name given to the method of drying hydrogen & collecting it in a gas jar . why hydrogen be collected this way

Answer 1

The method is called laboratory "syntheses" of hydrogen gas. A "Kipp Gas Generator" can be sued to make hydrogen using one of two methods:

1) Preparation of Hydrogen by the Action of Metals. Here, alkali metals like lithium, sodium, and potassium react violently with water to yield hydrogen. The reaction with lithium is: 2 Li + 2 H2O ==> H2 + 2 LiOH

2) Preparation of Hydrogen from Action of Acids. Here, sulphuric acid or hydrochloric acid is added to granulated zinc and hydrogen is evolved and collected over water in a trough. The reactions are:
Zn + H2SO4 ==> ZnSO4 + H2
or
Zn + 2 HCl ==> ZnCl2 + H2

Answer 2

The most simple method of preparing Hydrogen in laboratory is electrolysis - of acidic water in galvanic cell. Hydrogen is collected by displacement of water in inverted cylinder over the electrode, as it is much lighter gas.
Hydrogen prepared by electrolysis of water is chemically pure, and in most case do not require any purification. Because it is collected over water, it is saturated with moisture, and drying is not necessary in all case. Hydrogen can be dried over Sulphuric acid, Phosphoric acid or over Molecular sieve - as per need.

Answer 3

1. conc H2SO4 (doesn't react with it)
2. upward delivery (less dense than air)