Water density at 0 C is 0.999 g/cm3, and at 99 C it is 0.958 g/cm3, so why does ice float in hot water if ice is more dense that hot water.
A substance floats if it is less dense, or has less mass per
unit volume, than other components in a mixture. For example, if
you toss a handful of rocks into a bucket of water, the rocks,
which are dense compared to the water, will sink. The water, which
is less dense than the rocks, will float. Basically, the rocks push
the water out of the way, or displace it. For an object to be able
to float, it has to displace a weight of fluid equal to its own
weight.
Water reaches its maximum density at 4°C (40°F). As it cools
further and freezes into ice, it actually becomes less dense. On
the other hand, most substances are most dense in their solid
(frozen) state than in their liquid state. Water is different
because of hydrogen bonding.
A water molecule is made from one oxygen atom and two hydrogen
atoms, strongly joined to each other with covalent bonds.
Water molecules are also attracted to each other by weaker chemical
bonds (hydrogen bonds) between the positively-charged hydrogen
atoms and the negatively-charged oxygen atoms of neighboring water
molecules. As water cools below 4°C, the hydrogen bonds adjust to
hold the negatively charged oxygen atoms apart. This produces a
crystal lattice, which is commonly known as 'ice'.
Ice floats because it is about 9% less dense than liquid water. In
other words, ice takes up about 9% more space than water, so a
liter of ice weighs less than a liter water. The heavier water
displaces the lighter ice, so ice floats to the top. One
consequence of this is that lakes and rivers freeze from top to
bottom, allowing fish to survive even when the surface of a lake
has frozen over. If ice sank, the water would be displaced to the
top and exposed to the colder temperature, forcing rivers and lakes
to fill with ice and freeze solid.
Water density at 0 C is 0.999 g/cm3, and at 99 C it is 0.958 g/cm3,...
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