Why Does Ice Float on Water?

Drop an ice cube into a glass of water and it bobs at the top. That seems ordinary, but it is actually unusual. For most substances, the solid form is heavier than the same amount of liquid, so the solid sinks. Water does the opposite, and that one quirk shapes everything from your drink to life in frozen lakes.

Floating comes down to density

Whether something floats depends on density, which the US Geological Survey (USGS) Water Science School describes as the mass of a substance for a specific volume. Put simply, it is how much stuff is packed into a given space.

Liquid water has a density of roughly 1 gram per cubic centimeter. Anything less dense than the water around it floats, and anything denser sinks. So the real question is why ice ends up less dense than the water it came from.

Freezing spreads water molecules apart

The USGS explains that ice has a very different structure from liquid water. In liquid water, molecules move around and pack together fairly randomly. When water freezes, the molecules line up in a regular lattice, an orderly, repeating pattern. That lattice holds the molecules more spread out than they are in the liquid.

Same molecules, more space between them, means lower density. According to the USGS, the density of ice decreases by about 9 percent upon freezing. Turned around, the same amount of water takes up more room as ice. A liter of liquid water becomes roughly 1.1 liters of ice. That expansion is why a full water bottle left in the freezer can bulge or split.

How much of an ice cube sticks out

The USGS puts the density of ice at about 90 percent that of water, though it varies because ice can trap air bubbles. That number tells you how an ice cube or iceberg sits in the water: about 90 percent below the surface and about 10 percent above.

This is the origin of the phrase "the tip of the iceberg." What you can see is a small part of the whole.

NOAA's National Ocean Service gives a sense of scale for real icebergs. To be classified as an iceberg, a piece of floating ice must stand more than 16 feet above sea level and be 98 to 164 feet thick. Smaller fragments have their own names: "bergy bits" stand roughly 3 to 16 feet above the sea, and "growlers," which NOAA compares to the size of a truck or grand piano, stick up less than 3 feet. Because most of each piece is hidden, even a growler can be far bigger than it looks.

Water is densest just above freezing

Water has another oddity. As it cools, it gets denser, as most liquids do, but only down to about 4 degrees Celsius (39.2 degrees Fahrenheit). Below that, it starts getting less dense again, even before it freezes.

The USGS density table shows this:

Temperature	Density (grams per cubic centimeter)
0 C (32 F)	0.99987
4 C (39.2 F)	1.00000
10 C (50 F)	0.99975
21 C (70 F)	0.99802
100 C (212 F)	0.95865

The differences look tiny, but they are enough to make water layer itself by temperature.

Why lakes freeze from the top down

Put those two facts together. In winter, water near 4 C is the densest, so it sinks to the bottom of a lake. Colder water, closer to 0 C, stays on top, and that is where ice forms. The ice then floats as a lid on the surface.

The USGS points out what would happen otherwise. If water were most dense at its freezing point, the coldest water would sink, and a lake could freeze from the bottom up. Because water is also a good insulator, some frozen lakes might not fully thaw in summer. Instead, the floating ice layer helps shield the water below, which is why fish and other life can survive a winter under the ice.

Can ice ever sink?

Yes, in one special case. The USGS describes "heavy ice," made from heavy water, in which both hydrogen atoms in each water molecule are replaced by deuterium, a form of hydrogen with an extra neutron. Heavy ice is about 10.6 percent denser than normal water, so a heavy-ice cube sinks in an ordinary glass of water.

Saltwater makes the reverse point. The USGS notes that ocean water, with its dissolved salt, is denser than pure water. Since ice is less dense than either, it floats in the sea just as it does in a glass.

Try it at home

• Fill a clear glass with water, add an ice cube, and look from the side. Most of the cube sits below the waterline.
• Freeze water in a small plastic cup filled exactly to a line you draw. After it freezes, the top will have risen above the line.
• Put an ice cube in a glass of very salty water and another in plain water, and compare how high they float.

Key takeaways

• Ice floats because it is less dense than liquid water, which is unusual for a solid.
• Freezing locks water molecules into an open lattice, lowering density by about 9 percent.
• Ice is about 90 percent as dense as water, so roughly 90 percent of an iceberg sits underwater.
• Water is densest at about 4 C, so lakes freeze from the top down and life survives underneath.
• Heavy ice made from heavy water is denser than normal water and sinks.