CBSE Class 10 Science 2015 Term I Set 1

Question : 20

Total: 36

(a) Define a universal indicator. Mention its one use.
(b) Solution A gives pink colour when a drop of phenolphthalein indicator is added to it. Solution B gives red colour when a drop of methyl orange is added to it. What type of solutions are A and B and which one of the solutions A and B will have a higher pH value ?
(c) Name one salt whose solution has pH more than 7 and one salt whose solution has pH less than 7

Solution:

(a) Universal indicator is a mixture of many different indicators which gives different colours at different pH values of the entire pH scale.
It shows different colours at different concentrations of hydrogen ions in a solution.
(b) Solution A gives pink colour when a drop of phenolphthalein indicator is added, therefore A is a base.
Solution B gives red colour when a drop of methyl orange is added to it, therefore B is an acid.
Hence, solution A will have less concentration of hydrogen ion than B.
Thus, A will have pH more than 7 beause pH value of
(i) an acid solution <7
(ii) a base solution >7
(iii) a neutral solution =7
(c) (i) The salts of strong acids and weak bases give acidic solution having pH less than 7. Example, NH4‌Cl, Ammonium Chloride will have pH less than 7.
(ii) The salts of weak acids and strong bases give basic solution having pH more than 7. Example, Na2CO3 Sodium Carbonate will have pH more than 7 .

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