(a) A gas is produced when conc. `H_(2)SO_(4)` is added to solide sodium chloride taken in a test tube and the tube is heated. The gas coming out through the delivery tube is passed over a dry litmus paper and then over a moist litmus paper. What would you observe ? Explain your answer.
(b) Fresh mikl has pH of 6. When it changes to curd (yogurt(, will its pH value increase or decrease? Why ?
(c ) What will be the colour of blure limus in a solution of sodium carbonate ?

Explanation

(a) Upon heating sodium chloride with conc. `H_(2)SO_(4),` hydrogen chloride gas evolves.
`NaCl(s)+H_(2)SO_(4) (aq)to NaHSO_(4)(aq)+HCl(g)`
In the gaseous state, the acidic character is not shown because HCl dows not release and `H^(+)` ions, In the presence of moisture (moist litmus paper), the gas changes to hydrochloric acid i.e., HCl (aq). The aid releases `H^(+)` ions and that , shows acidic character. Therefore, moist blue litmus paper becomes red.
(b) When fresh milk changes to curd, the pH of the pH of the solution is likely to drcrease. Actually, lactose present in milk gets converted to lactic acid when curd or yogurt is formed from milk. Therefore, the medium becomes more acidic and its pH decreases.
(c ) The solution of sodium carbonate `(Na_(2)CO_(3))` is of basic nature. Actually, the salt dissolves in water to produce NaOH (strong base) and `H_(2)CO_(3)` (weak acid). The blue litmus will not undergo any change in the basic medium. It will remain blue.
`Na_(2)CO_(3)+2H_(2)tounderset(("strong"))(2NaOH)+underset(("weak"))(H_(2)Co\O_(3))`