Why does ice float?

Ice floats because it is less dense than water. Water has a density of 1.0 gm/cubic cm.

The density of ice Ih is 0.931 gm/cubic cm.

But, why is ice less dense than water if both are made up of molecules of H 2 O?

Note about 3D molecules -- Our files on this page now use Jsmol instead of Jmol. These files make use of Javascript which permits viewing of molecules on tablets, phones and easier use on Macs. For the Jmol version of this page click here. - Jsmol is best viewed with the Chrome browser.

[ Ice structure shown using the JMol Applet]

Try this!!

Click the right mouse button with the cursor over the image-->

Style --> Schemes --> CPK Spacefill

Click on the left mouse button and rotate the ice structure.

Do you see the open spaces in ice? They are formed at low temperatures when water molecules form many stable hydrogen bonds.

Compare this structure to that below of water at room temperature.

[ Water Molecules are shown using the JMol Applet]

Style --> Schemes --> CPK Spacefill

Hydrogen bond lengths between water molecules will vary since the molecules are in constant motion, unlike ice which is a rigid lattice structure (above)

To measure distance

Double click on one atom then drag to second atom-- double click again

To measure angles

Double click on first atom, once on middle atom, twice on atom three.

Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up.

The strong hydrogen bonds give water a high cohesiveness and, consequently, surface tension. This is evident when small quantities of water are put onto a nonsoluble surface and the water stays together as drops.

On the other hand ice has a rigid lattice structure.

In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules.

In ice each each molecule is hydrogen bonded to 4 other molecules.

Compare the two structures below. Notice the empty spaces within the ice structure.

In ice Ih, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. Because of ordered structure in ice there are less H20 molecules in a given space of volume.

Try this --

1) Measure the O-O distances between any two adjacent oxygen atoms in ice shown in the above structure..

Please enter your answer in the space provided:


2) Measure the O-O-O angle formed between adjacent oxygen atoms in ice.

3) What is the length of the hydrogen bond H-O in ice?


Unlike ice -- water at room temperature is in constant motion -- hydrogen bonds are constantly formed and being broken-- click here to see water molecules in motion

Multiple Choice Questions

1. Why does an ice cube float in water?
a) because the air bubbles in the ice make it lighter
b) because ice is a solid and water is a liquid
c) because ice has a lower density
d) because ice is packed tighter than water

2. At what temperature does water freeze?
a) 0 degrees centigrade
b) 100 degrees centigrade
c) 4 degrees centigrade
d) 32 degrees centigrade

3. Water is different than ice in that
a) they are made of different elements
b) ice has a rigid lattice structure
c) ice contains hydrogen bonds while water does not
d) water molecules in ice are more polar

4. The density of liquid water is 1.0 gm/cu.cm. The density of ice is closest to
a) 1.0 gm/cu.cm.
b) 0.93 gm/cu.cm.
c) 0.70 gm/cu.cm
d) 1.1 gm/cu.cm.

5. Which of the following statements is not correct
a) Liquid water has a partially ordered structure
b) water molecules are in constant motion forming and breaking hydrogen bonds
c) ice has a rigid lattice structure
d) a 10 cubic angstrom box containing ice will contain more H2O molecules than the same size box containing liquid water molecules.

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