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Acids Bases and Salts : NCERT Intext Questions


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Page Number – 18

Q.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?
Sol.

First of all, we will put one strip of red litmus paper in each test tube. In one of the test tubes, the litmus paper will turn blue. It is sure that this test tube contains basic solution. The remaining two test tubes contain distilled water and acidic solution in both of which red litmus remains red.

Now, we will pour out some basic solution from the test tube so that only a little amount remains in it. Then, we will pour the contents of one of the remaining test tubes into the test tube containing basic solution. Two things may happen :

(a) The litmus may turn red again : This indicates that the second test tube contains acidic substance. And the other one contains distilled water.

(b) The litmus may remain blue : This indicates that the poured liquid is distilled water. And the other one is the acidic solution.


Page Number – 22

Q.1 Why should curd and sour substances not be kept in brass and copper vessels?
Sol.

Curd and sour substances contain acids. Acids react with metals to give salt and hydrogen gas. So, if such substances are kept in copper container, the acid will react and the container will corroded.


Q.2 Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Sol.

Usually, hydrogen gas is liberated when an acid reacts with a metal. Let us take the reaction between zinc and sulphuric acid.

13

Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning

The apparatus is set as shown in Fig.  About 5 ml of dil. H2SO4 is taken in the test tube and a few granules of zinc are added to it. The delivery tube is caped in the test tube so that the evolved gas passes through the soap solution. Soap bubbles filled with the evolved gas come out of the solution. The flame of a lighted candle is brought in contact with the bubble. The gas in the bubble burns with a pop sound. This proves that the gas evolved in the reaction between zinc and sulphuric acid is hydrogen.


Q.3 A Metal compound 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

Sol. CaC{O_3} + 2HCl \to CaC{l_2} + {H_2}O + C{O_2}


Page Number – 25

Q.1 Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Sol. 

HCl, HNO3, etc. have releasable H+ ion. When dissolved in water, the H+ ion gets separated and shows acidic character. In alcohol or glucose, there is no releasable H+ ion.


Q.2 Why does an aqueous solution of an acid conduct electricity?
Sol.  

When dissolved in water, acids dissociate to form ions e.g.,
HCl + {H_2}O \to Cl - + {H_3}{O^ + }

These ions are responsible for electrical conductivity.


Q.3 Why does dry HCl gas not change the colour of the dry litmus paper?

Sol. Dry HCl does not dissociate to give H+ ions. So, it does not show acidic character.


Q.4 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Sol.

The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.


Q.5 How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Sol. Concentration of H3O+ ion decreases.


Q.5 How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Sol. Concentration of H3O+ ion decreases.


Q.6 How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide ?

Sol. Concentration of OH group increases.


Page Number – 28

Q.1  You have two solutions, A and B. The pH solution A is 6 and pH of solution B is 8. While solution has more hydrogen ion concentration. Which of this is acidic and which one is basic ? 

Sol. 

The solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.


Q.2 What effect does the concentration of H+(aq) ions have on the nature of the solution?

Sol. As the concentration of H+ ions increases the solution becomes more acidic.


Q.3 Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Sol. This H+ ion comes from water.


Q.4 Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Sol. If the farmer finds his soil turned acidic, he should use the bases to neutralise it.


Page Number – 33

Q.1  What is the common name of the compound CaOCl2?

Sol. Bleaching powder.


Q.2  Name the substance which on treatment with chlorine yields bleaching powder.

Sol. Dry slaked lime [Ca(OH)2]


Q.3 Name the sodium compound which is used for softening hard water.

Sol. Washing soda i.e., sodium carbonate (Na2CO3).


Q.4 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Sol.

The following reaction take place when it is heated during cooking :


Q.5 Write an equation to show the reaction between Plaster of Paris and water.

Sol. CaS{O_4}.{1 \over 2}{H_2}O + 1{1 \over 2}{H_2}O \to CaS{O_4}.2{H_2}O



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