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

Compound Formed is Copper Chloride

Explanation:

• a.) pH = 6  

Since fresh Milk is changed to curd, curd comes under category of Lactic Acids which has Lower pH than 6.

• b.) Dil Hcl + Sodium Hydrogencarbonate,

$Hcl + NaHCO_3 \geq NaCl + H_2O + CO_2$

which shows that the new compound formed is Common Salt or Sodium Chloride. It can be recognized by the Brisk Effervescence (Milky) or Lime Water formation from $CO_2$.

• c.) On adding dilute Hydrochloric acid to copper oxide powder, the solution formed is blue-green, as we have got the formation of Copper Chloride.

$HCl + CuO \geq CuCl_2 + H_2O$

       dil                      Blue-Green

Answer 2

Answer:

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

Explanation:

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