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ACIDS BASES AND SALTS, 1) You have been provided with three test-tubes. One of them, contains distilled water and the other two contain an acidic, solution and a basic solution, respectively. If you are given, only red litmus paper, how will you identify the contents of, each test-tube?, Ans : (i) Red litmus paper is added in all the three test tubes,, turn by turn. The solution which turns red litmus to blue will, be basic solution., (ii) The blue litmus paper [ formed in test (i) ] is added in the, remaining two test tubes one by one. The solution which, turns blue litmus paper red will be the acidic solution., (iii) The solution which has no effect on red litmus paper as, well as on blue litmus paper will be neutral and therefore, it, will be distilled water., 2) Why should curd and sour substances not be kept in brass, and copper vessels?, Ans. Curd and other sour substances contain acids which, can react with the metals of brass and copper vessels to, form toxic (poisonous) metal compounds which make the, substances unfit for consumption. Hence, to prevent any, such possibility curd or any sour substances should not be, kept in copper and brass vessels., 3) Which gas is usually liberated when an acid reacts with a, metal? Illustrate with the help of an example. How will you, test the presence of this gas?, Ans. When an acid reacts with a metal, then hydrogen gas, is liberated., For example :, Zn (s) + H2SO4 (aq) ⎯⎯→ ZnSO4 (aq) + H2 (g).
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4) Metal compound A reacts with dilute hydrochloric acid to, produce effervescence. The gas evolved extinguishes a, burning candle. Write a balanced chemical equation for the, reaction if one of the compounds formed is calcium chloride., Ans : The gas evolved is carbon dioxide which extinguishes, a burning candle., The metal compound A is calcium carbonate and the, reaction is:, CaCO3 + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O(l), 5) Why do HCl, HNO3, etc., show acidic character in aqueous, solutions while solutions of compounds like alcohol and, glucose do not show acidic character?, Ans. The acidic character of acids is due to the presence of, hydrogen ions [H+ (aq) ions] in its aqueous solution. HCl and, HNO3 show acidic properties because they produce, hydrogen ions when dissolved in water., On the other hand, solutions of compounds like alcohol and, glucose do not show acidic character because they do not, ionise (or dissociate) in water to produce hydrogen ions., 6) Why does an aqueous solution of an acid conduct, electricity?, Ans : The aqueous solution of an acid possesses ions in it, and since ions are the carrier of charge, an aqueous, solution of an acid conducts electricity., 7) Why does dry HCl gas not change the colour of dry litmus, paper?, Ans. The colour of the litmus paper changes only in, aqueous solution due to the dissociation of an acid to, liberate H+ ions. Since, in this case, neither HCL is in, aqueous form nor is the Litmus paper wet, so the dry litmus, paper does not change its colour.
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8) While diluting an acid, why is it recommended that the acid, should be added to water and not water to the acid?, Ans. The dilution of an acid in water is a highly exothermic, one. The acid must always be added slowly to water with, constant stirring as so that the heat is evolved gradually and, easily absorbed by the large amount of water., If however, water is added to concentrated acid, then the, heat generated may cause the mixture to splash out and the, container may even break due to excessive local heating., 9) How is the concentration of hydronium ions (H3O+) affected, when the solution of an acid is diluted?, Ans. On diluting an acid, the overall volume of the solution, increases and so the concentration of hydronium ions, (H3O+) per unit volume decreases., 10) How is the concentration of hydroxide ions (OH–) affected, when excess of base is dissolved in a solution of sodium, hydroxide?, Ans. When excess of base is dissolved in a solution of, sodium hydroxide, the concentration of hydroxide ions, (OH-) increases., 11) You have two solutions A and B. The pH of solution A is 6, and pH of solution B is 8. Which solution has more hydrogen, ion concentration? Which of these is acidic and which one, basic?, Ans : Solution A has more hydrogen ion concentration as, the less is the pH value, the more is the hydrogen ion, concentration is., Solution A is acidic and solution B is basic., 12) What effect does the concentration of H+ (aq) ions have on, the nature of the solution?, Ans. Higher the concentration of H+(aq) ions in a solution,, more acidic the solution will be.
