Solved examples for class 7 science : Chapter 5

    Acids, Bases and Salts

    I. Multiple Choice Questions (MCQs).
    Q.1. The word acid is derived from the Latin word, acere, meaning
    a) Bitter
    b) sour
    c) sweet
    d) salty
    Answer: (b) sour
    Q.2. Coffee which is bitter, in fact contains a/an
    a) acid
    b) base
    c) salt
    d) none of these
    Answer: a) acid
    Q.3. Acid changes blue litmus paper to
    a) colourless
    b) green
    c) red
    d) purple
    Answer: c) red
    Q.4. Base changes colour of turmeric indicator into
    a) blue
    b) red
    c) green
    d) colourless
    Answer. b) red
    Q.5. A pH value equal to 7 indicates a/an
    a) acidic solution
    b) basic solution
    c) neutral solution
    d) none of these
    Answer. (c) neutral solution
    II. Fill in the blanks by selecting appropriate words from the given list :
    List : weak, lactic, organic, mineral, hydrochloric
    Q.1. The acid present in fruits, vegetables, curd, and milk are known as _________ acids.
    Q.2. Sulphuric acid, hydrochloric acid, nitric acid and phosphoric acid are some of the ______________ acids.
    Q.3. When we exercise a lot, ______________ acid gets accumulated in our muscles, which leads to cramps.
    Q.4. Our stomach contains ______________ acid which helps in digestion of food.

    Q.5. The base used for making window cleaners is a ______________ base.

    Answer: 1-organic,
    2- mineral,
    3- lactic,
    4- hydrochloric,
    5- window cleaners

    III. Match the items of Column A with those of Column B.

    Answer: 1- c, 2-d , 3- a 4-e 5 -b

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Ammonium hydroxide is a very useful weak base used in the manufacture of fertilisers.
    Q.2. Bases are used in the manufacture of soaps, detergents, chemicals like bleaching powder, fertilisers, etc.
    Q.3. If a turmeric paper is dipped in vinegar or lemon juice its colour changes to blue.
    Q.4. Purple cabbage juice acts as an indicator for acids only.
    Q.5. Acids are corrosive in nature and dissolve many substances.

    Answer: 1-T, 2-T, 3-F, 4-F, 5-T

    V. Very Short Answer Questions.
    Q.1. Which gases present in the atmosphere are responsible for causing acid rain?
    Ans: Carbon dioxide, nitrogen dioxide, sulphur dioxide
    Q.2. Discuss the use of limewater.

    Answer: limewater is used as a Laboratory reagent. On a large scale, a dilute solution of lime in water is used for white washing.

    Q.3. What is the natural colour of litmus?
    Answer: Litmus is a natural dye extracted from lichens. It is purple or mauve in colour in distilled water.

    Q.4. Name the products formed in a neutralisation reaction.
    Answer: The salt formed in a neutralisation reaction may be acidic, basic or neutral. A neutral salt is formed by the neutralisation of a strong acid and a strong base. Sodium chloride (NaCl) is a neutral salt formed by the reaction between sodium hydroxide and hydrochloric acid.

    Q.5. Which acid is present in vinegar?
    Answer: Acetic acid
    VI. Short Answer Questions.
    Q.1. What is the difference between organic acid and mineral acid?

    Organic Acid 

    Mineral Acid 

    The acids present in fruits, vegetables, curd and milk are naturally occurring acids known as known as organic acids.

    Some of the acids are derived from minerals. They are known as mineral                                            acids. 

    They are weak acids and are present in varying amounts in these substances.

    They are strong acids and are used in the industry for the manufacture of
    fertilisers, chemicals, dyes, explosives, detergents, plastics and various
    other substances.

    Citric acid, Lactic acid, Maleic acid are example of organic acid.

    Sulphuric acid (H2SO4) hydrochloric acid (HCl), nitric acid (HNO3) and phosphoric acid (H3PO4) are some of the mineral

    Q.2. Why do we feel burning sensation when we suffer from indigestion?
    Answer: Our stomach secretes dilute hydrochloric acid which helps us to digest the
    food. But sometimes during indigestion, excess of this acid is secreted. It is very painful since the acid penetrates the lining of the stomach and damages the cells below. This causes discomfort and a burning sensation in the stomach.

    Q.3. How do the factory wastes discharged into the water bodies, affect aquatic life?
    Answer: The wastes of the factories contain mainly acids. When these are discharged into a river, sea and other water bodies, they kill the fishes and other aquatic organisms.

    Q.4. (a) What do you mean by acid? Explain briefly.
    (b) What do you mean by base? Discuss with examples.
    Answer.(a) Acid is a substance which tastes sour and turns blue litmus red.
    (b) Base is a substance which is bitter, feels soapy to touch and turns red litmus blue. Some examples of base are ammonium hydroxide, calcium hydroxide and sodium hydroxide etc.

    Q.5. What is acid rain? How is it formed? What are its adverse effects?
    Answer: In regions of heavy pollution, gases like carbon dioxide, nitrogen dioxide, sulphur dioxide are released in the atmosphere. Rainwater dissolves these gases and forms carbonic acid, nitric acid and sulphuric acid respectively. Thus rainwater becomes acidic and is called acid rain. It damages buildings, historical monuments, plants and animals.

    Q.6. What do you mean by indicator? Name some common indicators and also discuss their effective colour changes with acidic, basic and neutral solutions.
    Answer: Indicator is a substance that shows a change in colour when brought in contact with an acid, base or a neutral substance.
    Turmeric, litmus, China rose petals, purple cabbage juice, are some of the naturally occurring indicators. Phenolphthalein and methyl orange are some synthetic indicators.

    Litmus: Acids or their solutions change the colour of blue litmus to red while bases or their solutions change the colour of red litmus to blue. Neutral solutions do not change the colour of red or blue litmus.

    Turmeric: In a basic solution, this indicator changes to red but in acidic solution, its colour does not change. Turmeric indicator does not change its colour even in a neutral solution.

    Phenolphthalein: It is used in an acid-base reaction (neutralisation reaction) to indicate the completion of the reaction by its colour change. In a basic solution, phenolphthalein gives a pink colour and in an acidic and neutral solution it remains colourless. Methyl orange: It changes to yellow colour in neutral or basic solutions. It indicates an acidic solution by changing to a red colour.

    China rose: This indicator changes to dark pink [magenta] colour in acidic solutions and to green colour in basic solutions.

    VII. Long Answer Questions.

    Q.1. What is an indicator used for? How is pH used to measure the strength of acids or bases?
    Answer: An indicator is used to indicate whether a substance is an acid or a base. The strength of the acids or bases is measured by a scale known as the pH scale. It varies from 0 to 14. A pH value Less than 7 indicates an acid while pH value more than 7 indicates a base. A pH value equal to 7 indicates a neutral solution.

    Q.2. What do you mean by neutralisation reaction? Explain with examples.
    Answer: When an acidic solution and a basic solution are mixed with each other, they neutralise the effect of each other. This reaction is known as neutralisation reaction.

    For example, the reaction between hydrochloric acid and sodium hydroxide to form sodium chloride and water with the evolution of heat energy is a neutralisation reaction.

    Hydrochloric acid + Sodium hydroxide -> Sodium chloride + Water + Heat 
    Q.3. What are the applications of neutralisation reaction in everyday life? Discuss with some examples.
    Answer: Following are the some of the applications of neutralisation reaction.
    (a) Indigestion : Our stomach secretes dilute hydrochloric acid which helps us to digest the food. But sometimes during indigestion, excess of this acid is secreted. To neutralise the effect of excess acid, we take an antacid, like milk of magnesia or magnesium hydroxide (base). Baking soda or sodium hydrogen carbonate, which is a salt of a strong base, is also taken to cure indigestion.
    (b) Treatment of soil : The soil of an agricultural land becomes acidic on excessive use of chemical and organic fertilisers. Plants do not get nutrients when the soil is excessively acidic or basic. This hinders their proper growth. When a soil is extremely acidic, quick lime (calcium oxide) or slaked lime (calcium hydroxide) is added to it to neutralise the effect of the acidic substances present. Similarly basic soil is treated with acidic substances like an organic matter. Organic matter releases acids and neutralises the effect of the basic substances present in soil.

    (c) Ant sting: When an ant stings, it releases a liquid into the body of the person. The liquid contains a weak acid such as formic acid. This causes swelling and a burning sensation in the skin. To neutralise the effect of the sting, moist baking soda (sodium hydrogen carbonate) or calamine is rubbed on it.

    (d) Factory waste: The wastes of the factories contain mainly acids. When these are discharged into a river, sea and other water bodies, they kill the fishes and other aquatic organisms. These acids should, therefore, be neutralised by adding basic substances such as slaked lime, before being discharged into the water.

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