Why Does Ice Float on Water? The Science Behind a Very Unusual Property
Ice floats because it is less dense than liquid water. The density of ice is about 90 percent that of water, according to the US Geological Survey (USGS). Almost every other substance does the opposite: its solid form is denser than its liquid, so it sinks.
What density means
Density is the mass of a substance in a given volume. The USGS gives the density of water as roughly 1 gram per milliliter, and notes it changes with temperature and with anything dissolved in it. Something less dense than the liquid it is placed in will float, and something denser will sink.
What happens when water freezes
In liquid water, molecules move around more randomly. In ice, the USGS explains, the molecules align in a regular lattice. That lattice holds the molecules further apart than they are in the liquid. So the same amount of water takes up more space as ice, and its density drops. Ice can also contain trapped air, which is why its density can vary a little.
Why most of an iceberg is underwater
Because ice is about 90 percent as dense as water, a floating piece of ice sits low, with most of its volume below the surface. The USGS notes that almost all the volume of an iceberg is below the waterline.
Water is densest at 4 degrees Celsius, not at freezing
The USGS table shows water is densest at about 4 degrees Celsius (39.2 degrees Fahrenheit), at essentially 1.00000 grams per cubic centimetre. At 0 degrees Celsius, liquid water is slightly less dense, at 0.99987.
This has a big effect on lakes. In winter, water near 4 degrees Celsius is denser than colder water, so it sinks and the coldest water stays on top, where it freezes. The USGS says that if water were densest at freezing, the cold surface water would sink and a lake could freeze from the bottom up. Because water is such a good insulator, some frozen lakes might not fully thaw in summer.
What would change if ice sank
The USGS puts it directly: if ice were denser than water, we could not ice skate or ice fish without scuba equipment. More seriously, it calls this property of water critical for life on Earth, because fish and other organisms survive winters in liquid water under the ice.
Heavy ice does sink
There is one exception. "Heavy ice" is made from heavy water, where the hydrogen atoms are replaced by deuterium, a heavier form of hydrogen. The USGS says this ice is 10.6 percent denser than normal water, so heavy ice cubes sink in a glass of ordinary water.
Try it at home
Freeze a small sealed container of water, such as a plastic bottle filled almost to the top, and watch the ice push outward as it expands. Leave room at the top and never use a glass bottle, which can crack. Then drop a regular ice cube into a glass of water and note how low it sits.
Quick answers
Does ice float because it contains air? Air can make ice slightly lighter, but the main reason is the open lattice structure of frozen water.
Is ice less dense in every case? For ordinary water ice, yes. The one exception noted by the USGS is heavy ice.
Facts checked October 2, 2026.
Sources
- US Geological Survey, Water Science School, "Water Density" (August 24, 2026): https://www.usgs.gov/water-science-school/science/water-density