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Class VII - Science: Acids and Bases
One Word Answer Questions:
Q) An acid and a base neutralise each other to form?
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Q) The substances which are sour in taste are called?
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Q) What are the substances called which are used to test whether a substance is acidic or basic?
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Q) Name the most common natural indicator?
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Q) Name the source from which litmus solution is obtained. What is the use of this solution?
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Short Answer Questions:
Q) What are neutral solutions?
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Q) What happens to the skin when an ant bites? How can the effect be cured?
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Q) What is indigestion?
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Q) Why factory waste is neutralized before disposing it into the water bodies?
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Q) Distinguish between acids and bases?
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Q) Is the distilled water acidic/basic/neutral? How would you verify it?
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Long Answer Questions:
Q) Name the substances which are neither acidic nor basic?
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Q) Explain the acidic and basic nature of soil?
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Q) Describe the process of neutralization with the help of an example?
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Q) What is the effect of china rose indicator on acidic, basic and neutral solutions?
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Acids and Bases

Taste of acid is sour. There are many substances that contain acid and so taste sour.
Example: Lemon, curd, pickles, orange juice, vinegar, etc.
Substances that taste sour are called acidic. The chemical nature of such substance is known as Acidic. The word acid comes from Latin Acere which means sour.

Taste of base is bitter. Substances that contain base taste bitter.
Example: Soap or soap solution, baking soda, washing soda, etc.
The chemical nature of substances that contain base is known as basic.


  • It is not always possible to know the acidic or basic nature of substances by tasting them. Tasting a substance in laboratory is not also advisable because it may be harmful.
  • Thus, to test the chemical nature (acidic or basic nature) of substance a special kind of substance is used. This special kind of substance which is used to test the acidic or basic nature of anything is known as Indicator.
  • Indicator is a substance that shows the acidic or basic nature of a substance by change in its colour.
Types of Indicator: Indicators can be divided into two types.

Natural Indicator:
Indicators that are obtained from naturally occurring substance are called Natural Indicators.
Example: Litmus, turmeric, China rose, etc.

Synthetic Indicator:
Indicators that are made in laboratory are called Synthetic Indicators.
Example: Phenolphthalein, methyl orange, etc.

Natural Indicator

Litmus A Natural Dye:

  • It is the most commonly used natural indicator. Litmus is extracted from Lichens. Lichen is a composite organism. Lichens consist of fungi and algae living in symbiotic relationship.
  • lichen On A Branch Of Tree
    Lichen On A Branch Of Tree
  • Litmus is a purple coloured liquid in distilled water. Litmus comes in the form of strips of two colours. One is called blue litmus paper and another is called red litmus paper.


Litmus liquid and litmus paper are used to detect the acidic or basic nature of a substance.

  • Colour of litmus paper in acid: Blue litmus paper turns into red when dipped in acidic solution.
  • Colour of litmus paper in base: Red litmus paper turns into blue when dipped in basic solution.
  • paper

    TURMERIC: Turmeric is used as another natural indicator. It is of yellow colour. Turmeric paper turns
    into red when it is dipped into basic solution. Turmeric paper does not change its colour with acid.

    Turmeric With Base
    Turmeric with Acid

    CHINA ROSE: China rose is another natural indicator. China rose solution gives dark pink (magenta)
    colour with acid and green colour with base.

    magenta Litmus

    Acid Rain: Carbon dioxide, sulphur dioxide and nitrogen dioxide which are released from vehicles and chimneys mix with droplets of rain and turn the rain water acidic. When this acidic rain water falls over earth, it is known as acid rain. Acid rain damages the buildings and is harmful for plants and animals. Taj Mahal; which is made of marble is in threat because of acid rain. Many parts of Taj Mahal and other many historical buildings and monuments have got damaged due to acid rain.

    Characteristics of Acid:
    • Sour in taste.
    • Turns blue litmus paper red.
    • Turns the solution of China rose to dark pink colour (magenta).
    Characteristics of Base:
    • Bitter in taste.
    • Turns red litmus paper blue.
    • Turns solution of China rose to green.
    • Turns turmeric paper to red.

    When solution of acid is mixed with the solution of base, both of them neutralize each other and a third substance; called salt; is formed.

    Such phenomenon is called neutralisation or neutralisation reaction.

    • The solution formed because of mixing of solution of acid and base is neither acidic nor basic in nature.
    • Such solution is known as neutral solution.
    • Salt formed because of neutralization reaction may be acidic or basic in nature.
    • Acidic or Basic nature of salt depends upon the strength of acid and base.
    • In neutralization reaction heat is evolved. Reactions in which heat is evolved are known as exothermic reactions.
    • Thus, neutralization reaction is an exothermic reaction.


    • Sodium hydroxide is a base and hydrochloric acid is an acid.
    • When solution of sodium hydroxide is mixed with the solution of hydrochloric acid, both neutralize each other and common salt (Sodium chloride) is formed.
    • Since, it is an exothermic reaction, so reaction mixture becomes slightly hot.
    • The reaction involved in this can be written as follows:
    • sodium
    • Sodium chloride is the chemical name of common salt which is used in household.

    Neutralization Reaction in Everyday Life:

    Neutralization Reaction in Everyday Life:

    There are many uses of neutralization reaction in everyday life.


    • Our stomach releases hydrochloric acid to kill bacteria; if any; present in food.
    • Hydrochloric acid released in our stomach also helps in the digestion of food.
    • Sometime our stomach produces more hydrochloric acid than required. Production of more hydrochloric acid in the stomach manifests as indigestion.
      This condition can be painful and causes pain in the stomach.
    • To get rid of such symptoms, a medicine made of milk of magnesia (a base) is taken orally.
    • Milk of magnesia, being a base neutralizes the hydrochloric acid and gives relief from pain because of indigestion.

    Ant sting:

    • Ant sting or bee sting contains methanoic acid.
    • Methanoic acid is also known as formic acid. Bee or ant injects formic acid into our skin, while biting.
    • Injection of acid by ant or bee results in pain at the place of bite.
    • Rubbing baking soda over the skin gives relief from pain due to ant or bee sting.
    • Baking soda, which is a base, neutralizes the effect of acid injected by bee or ant.
    • Another base, such as zinc carbonate (Calamite solution) is also used in the case of ant or bee sting.

    Soil Treatment:

    • Sometimes soil becomes acidic or basic due to excess use of fertilizers or wrong method of harvesting.
    • Acidic or basic nature of soil affects the yield as plants do not grow properly on such soil.
    • Acidic soil is treated with slaked lime or quick lime. Slaked lime (Calcium hydroxide) and quick lime (Calcium oxide) are bases.
    • Use of slaked lime or quick lime neutralizes the acidic nature of soil.
    • Basic soil is treated with organic matter. Organic matter releases acid and neutralizes the basic nature of soil.

    Factory Waste:

    • Wastes of most of the factories are acidic.
    • If such acidic wastes are flushed into rivers, acid present in them kills the aquatic organisms and pollutes the water.
    • Thus, factory wastes are treated with basic substances to neutralize the acid present in them before being flushed in the river.

    Important Points:

    • Acid is sour in taste.
    • Acid turns China rose solution to dark pink.
    • Base is bitter in taste.
    • Base turns red litmus paper to blue.
    • Base turns turmeric paper or solution to red.
    • Base turns China rose solution into green.
    • Indicator is a special substance which shows the acidic or basic character of a substance by change in colour.
    • Litmus, turmeric and solution of China rose are natural indicators.
    • Litmus is obtained from lichens.
    • Solutions of acid and base neutralize each other when mixed in proper ratio. This is called neutralization or neutralization reaction.
    • In neutralization reaction between the solutions of acid and base, a new substance called salt is formed.
    • Salt formed because of neutralization reaction may be acidic or basic in nature. The acidic or basic nature of salt depends upon the strength of acid and base used for neutralization reaction.
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