Published: 29 April, 2004

What's more dense: Ice or water?

What is the volume of water in a gallon of ice?

A very old joke asks, "What weighs more - a pound of lead or a pound of feathers?" Think about this for a minute, and you may realize that a pound of one thing weighs the same amount as a pound of something else because a pound is a measure of weight. The difference between a pound of feathers and a pound of lead is the volume. A pound of feathers would be a large bag of feathers, but a pound of lead can easily be held in your hand.

Density is the measurement we use to compare volume and weight. Density is an objects mass divided by its volume (kg/liter), but I'll use a similar measure: weight divided by volume (pounds/gallon). For example, feathers are much less dense than lead although a pound of each still weighs a pound.

Similarly, a gallon of ice has the same volume as a gallon of water. But if we let a gallon of ice melt, would we get a gallon of water? Density is the key. If the density of ice is different than the density of water, then the volumes will also be different.

You can guess at the answer by taking a cup of water and putting an ice cube and a small pebble in it. What happens? The ice floats but the rock does not. The rock sinks because its density is greater than that of water. The ice floats because its density is less than that of water. Aha!

We now know that the density of ice is less than that of water. Ice takes up more space than water for a given weight. In fact the density of ice is 7.65 pounds/gallon compared to 8.35 pounds/gallon for water. Therefore, one gallon of ice weighs 7.65 pounds, but 7.65 pounds of water is only 7.65/8.35 = 0.92 gallons of water.

You can experiment with density this summer with an adult watching: Do you float when you swim? Does it matter if you have a big breath of air? Are you more or less dense than water? Have fun!