic.
  • Hence substances are added to bring the pH at a neutral level.

Uses of Acids and Bases

  • Food preservation – Citric acid.
  • Aerated drinks – Carbonic acid.
  • Baking powder – Tartaric acid.
  • Cooking – Acetic acid(vinegar)
  • Manufacture of soaps – Sodium hydroxide.
  • Manufacture of bleaching powder – Calcium hydroxide
  • As a foaming agent in fire extinguishers – Aluminium hydroxide.

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