CBSE – 7th Standard Science

Chapter 5 – Acids bases and salts


Acids bases and salts

  • Curd, lemon juice, orange juice and vinegar taste sour.
  • These substances taste sour because they contain acids.
  • The chemical nature of such substances is acidic.
  • The word acid comes from the Latin word acere which means sour.
  • The acids in these substances are natural acids.
  • Since, it does not taste sour it means, that it you rub its solution between fingers, it feels soapy.
  • Substances like these which are bitter in taste and feel soapy on touching are known as bases.
  • The nature of such substances is said to be basic.
  • Special type of substances are used to test whether a substance is acidic or basic.


  • These substances are known as indicators.
  • The indicators change their colour when added to a solution containing an acidic or a basic substance.
  • Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.
  • The most commonly used natural indicator is litmus.
  • It is extracted from lichens


  • It has a mauve (purple) colour in distilled water.
  • 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 in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper.


  • As the name indicates the rain containing excess of acids is called an acid rain.
  • The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively.
  • Acid rain can cause damage to buildings, historical monuments, plants and animals.


  • When an acidic solution is mixed with a basic solution, both the solutions neutralise the effect of each other.
  • When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed.
  • The resulting solution is neither acidic nor basic. Touch the test tube immediately after neutralisation.


  • In neutralisation reaction, heat is always produced, or evolved.
  • The evolved heat raises the temperature of the reaction mixture.
  • In neutralisation reaction a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature
  • The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat.
  • Our stomach contains hydrochloric acid. It helps us to digest food
  • Too much of acid in the stomach causes indigestion.
  • Sometimes indigestion is painful.
  • To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide.


  • It neutralises the effect of excessive acid.
  • When an ant bites, it injects the acidic liquid (formic acid) into the skin.
  • The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.
  • Excessive use of chemical fertilisers makes the soil acidic.
  • Plants do not grow well when the soil is either too acidic or too basic.
  • When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide).
  • If the soil is basic, organic matter is added to it.
  • Organic matter releases acids which neutralises the basic nature of the soil.
  • The wastes of many factories contain acids.
  • If they are allowed to flow into the water bodies, the acids will kill fish and other organisms.
  • The factory wastes are, therefore, neutralised by adding basic substances.
  • Great care should be taken while handling laboratory acids and bases because these are corrosive in nature, irritating and harmful to skin.
  • To prepare limewater, dissolve some lime (chuna) in water in a bottle. Stir the solution and keep it for some time. Pour a little from the top. This is lime water.

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  • Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water. If possible make solutions in distilled water