Answer for your questions XA&B

CBSE NCERT textbook page no:35
Qn 10.
Answer:
More vigourous fizzing wiil occur in test tube A(HCl).
Bcoz,HCl is stronger than CH3COOH.
The liberation of H2 wiil be faster in HCl than CH3COOH.
Mg + 2 HCl ------> MgCl2 + H2 Fast
Mg + 2 CH3COOH -----> (CH3COO)2Mg + H2 Slow
Qn 11.
Answer:
When milk changes into curd,pH will decrease. This is bcoz curd is lactic acid i.e more acidic than milk. If More acidic pH is less.
Qn 12.
Answer:
(a) Fresh milk is acidic and turns sour easily to become more acidic. In presence of baking soda(weak base) milk become alkaline and does not turn sour easily because alkali does not allow the milk to become more acidic easily.
(b) The presence of alkali will not allow the milk to turn sour even @the b.pt.
(c) When the milk sets curd, the pH decreases,i.e., the milk becomes more acidic.The presence of alkali does not allow the milk to become more acidic easily. Hence it will take a long time to set as curd.