Answer – Ice has a lower density than water. Therefore, it floats in it instead of sinking to the bottom.
Explanation:
The density of a substance is its mass per unit volume. Another way to look at it is as the number of atoms of a substance that are required to occupy and fill up a unit of volume. The fewer the atoms needed, the lower is the density and vice versa.
Solids tend to have higher densities as the atoms are tightly packed in crystal lattice structures. In fluids (liquids and gases), on the other hand, atoms are not so closely bound and can move around relatively freely (more so in gases than in liquids). Thus, the solid states of most materials are denser than their fluid counterparts.
However, in water, it is not so. When the liquid state of water is cooled, initially it does start to become denser. However, once the temperature falls below -4oC, ice (solid state of water) begins to expand outward, becoming less dense.
This is due to the molecular structure of water. Each water molecule is V-shaped and contains two hydrogen atoms and one oxygen atom. The bonds between these atoms are covalent (electrons are shared between them). Oxygen atoms, being larger, are able to exert a greater pull over the shared electrons, resulting in the oxygen section of the molecule having a slightly negative charge, and the hydrogen section having a slightly positive charge.
These molecules then interact with each other and can bond when sections of opposite charges attract one another. In the fluid states of water, this happens easily and frequently, with bonds between molecules forming and breaking as they move around, flowing close to one another and then separating. However, in solid state (below -4oC) the molecules get too close and the similar charges begin to repel one another, forcing them apart. This causes ice to be about 9% less dense than (liquid) water.
For any object to sink into water (or any other liquid), its density needs to be higher than that of water (or the liquid). Objects of lower density (than that of water) float. And since ice is of a lower density than water, it floats in it instead of sinking.
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