Page 87 - Chemistry ICSE Class X
P. 87

Study of Acids, Bases and Salts                                                                       73

          How will you find out if the given solution is acidic or basic
          The acidic or basic character of any solution can be determined by using
          acid-base indicators.
              An acid-base indicator is a compound or mixture of compounds             hŶŝǀĞƌƐĂů ŝŶĚŝĐĂƚŽƌ ŝƐ Ă ŵŝdžƚƵƌĞ ŽĨ
          which changes its colour with the change in the relative concentration of    ŽƌŐĂŶŝĐ ĚLJĞƐ ƚŚĂƚ ŐŝǀĞƐ Ă ƐƉĞĐŝĮĐ
                                                                                       ĐŽůŽƵƌ ĐŚĂŶŐĞ ŽǀĞƌ Ă ƌĂŶŐĞ ŽĨ Ɖ,
                              +
                                                       –
          hydronium ion (H O ) and hydroxide ion (OH ).                                ǀĂůƵĞƐ͘
                            3
              Some commonly used acid-base indicators are
            ƒ  Litmus
            ƒ  Methyl orange
            ƒ  Phenolphthalein
              Litmus contains a colouring substance called  azolitmin.  Azolitmin
          gives blue colour in alkaline solutions and red colour in acidic solutions.
          Litmus is commonly available in two colours — Blue litmus and Red litmus.
            ƒ  Acids change colour of the blue litmus to red.
            ƒ  Bases/Alkalis change the colour of the red litmus to blue.
              Thus, by observing the change in the colour of litmus solution or pH
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          Using litmus solution

                         Test                 Observation         Conclusion
           1. Take about 2 mL of the given  Colour changes  The given substance/
              substance/solution. Add 2 drops  to red.      solution is an  acid or
              of blue litmus to it, and observe             acidic in nature.
              any change in colour.
           2. Take about 2 mL of the given  Colour changes  The given substance/
              substance/solution in a test  to blue.        solution is a  base  or
              tube. Add 2 drops of red litmus               basic in nature.
              to it, and observe any change in
              colour.

          Using pH paper

                         Test                 Observation         Conclusion
          1.  Take a small piece of pH paper.  Colour changes  The given solution is an
              Place a drop of the given solution  from  yellow to  acid or acidic in nature.
              over it with a dropper. Observe  red.
              any change in colour.
           2. Take a small piece of pH paper.  Colour changes  The given solution is
              Place a drop of the given solution  from  yellow to  an alkali or alkaline in
              over it with a dropper. Observe  blue.        nature.
              any change in colour.

          Testing the Acidity, Neutrality and Alkalinity of a

          Solution using pH Paper and Universal Indicator
          The acidity, neutrality and alkalinity of a solution can be tested by using
          either of the following:
            ƒ  A pH paper
            ƒ  The universal indicator
              The following activity describes the experiment:
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