Why Does Ice Float on Water?

How much less dense is ice than liquid water?

Density is the mass of a substance in a given volume.^[1] Liquid water has a density of roughly 1 gram per milliliter, and ice is less dense than that, which is why ice cubes float in a glass.^[1]

The U.S. Geological Survey (USGS) says density falls by about 9 percent when water freezes, and also that ice is about 90 percent as dense as water, a figure that can vary because ice can contain air.^[1] A NOAA classroom handout gives 0.92 g/mL for ice against 1.0 g/mL for liquid water, and the U.S. Coast Guard lists normal ice at 0.917 g/cm³.^[4][6] By BodaWiki's own calculation, these figures put ice roughly 8 to 10 percent below liquid water in density: 0.917 against 1 is about 8 percent lower, and USGS's "about 90 percent" is 10 percent lower.^[1][4]

The gap exists because water expands as it freezes.^[6] The amount of matter does not change, but it is spread over a larger space.^[6]

In most substances the solid is denser than the liquid, and USGS notes that ice would sink if water followed the same rule.^[1] A NOAA laboratory essay calls ice one of very few substances whose solid form is less dense than its melted form.^[2]

Floating itself comes down to buoyancy.^[4] Archimedes' principle says the upward force on an object in a fluid equals the weight of the fluid it displaces.^[4] A floating object settles at the level where that force exactly balances its weight.^[4]

What do hydrogen bonds do when water freezes?

In a water molecule, the hydrogen atoms carry a slightly positive charge and the oxygen atom a slightly negative one.^[3] A hydrogen bond forms when the oxygen of one molecule is attracted to a hydrogen of another.^[3]

In liquid water these bonds keep forming and breaking as the molecules move past one another.^[3] When water freezes, there is not enough energy left to break them, and the molecules lock into a crystal held together by hydrogen bonding.^[3] The way the bonds orient pushes the molecules farther apart than they were in the liquid.^[3]

Normal ice has an open hexagonal structure.^[2] Each oxygen atom sits at the centre of a tetrahedron with four other oxygen atoms at its corners, and the overall pattern resembles a beehive built from layers of slightly crumpled hexagons.^[2] The length of the hydrogen bonds is what creates this open framework.^[2] USGS puts it more simply: molecules in ice line up in a regular lattice, which lets them be more spread out than in the liquid.^[1]

When ice melts, some of the bonds break, leaving a disordered structure with a higher density.^[2] Most other liquids do the opposite as they solidify: their molecules pack more tightly, so the solid ends up denser than the liquid.^[3]

Why is water densest at about 4 °C?

Water at about 4 °C (39 °F) is denser than water at 0 °C (32 °F).^[1] A USGS table lists the density of water as 0.99987 grams per cubic centimeter at 0 °C, 1.00000 at 4.0 °C and 0.95865 at 100 °C.^[1]

The NOAA essay explains the peak this way: even in liquid water, a few molecules keep the crystal-like bonded arrangement until thermal motion destroys it, which produces a maximum density at 4 °C in fresh water.^[2]

Salt changes the pattern: adding it lowers the temperature of maximum density, and once salinity exceeds 24.7 parts per thousand the maximum disappears.^[2] Typical sea water freezes at about -1.8 °C.^[2]

Why do lakes freeze from the top down?

When cold air chills a lake at the end of summer, the surface water grows denser and sinks, and warmer water from below takes its place and is cooled in turn.^[2] This goes on until the lake reaches 4 °C, its maximum density.^[2] Further cooling makes the surface water less dense, so it stays on top.^[2] That thin cold layer can then cool quickly to the freezing point, and ice forms on the surface while the water underneath may still be close to 4 °C.^[2]

USGS points out that if water were densest at its freezing point, the coldest water would sink in winter and a lake could freeze from the bottom up.^[1]

How do fish survive under the ice?

The layer of ice on top of a lake, pond, river or stream provides some insulation that helps the water hold its heat.^[5] Fish in very cold fresh water often gather in groups near the bottom, and most simply school in the deepest pools and take what NOAA calls a "winter rest".^[5] In that state their hearts slow down, they need less food and oxygen, and they move very little.^[5] Some species, such as koi and gobies, may burrow into soft sediment and go dormant.^[5]

The OpenStax biology textbook adds that without this insulating layer of ice, plants and animals living in a pond would freeze in a solid block of ice and could not survive.^[3]

How much of an iceberg is under water?

USGS says almost all of an iceberg's volume is below the water line, with about 10 percent above it.^[1] The U.S. Coast Guard puts the hidden share at about 7/8ths and stresses that the figure is approximate, because iceberg density and water density vary.^[4]

The Coast Guard's worked example shows where the number comes from: the fraction of ice under water equals the density of the ice divided by the density of the water.^[4] Using 0.90 g/cm³ for the iceberg's ice and 1.035 g/cm³ for seawater, it arrives at a ratio of 0.87, meaning about 87 percent of the iceberg is underwater.^[4]

Seawater is slightly denser than fresh water, and the NOAA handout puts it at approximately 1.03 g/mL.^[6] Because the densities of ice and seawater are so close, ice floats low in the water.^[6]

• Normal ice0.92 g/cm³
• Fresh water1 g/cm³
• Seawater1.04 g/cm³

Source: U.S. Coast Guard Navigation Center [4]