F is a smaller and more electronegative than Cl, so it forms stronger H-bonds as compared to Cl. That's why HF is liquid and Hcl is a gas.

In metallic hydrides, hydrogen is absorbed as H-atoms. This property of adsorption of hydrgen on transition metals is widely used as its storage media. Some of the metals such as Pd, Pt can accommodate a very large volume of hydrogen storgae and as a source of energy. Metalic hydrides on heating decompose to form hydrogen and very finely divided metal.

Density of ice is less than that of liquid water. During sereve winter, the temperature of surface water keeps on decreasing and ultimately it freezes.
Thus, the ice layer at lower temperature floats over the water below it.Due to this, freezing of water into ice takes place continously from top towards bottom.

\(PbS(s)+{ H }_{ 2 }{ O }_{ 2 }(aq)\rightarrow PbSO_{ 4 }(s)+{ 4H }_{ 2 }O\)

Hydrogen because of the presence of only one electron in the valence shell can either lose or gain or share it to acquire noble gas, i.e. helium gas, configuration. Therefore, in principle, it can form either ionic or covalent bonds. But the ionisation enthalpy of His very high (1312 kJmol^-1) and its electron gain enthalpy is only slightly negative (-73 kJmol^-1). As a result, it does not have a high tendency to form ionic bonds but rather prefers to form only covalent bonds.