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 H2O?

-------------->spin on -------->- spin off
------>space fill/cpk -------->stick ----> ball-and-stick

To Rotate the Molecule--->Left Click and Drag

To Zoom-->>Left Click + hold Shift button and Drag Vertically

Jsmol Menu --->>Right-Click

Try this!! 

Change to wireframe first:

Click on right mouse button over box

Style --> Schemes --> wireframe

Label atoms:

Style -->Label ---> atom number

Measure the following angles:


To measure angles: hold left mouse bottom over atom and double click on atom 1, drag to second atom (center atom) single click center atom, drag and double click atom 3.


-------------->spin on -------->- spin off
------>space fill/cpk -------->stick ----> ball-and-stick

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

water dimer molecule

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.

water structure   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 --

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.