Which is more dense - Ice or Liquid Water?

DENSITY is defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume. To understand the properties of ice, we need to think in terms of a change in volume with a constant weight or mass.

Density Comparison to Water: In chemistry, the density of many substances is compared to the density of water. Does ice float on water or sink in the water?

Everyone knows what happens in the case of ice cubes in a glass of water.

Look at the graphic on the right.

Why does ice float?

What other observations or facts are known?

  • Because ice floats, we can infer that ice must be less dense than water.
  • If water is frozen in a glass jar, the glass jar breaks.
  • If a pop can freezes, it will also burst.
  • From both of the above we infer that the volume of the ice has increased.

Conclusion: The volume of ice must be greater than the same mass of liquid water. Why does the volume increase?

 Molecular basis for the Volume Increase of Ice:

The normal pattern for most compounds is that as the temperature of the liquid increases, the density decreases as the molecules spread out from each other. As the temperature decreases, the density increases as the molecules become more closely packed. This pattern does not hold true for ice as the exact opposite occurs.

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. Hydrogen Bonding in Water Graphic

Compare the structures of liquid water and that of solid ice. Notice the empty spaces within the ice structure, as this translates to a more open or expanded structure. The ice structure takes up more volume than the liquid water molecules, hence ice is less dense than liquid water.

 What is the exact change in volume of the water when it freezes as ice?

Comparison of:

 Liquid water

 Ice

 Mass = 100 g

 Mass = 100 g

 Volume = 100 mL

 Volume = ? mL
 Density = 1.0 g/mL Density = 0.92 g/mL

 Mathematical Definition of Density

The formal definition of density is mass per unit volume. Usually the density is expressed in grams per mL or cc. Mathematically a "per" statement is translated as a division. cc is a cubic centimeter and is equal to a mL Therefore,

 Density =

 mass =
  g/mL
 

  volume
 

Example: Calculate the volume in a 100 g ice cube with a density of 0.92 g per mL.

Solution: The density translated as a conversion factor is:

0.92 g = 1 mL - "per" is equivalent to an equal sign.

 100 g x

 1.0 mL =
 108.7 mL
 

0.92 g
 

The increase in volume of ice is about 9%. This increase causes enough force to break most rigid containers. This is the same force, repeated on a daily basis, that creates "pot holes" in the roads in the winter time.