Why does Ice Floats on Water ?

It’s a known fact that solid objects are denser and have more weight than liquids. However, as we see, the ice floats on water.

But why does Ice Floats on Water ?

First we should know why an object floats on a liquid ?

An object floats on a liquid when it has lower density than said liquid.

Does that mean Ice has lower density than water ?

Yes, we got it right. Ice has less density than water. Water has a density of 1.0 gm/cubic cm wherein the density of ice is 0.931 gm/cubic cm.  Hence, ice is actually about 9% less dense than water. 

But why ice has lower density than water despite solid structure ?

In general, most liquids contract on cooling. Hence, their volume decrease and density increases because density is inversely proportional to its volume. However, ice has lower density than water. Here comes the concept of ‘Anomalous expansion of water’.

Water behaves abnormally when cooled due to its molecular structure. When water cools down, it contracts and reduces its volume until 4 degree centigrade. Beyond this point, when the water cools down, its volume increases and becomes more than the initial volume.  Hence its density decreases. This concept is called ‘Anomalous expansion of water’. Hence, this makes density of ice lighter than water. Because of this reason, Ice Floats on Water.

What is the reason behind ‘Anomalous expansion of water’ ?

Ice Floats on Water because it is about 9% less dense due to stronger O-H bond at temperature below 4 degree centigrade.
Ice floats on water because it is about 9% less dense due to stronger O-H bond at low temperature. Image Credit : Techie Scientist.

At normal temperature, the water molecules i.e. one oxygen atom and two hydrogen atoms, are held together in a liquid state because of the inter-molecular attraction. In a liquid state, the water molecules are constantly whizzing around and are constantly being rearranged. Further, Hydrogen also contributes to the inter-molecular attraction. The distance between the H-O bonds is more than the O-O attraction in the water. On cooling, water molecules lose energy and decreases whizzing around. On further cooling, the water molecules start squeezing together, increasing their density. At 4°C the density reaches its maximum and after this, the water molecules can squeeze no further.

So, the water freezing over into ice is held together not by the O-O attraction but by the H-O attraction. The lattice arrangement of ice prevents the movement of water molecules. But since the H-O is not quite as tight as the O-O bond, it experiences a little expansion once the H-O bond takes over. This structure requires more space as H-O bond tries to be rearranged in specific structure (as shown in the image) and thus increase the slight volume of ice. Hence, the density of ice slightly decreases as compare to the liquid water. This makes Ice Floats on Water.

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