What is the answer for 1,5

Dear student,
1) The reaction proceeds as,

H₂SO₄ + 2 NaCl -------> 2 HCl + Na₂SO₄

Formation of hard crust of Na₂SO₄ which sticks to glass
2) The dense white fumes are formed when a bottle full of HCL gas is inverted over a jar of ammonia gas. This happens because of the formation of ammonium chloride. 
HCl + NH₃ →NH₄Cl

3) When the HCl gas dissolves, water is sucked up to create a fountain, this creates a lot of vaccum which may lead to imploding of the flask. In R.B the pressure is equally distributed and hence is safer to conduct the exptt.

4) It is done to remove existing rust and prevent oxidation till galvanizing process begins.
5) HCl gets fumes in moist air but H2S gas does not. Hydrogen chloride gas fumes in moist air because it has high solubility. It forms a mist of droplets of hydrogen chloride. The HCl gas mixes with the atmospheric vapour to form a mist which appears as white fumes. 
Regards