There are two parts to the answer for this question. First, let's take a look at why anything floats. Then, let's examine why ice floats on top of liquid water, instead of sinking to the bottom.
A substance floats if it is less dense, or has less mass per unit volume, than other components in a mixture. For example, if you toss a handful of rocks into a bucket of water, the rocks, which are dense compared to the water, will sink. The water, which is less dense than the rocks, will float. Basically, the rocks push the water out of the way, or displace it. For an object to be able to float, it has to displace a weight of fluid equal to its own weight.
Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.
A water molecule is made from one oxygen atom and two hydrogen atoms, strongly joined to each other with covalent bonds. Water molecules are also attracted to each other by weaker chemical bonds (hydrogen bonds) between the positively-charged hydrogen atoms and the negatively-charged oxygen atoms of neighboring water molecules. As water cools below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart. This produces a crystal lattice, which is 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 water. The heavier water displaces the lighter ice, so ice floats to the top. 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 over. If ice sank, the water would be displaced to the top and exposed to the colder temperature, forcing rivers and lakes to fill with ice and freeze solid.
Ice is less dense than liquid water
Water is a member of a very exclusive group of substances that are less dense as a solid than as a liquid. It is very important that ice floats and has numerous biological impacts; namely, all life on earth. Most substances will contract as they cool, their individual molecules slowing down and "staying put" until finally forming a solid. Water will also do this - up to a point. As the temperature of water drops, the molecules slow down and contract just like any other substance. But once it reaches 4° C, water will start to expand. The reason water does this lies in its hydrogen bonds.
Every molecule of H2O is bent - the two hydrogens hanging off the oxygen at angles (104.5° between hydrogen atoms in case you're interested) to make a rough "L" shape. This bent shape causes the molecule to be overall polar. Oxygen has a higher electronegativity and "likes" electrons more, which results in the electrons spending more time with the oxygen. So the oxygen is slightly negative, and the electron-deprived hydrogens are slightly positive. Because the molecule is bent with one end slightly negative and one end slightly positive, the molecule is said to be polar.
So now if we expand our view to, say, a glass of water, we will see water molecules running around and bumping into each other. Since every molecule in this glass has a slightly negative and a slightly positive end, there is going to be attraction between molecules. As one water molecule runs around, its negative oxygen is going to be pulled towards the hydrogen atoms of different molecules, and the hydrogens will be pulled towards the oxygen. Now what we have between the hydrogen and the oxygen is a hydrogen bond. Technically, a hydrogen bond is said to be formed when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom (usually between a hydrogen and a oxygen or nitrogen in living cells).
Hydrogen bonds separate molecules
We know what a hydrogen bond is, so now we apply it to the freezing of water. When the molecules of water are still a liquid, they are free-moving and make/break hydrogen bonds very easily and frequently. The molecules can slip in and out at close proximity due to its high energy. But when the temperature drops, the molecules lose energy, slow down, and keep their hydrogen bonds for longer. Soon enough, at 4° C, the hydrogen bonds start altering the layout of the molecules. A single water molecule can only form a maximum of four hydrogen bonds with its neighboring molecules. At 4° C and below, a molecule will "keep" its bonds and lock into a crystalline lattice with its four neighbors. Then the molecules are at "arms length" so to speak and there are less molecules in a given space; the density is now lower.
When the hydrogen bonds are able to form and be retained (as in ice), the density becomes 10% less dense than the water above 4 °C. And then, because a substance less dense than the liquid surrounding it will float in it, ice will float when placed in water.
Read more: http://wiki.answers.com/Q/Why_does_ice_float_in_water#ixzz1Iw7S4hbZ
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NAME : VEBRIA ARDINA
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STUDY PROGRAMME : ISSTE OF CHEMISTRY 2010