# Why does ice float on water?

Jan 21, 2015

Ice floats on water because it is less dense than water.

When water freezes into its solid form, its molecules are able to form more stable hydrogen bonds locking them into positions. Because the molecules are not moving, they're not able to form as many hydrogen bonds with other water molecules. This leads to ice water molecules not being as close together as in the case of liquid water, thus reducing its density.

Most substances in their solid form are more dense than their liquid forms. The opposite is true in water. This property of water is somewhat unusual and rare.

Water is actually most dense at 4ºC. At any temperature below or above 4ºC, water becomes less dense.

Jan 21, 2015

Ice floats on water because it is less dense than water.

Density is defined as mass per unit volume of a substance. By saying that ice is less dense than water, we mean that a sample of ice will take up more space than a sample of water that has the same mass.

Ice and water are both made of the same element ${H}_{2} O$, otherwise known as Hydrogen Dioxide. At sufficiently cold temperatures, usually around 0 degrees Celsius (32 degrees Fahrenheit), water undergoes a phase transition to ice called freezing. This is because as the temperature becomes colder, the water molecules lose energy and move less.

The hydrogen bonds that form when water freezes into ice allow the molecules to be spaced farther apart, thus making them take more space, decreasing the overall density and making it float in the water.

The reason that density determines if something will float or sink is because, as stated by Newton's third law:

$F = m a$ where $F$ is force, $m$ is mass and $a$ is acceleration.

and so the gravitational force for two substances with the same volume will be greater for the substance with higher mass and thus higher density.