H₂O₂ can be prepared by adding a metal peroxide to dilute acid.
(i) BaO₂₍s) + H₂SO₄(aq) ➝ BaSO₄(s) + H₂O₂(aq)
(ii) Na₂O₂(s) + H₂SO₄(aq) ➝ 2Na+(aq) + SO₄^2-(aq) + H₂O₂(aq)

HF is a liquid due to the strong intermolecular hydrogen bonds between two molecules. Whereas in HCI, due to large size of chlorine atom and low electron density it does not exhibit hydrogen bonding so exists as gas.

Water (H₂O) is polar because of the bent shape of the molecule.

The shape means most of the negative charge from the oxygen is on one side of the molecule and the positive charge of the hydrogen atoms is on the other side of the molecule. This is an example of polar covalent chemical bonding.

Small amounts of hydrogen are conveniently prepared in laboratory by the reaction of metals, such as zinc, iron and tin, with dilute acid.
Zn_(s) + 2HCl_(aq) ➝ ZnCl_2(s) + H_2(g)

(i) WO₃ + 3H₂ ➝ W + 3H₂O
(ii) H₂ + Cl₂ ➝ 2HCl.

a) HCl is a gas. It is an electron rich covalent hydride. It is a covalent molecule with strong covalent bond. However the forces between the molecules are not as strong.
b) NaH is a solid. It is an ionic hydride of group I sodium metal. It is an insoluble hydride

Hydrogen has electronic configuration 1s¹. So it resembles ns¹ general valence shell configuration of alkali metals.
Its similarity with alkali metals are:
1. Form uni positive (H⁺) ions like Na⁺, K⁺, Cs⁺
2. Form halides, (HX), oxides (H₂O) peroxides (H₂O₂) and sulphides (H₂S) like (Nax, Na2O, Na₂O₂, Na₂S)
3. Acts as reducing agent. 
4. Has +1 oxidation state.
5. If it is placed in the halogen family it should be the most electro negative element which is not true.

a) Electron-precise hydrides:
These have required number of electron to represent their conventional d lewis structure. All the elements of carbon group (14) form such hydrides.
Eg CH₄, C₂H₆, SiH₄, GeH₄
b) Electron - deficient hydrides:
There act as Lewis acids (ie) electron acceptors. The elements from group (13) form such hydrides.
Eg: B₂H₆
c) Electron - rich hydrides:
There have excess electron, which are present as lone pairs. Elements of group 15-17 form such hydrides. They behave as Lewis bases (ie) electron donors.
Eg: NH₃ ,H₂O, HF

NOTE:
At any condition ice cube does not sink in water.
At 0⁰C, ice and liquid water will be in equilibrium and will coexist. Hence no freezing or melting will occur.

(i) Atomic hydrogen and oxy-hydrogen torches finds use for cutting and welding purposes.
(ii) Atomic hydrogen atoms are allowed to recombine on the surface to be welded to generate temperature of 4000 K.

When conc. H₂SO₄ absorbs H₂O from moist H₂ it produces so much of heat since its highly exothermic and so it may catch fire.

(i) Resemblance with alkali metals :
Its electronic configuration is 1s¹ which is similar to alkali metals. 
It acts as a good reducing agent. 
Like alkali metal, it loses one electron to form H⁺, 
It is liberated at cathode when HCI is electrolysed.
It is electropositive in nature.
(ii) Resemblance with halogens :  
It is a non-metal. 
It is a gas like F₂ and Cl₂ 
It is slightly electronegative in nature 
Like halogens it is ready to accept one electron and attain the stable inert gas configuration.