CBSE Class 10 Science 2019 Outside Delhi Set 1

Question : 7

Total: 27

(a) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
(b) Dry hydrogen chloride gas does not change the colour of dry litmus paper why?
OR
How is sodium hydroxide manufactured in industries? Name the process. In this process a gas X is formed as by-product. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and Y and write the chemical equation of the reaction involved.

Solution:

(a) The process of mixing the concentrated acid with water is highly exothermic. So, when a concentrated acid is added to water then heat is easily absorbed by the large amount of water. Thus it is recommended to add acid to water and not water to the acid.
(b) Dry hydrogen chloride does not contain any hydrogen ions in it, so it does not show acidic behaviour and thus does not change the colour of dry litmus paper.
OR
Sodium hydroxide is manufactured by the electrolysis of concentrated aqueous solution of sodium chloride.

2‌NaCl(aq)

Sodium chloride

(Brine)

2H2O(l)

water

‌ Electricity ‌

────────▶

‌ (Electrolysis )‌

2‌NaOH(aq)

Sodium hydroxide

(Caustic Soda)

Cl2(g)

Chloride

H2(g)

Hydrogen

The process of manufacturing sodium hydroxide by electrolysis process is called chloro-alkali process.
Gas X is chlorine gas and compound Y is calcium oxychloride (Bleaching powder).

2‌NaCl(aq)+2H2O(l)→2‌NaOH(aq)+Cl2(g)+H2(g)‌Ca(OH)2+Cl2→‌CaCl2+H2O

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