http://i64.tinypic.com/23hvss8.jpg - Question 2 why is the answer (c)..?

2.
Equation for the reaction :
NH₄Cl(s) → NH₃(g) + HCl(aq)

Molar mass of NH₃ = (14.01 + 1.008×3) g/mol = 17.03 g/mol
Molar mass of HCl = (1.008 + 35.45) g/mol = 36.46 g/mol

Since the molar mass of NH₃ is greater than that of HCl, the density of NH₃ is lower than that of HCl. Hence, NH₃ diffuses out of the test tube in a faster rate than HCl.

Firstly, NH₃ diffuses out of the test tube and turns the damp red litmus paper blue because NH₃ is a basic gas. After some time, HCl diffuses out of the test tube and turns the damp litmus paper red again because HCl is an acidic gas. Neither NH₃ nor HCl is a bleaching agent, and thus the litmus paper will not be bleached.

...... The answer is : C. damp red litmus paper turns blue and then turns red.