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

Hydrogen is a gas at room temperature. Because of it occupying large volume, it is difficult to transport it either by rail or by road. However, by cooling and applying high pressure, gaseous \({ H }_{ 2 }\) can be converted into liquid \({ H }_{ 2 }\)which has much smaller volume and hence, can be transported easily. Thus, the basic property of hydrogen which is useful for hydrogen economy is that it can be converted into a liquid by cooling under high pressure.

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.

Density of ice is less than that of liquid water, so it floats over water.

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.