Acids Bases and Salts

Class 10 NCERT Science

NCERT

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?

Solution :

If the colour of red litmus paper gets changed to blue, then it is a base and if there is no colour change, then it is either acidic or neutral. Thus, basic solution can be easily identified. Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic.This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

2   Why should curd and sour substances not be kept in brass and copper vessels?

Solution :

Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.

3   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?

Solution :

Hydrogen gas is usually liberated when an acid reacts with a metal.$\\$ Take few pieces of zinc granules and add $5 ml$ of dilute $H_2 SO_4 .$ Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.$\\$ $H_2SO_4+Zn \rightarrow Zn SO_4 + H_2 \uparrow $$\\$ We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

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.

Solution :

$\underset{\text{Calcium carnonate}}{CaCo_{3(s)}} +\underset{\text{Hydrochloric acid}}{2HCl_{(aq)}} \rightarrow \underset{\text{Calcium chloride}}{CaCl_{2(aq)}}+ \underset{\text{Carbon dioxide}}{CO_{2(g)}}+\underset{\text{Water}}{H_2O+{(l)}}$

5   Why do $HCl, HNO_3$ , etc., show acidic characters in aqueous solutions while solutions ofcompounds like alcohol and glucose do not show acidic character?

Solution :

The dissociation of $HCl $ or $HNO_3$ to form hydrogen ions always occurs in the presence ofwater. Hydrogen ions $(H^ + )$ combine with $H_2 O$ to form hydronium ions $(H_3 O^ + ).$ The reaction is as follows:$\\$ $HCl_{(aq)} \rightarrow H^++Cl^-$$\\$ $ H^++H_2O \rightarrow H_3O^+$$\\$ Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.