Question

(a) Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increase or decrease? Why?
(b) Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How is it recognised?
(c) On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.

Answer 1

Answer:

A) Decrease

B) NaCl,CO2

C)CuCl2

Explanation:

a) When fresh milk changes into curd, its pH value decreases because of an increase in the acidity of the milk. Bacteria usually change fresh milk to curd by producing lactic acid.

(b) When sodium hydrogen carbonate reacts with dilute hydrochloric acid, it gives sodium chloride, carbon dioxide, and water. When the gas liberated is passed through lime water, lime water turns milky.

NaHCO3+HCl  → NaCl+CO2+H2O

(c) When dilute hydrochloric acid is added to copper oxide powder, a blue-green coloured compound called copper chloride will be obtained.

CuO+2 HCl    → CuCl2blue green+H2O

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Answer 2

Explanation:

(a) pH of curd:

• The pH of the curd decreases and the pH value varies from 4.5 to 5.5.
• The milk is covered to curd with the help of bacteria in the process fermentation. This bacteria produces an acid named lactic acid. The pH of the acid is less than 7.

(b) Chemical reaction:

• The name of the gas evolved is carbon dioxide.
• The reaction between dilute HCl and sodium hydrogencarbonate is given below:

       $\mathrm{NaHCO}_{3}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}$

• The carbon dioxide is recognised with the help of calcium hydroxide (lime water). The gas is passed over lime water which turns it into milky white substances slaked lime (calcium carbonate). The reaction is given below:

       $\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{CO}_{2} \rightarrow \mathrm{CaCO}_{3}+\mathrm{H}_{2} \mathrm{O}$

(c) Chemical reaction:

• The reaction between dilute hydrochloric acid and copper oxide powder is given below:

        $\mathrm{CuO}+2 \mathrm{HCl} \rightarrow \mathrm{CuCl}_{2}+\mathrm{H}_{2} \mathrm{O}$

• The products formed in the reaction includes copper (II) chloride and water. The copper (II) chloride is the compound which is in blue green colour which imparts that colour to the solution.