Why is Water Denser than Ice?

by Jay | Updated on June 7th, 2023

When it comes to the unique properties of water, one question often asked is why water is denser than ice. This counterintuitive fact explains why ice floats on water. The answer lies within the molecular structure and behavior of water in its different states. Let’s delve into this fascinating subject.

The Principle of Density

The density of a substance is defined as its mass per unit volume. The density of most substances changes with temperature. It’s not that the density of a substance changes with temperature, but rather the space it occupies does.

photo of icebergs during daytime

When the temperature rises, the acceleration of molecules increases, consuming more energy. This causes the molecules of certain substances to spread out, meaning warmer liquids tend to be less dense than colder solids. But, if we compare equal volumes of water and ice in the same container, the water weighs more. This is because water is denser than ice and takes up less volume than ice. Consequently, due to its lower mass, ice floats on water.

The Role of Hydrogen Bonds

Water behaves differently due to the unique properties of hydrogen bonding. In liquid water, hydrogen bonds link each water molecule to approximately 3.4 other water molecules. But as water freezes into ice, it crystallizes into a rigid lattice, which increases the distance between molecules. In this state, each water molecule is hydrogen-bonded to four others, which lowers the density of ice compared to liquid water.

The Anomaly of “Heavy Ice”

While it’s established that ice floats on water due to its lower density, there are certain types of ice that can be denser than regular water. This can be observed when ice is formed from “heavy water,” which is 10.6% denser than ordinary water.

Heavy water, or D2O instead of H2O, is a variant of water in which all hydrogen atoms have been replaced by deuterium, a hydrogen isotope with one proton and one neutron. Heavy water is denser than ordinary water, and consequently, heavy-water ice can sink in regular water.

When water is heated, the kinetic energy breaks the hydrogen bonds, freeing water molecules to escape into the air as steam or water vapor. As water freezes, the molecules form a crystalline structure held together by hydrogen bonds.

Hypothetical Consequences: If Ice was Denser than Water

Water becomes denser as the temperature decreases, reaching maximum density at 4° C. Below this temperature, water has the unusual property of becoming less dense until it freezes at 0° C. This is crucial for the survival of many aquatic creatures during the winter.

If ice were denser than water, it would sink as it forms, potentially freezing the entire body of water from the bottom up. This could lead to the death of many aquatic species, drastically reducing biodiversity in bodies of water that experience regular freezing.

Changes in water density also cause vertical circulation, known as turnover, in lakes and other bodies of water. When ice melts in the spring, the surface water warms to 4 degrees Celsius, becoming denser and sinking. This sinking action displaces nutrient-rich water from the bottom, promoting a healthier aquatic ecosystem.

In conclusion, understanding the unique properties of water and ice is more than just a scientific curiosity. It is fundamental to the preservation of our planet’s biodiversity, particularly in aquatic environments.


Jay is a health and wellness enthusiast with expertise in water quality and nutrition. As a knowledgeable advocate for holistic well-being, Jay successfully manages Type 2 Diabetes through informed lifestyle choices. Committed to sharing reliable and authoritative insights, Jay combines firsthand experience with a passion for enhancing health."