Water is a polar molecule. The force of attraction between positively charged hydrogen atoms on one water molecule and the negatively charged oxygen atom on another water molecule results in intermolecular forces. The intermolecular force is called a hydrogen bond. Other molecular forces in water are dipole van der Waals forces and dispersion forces. The water or H two O molecule has a 104 degree angle between the oxygen and the hydrogen atoms. The angle shape of water molecules and the binding forces which remain strong at these angles allows water molecules to form an open hexagonal crystal structure when water freezes. The water crystalline structure resembles the carbon crystal forming a diamond. The open hexagonal crystal structure of water results in the expansion of the water as it freezes. This expansion makes ice less dense than water. This makes ice float on water. Thus ice forms on lakes and rivers from the top down.
Most crystalline arrangements of molecules in a solid result in a smaller volume for a solid than its liquid which makes most solids denser than their liquids. The molecules in most solids are packed tightly together. So the solid form of most substances sinks to the bottom of the container.
Water exerts an upward force on objects placed in it. An object floats if the upward force exerted by the water is greater than the weight of the object. Objects denser than water are partly submerged in the water. The upward force exerted by water is called buoyancy.
Water has its smallest volume and greatest density at four degrees Celsius or 39.2 degrees Fahrenheit. Before any ice can form, all the water in an ice cube tray, lake, or river must be cooled to this temperature. Then the ice can form. It is good for wildlife that ice floats and forms at the surface of bodies of water from the top down. This allows amphibians and fish to survive at the bottom of lakes and rivers through the winter. When the ice gets thick enough for man to safely stand on it, it is possible to go ice fishing and continue to eat through the winter.
Another reason why it is good that ice floats on water is that this makes it possible for the ice to melt in the summer when the temperature increases. If ice sank to the bottom of a large body of water, it might never warm enough to melt.