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Why does ice float on top of water instead of sinking like most solids? There are two parts to the answer to this question: first we’ll find out why something floats, and then we’ll examine why ice floats on top of liquid water instead of sinking to the bottom.
Why does ice float?
A substance floats if it is less dense or has less mass per unit volume than the other components in the mixture. For example, if a handful of rocks is dropped into a bucket of water, the rocks, which are denser compared to water, sink. Water, which is less dense than rocks, will float. Basically, the rocks push the water out of the container or displace it. For an object to float, it has to displace a weight of fluid equal to its own weight.
Water reaches its maximum density at a temperature of 4°C (40°F). As it gets colder and freezes, turning into ice, which is actually less dense than water. For most substances, the density increases in their solid (frozen) state, that is, they are usually more dense than in their liquid state. Water is different because of the hydrogen bonds.
A water molecule is made up of one oxygen atom and two hydrogen atoms strongly bonded together by covalent bonds. Water molecules are also attracted to each other by chemical bonds much weaker than covalent ones, called hydrogen bonds . This attraction occurs between the hydrogen atoms that have positive charges and the oxygen atoms of neighboring water molecules that have negative charges. As the water cools below 4 °C, the hydrogen bonds rearrange to keep the negatively charged oxygen atoms apart. All this produces a network of crystals 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 of water. The heavier water displaces the lighter ice, so the ice floats on the surface of the water. One consequence of this is seen in lakes and rivers, which freeze from the top down, allowing fish to survive even when the surface of a lake has frozen. If the ice were to sink, water would move to the surface and be exposed to colder temperatures, causing rivers and lakes to freeze over.
Heavy water ice sinks into liquid water
However, not all water in the form of ice or in a solid state floats on the surface of normal water. Ice made with heavy water, that is, that which contains the hydrogen isotope deuterium, sinks in normal water. Hydrogen bonds still occur in this type of water, but not in sufficient numbers to make up for the difference in mass between normal water and heavy water. Therefore heavy water ice also sinks in liquid water.
