Why does ice float on water rather than sink like most solids? There are two parts to answer this question. First, let’s look at why everything floats. Next, let’s examine why the ice floats above the liquid water, instead of sinking to the bottom.
Why ice floats
A substance floats if it is less dense, or has less mass per unit volume, than the other components of a mixture. For example, if you throw a handful of rocks into a bucket of water, the rocks, which are dense with respect to water, will sink.
The water, less dense than the rocks, will float. Basically, rocks repel or displace water. In order for an object to float, it must move a weight of fluid equal to its own weight.
Water reaches its maximum density at 4 C (40 F). As it cools more and freezes in the ice, it becomes less dense. On the other hand, most substances are denser in their solid (frozen) state than in their liquid state. Water is different due to the hydrogen bond .
A water molecule consists of an oxygen atom and two hydrogen atoms, strongly linked to each other by covalent bonds . Water molecules are attracted to each other by weaker chemical bonds (bonds hydrogen ) between the atoms of hydrogen and the positively charged oxygen atoms negatively charged neighboring water molecules. When the water cools below 4 ° C, the hydrogen bonds adjust to keep the negatively charged oxygen atoms separate.
This produces a crystal lattice, commonly known as “ice”.
Ice floats because it is about 9% less dense than that of liquid water. In other words, ice occupies about 9% more space than water, so one liter of ice weighs less than one liter of water. Heavier water moves lighter ice, so ice floats upward.
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. If the ice flowed, the water would be moved upward and exposed to the cooler temperature, forcing rivers and lakes to fill with ice and freeze.
Heavy water ice sinks
However, not all water ice floats on ordinary water. Heavy water ice, which contains the isotope of hydrogen deuterium, flows into ordinary water . The hydrogen bond still occurs, but this is not enough to compensate for the mass difference between normal water and heavy water. Heavy water ice flows into heavy water.
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