Why Does Ice Float?

It is very likely that most everyone would know the answer to this question and answer: "Because it is less dense than liquid water." and this is essentially correct, but not absolutely definitive. Density is the amount of matter in a given volume of space. Usually scientists would compare everything on a volume of 1 cubic centimeter. You can't measure liquids as if they were little solid cubes, so we would use a graduated cylinder, measure the water and convert it to cubic centimeters. The density of water is established as 1 under normal temperatures and pressures and ice does have a lower density because water slightly expands as it gets colder (nearing freezing) and then at freezing has a temperature of about .9 grams per cubic centimeter.

Water is composed of two hydrogen atoms and one oxygen atom to form the water molecule. Hydrogen is the lightest of the elements in the universe and oxygen is a significantly heavier atom than hydrogen. Hydrogen itself has different forms. The number of neutrons in the nucleus of hydrogen affects it's density per cubic centimeter. The most common form of hydrogen in fact has no neutrons and is called "protium". Another form of hydrogen "deuterium" has one neutron in the nucleus of the atom. Still another form of hydrogen called "tritium" has two neutrons in the nucleus. A form of hydrogen has been produced in a laboratory that has four neutrons in the nucleus, but this is unstable and not found in nature. In fact scientists have been able to create more forms of hydrogen by forcing neutrons into the atom. All of the forms of hydrogen have one proton in the nucleus of the atom and one electron in the first electron cloud layer. Because the heavier isotopes are unstable, they are not important for this particular discussion.

Protium, deuterium, and tritium then could be the types of hydrogen atoms that could make water molecules. If enough deuterium or tritium could be isolated then a scientist might be able to make water molecules that are heavier than most water molecules or if enough water molecules could be isolated that have been formed with the heavier naturally formed heavier isotopes of hydrogen, then you would have "heavy" water.

This was a long way to go, to get to this point, but finally here we go - if you were to make ice cubes with "heavy water", they would sink in a glass of regular water. Unfortunately because production of "heavy" water is only done at one facility today and because of the expense, it prevents doing this in a classroom, still it points out even more dramatically how a slight difference in atomic structure can affect density.

The density of cold water is different from the density of warm water and this is very important in "marine science". Other experiments can be done in a laboratory however and in particular because salt is chemically NaCl and a fairly large molecule, it affects the density of ocean or any body of salt water. If you check the internet for the term "density" you can get a lot more information on the subject. Heavy water is used by scientists in many experiments and at nuclear facilities around the world.

For now, if you had answered ice floats because it is less dense than water, you are correct, but remember there are ways that it can sink.