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CLASS 7-Acids, Bases and Salt


Acids, Bases and Salt


·         The substance which taste sour contain acid and substance are called acidic substance. Some of naturally occurring acidic substance are:- lemon juice, orange juice, vinegar and curd.

·         Acids are corrosive in nature and are soluble in water.

·         Acids turn red litmus to blue.



·         The substance which are bitter in taste and are soapy to touch contain bases in them and are called as basic substance.

·         All bases are not soluble in water. The bases which are soluble in water are called as alkalis.

·         They turn blue litmus to red.



·         An indicator is a substance which changes colour when added to an Acid or base. If there is no change then it is said to salt or neutral.

·         There are two types of indicators Natural Indicators and Synthetic Indicators.

Natural Indicators


    ·       Litmus is a natural dye .It is extracted from lichens.

    ·      Litmus solution is prepared from this extract.

    ·     It has a purple colour in distilled water.

     ·    It is generally available in the form of a solution, or in the form of strips of paper, known as litmus paper.

·         Generally, it is found as red and blue litmus paper.


China rose:-

     ·         China rose is a flower also called as hibiscus.

·         It is prepared by soaking the petals of the flower in water.

·         In acidic substance it changes itself to dark pink or magenta colour.

·         In basic substance it changes itself to green.



·         Turmeric is another natural indicator.

·         It is made with turmeric paste.

·         It is also available in paper strips.

·         It remains yellow in acidic substance.

·         It turns brownish-red color on basic solution.

Synthetic Indicators


·         Phenolphthalein is colorless in acidic solution.

·         Turns pink in base solution.


·         The reaction between acid and base is known as neutralization.

·         Salt and water are produced in this process with the evolution of heat take place.

·         Acid + Base  Salt + Water

                         (Heat is evolved)


            Hydrochloric acid + Sodium hydroxide  Sodium chloride + Water

            HCl + NaOH → NaCl + H2O

Neutralization in everyday life


·         Our stomach contains hydrochloric acid. It helps us to digest food.

·         But excess quantity of acid in the stomach causes indigestion

·         Sometimes indigestion is painful

·         To relieve indigestion,  an antacid is taken. Which is  the milk of magnesia & contains magnesium hydroxide.

·         It neutralizes the effect of excessive acid.

Ant bite:-

·         When an ant bites, it injects the formic acid into the skin.

·         Baking soda (sodium hydrogen carbonate) or calamine solution, which contains   zinc carbonate is used to neutralize the effect of the acid can be by rubbing.

Soil treatment:-

·          Soil became acidic due to excessive use of chemical fertilizers.

·         Plants growth is not proper, when the soil is either too acidic or too basic.

·      Too acidic soil is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide).

·         If the soil is basic in nature, organic matter such as compost is added to it.

·         Organic matter releases acids that neutralize the basic nature of the soil.


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