Share with the children that this is a very special property of water. As most substances get colder, their material (molecules) scrunch closer and closer together, like penguins snuggled together to keep warm. Water molecules, however, don't like to scrunch. They spread out like acrobats in formation to form ice crystals like we see in snowflakes and frost.
Density is mass per unit volume, or the ratio of the amount of matter in an object compared to its volume, or simply put:
- Mass = the amount of "stuff"
- Weight = how heavy the "stuff"is (Weight is determined by the amount of gravitational pull on an object, which is a property of the mass of the planet the "stuff" is on. The greater the gravitational pull, the more the "stuff" weighs.)
- Density = how tightly packed the "stuff" is
- Volume = the area of space the "stuff" takes up
If we apply this information to the cups, the cup of water and the cup of ice have the same mass, i.e. amount of "stuff.” Because they had the same amount of "stuff," they have the same weight.
The "stuff" (molecules) in water is more tightly packed than in ice, so water has greater density than ice. Don't let the fact that ice is a solid fool you!
As water freezes it expands. So, ice has more volume (it takes up more space, but has less density) than water.
Water molecules in the liquid state like hanging out together as droplets, but in the solid (frozen) state, they need their space.
A water molecule is similar in outline to Mickey Mouse, with two hydrogen atoms placed on one end of an oxygen atom. The oxygen atoms are positively charged and hydrogen atoms negatively charged, so water molecules stick to each other like tiny magnets. The hydrogen "ears" make loose bonds, called hydrogen bonds, with the oxygen "faces" of other water molecules.
Each molecule of water is made of two hydrogen atoms (white) and one oxygen atom (red). The hydrogen atoms are not placed on opposite sides of the oxygen atom; they occur at one end. Image modified from: http://commons.wikimedia.org/wiki/File:H2O_(water_molecule).jpg.
In ice crystals, their hydrogen bonds link them together like acrobats, stacked one on top of the other, with their arms and legs outstretched. This provides more space between the molecules in the crystal structure of ice than the molecules of liquid water. As a result, ice is less dense than the liquid form.
Water is the only known non-metallic substance that expands when it freezes; its density decreases and it expands approximately 9% by volume.