If same mass of liquid water and a piece of ice is taken, then why is the density of ice less than that of liquid water?

Complete the following equation:

 $\mathrm{PbS} \left(\mathrm{s}\right) + {\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)\to$

Complete the following equation:

$\mathrm{CO} \left(\mathrm{g}\right) + 2{\mathrm{H}}_2 \left(\mathrm{g}\right)\underset{\mathrm{Catalyst}}{\overset{\mathrm{Cobalt}}{\to }}$

Give reason: Lakes freeze from top towards bottom.

Give reason: Ice floats on water.