Why does ice float, and why does that matter for life?

Ice floats because it is less dense than liquid water. Most other substances become denser when they solidify, which makes water unusual. Because ice stays at the surface, bodies of water freeze from the top down, and the layer of ice helps protect the organisms living below it. [1] [2]

A water molecule has two hydrogen atoms and one oxygen atom. The positively charged hydrogen of one molecule is attracted to the negatively charged oxygen of another, forming a hydrogen bond. In liquid water these bonds constantly form and break as molecules slide past one another, driven by their motion. [1] [2]

When the temperature drops, molecules have too little energy to break those bonds, and they settle into a rigid crystal lattice. Most ice on Earth's surface forms repeating hexagonal crystals. The lattice holds the molecules farther apart than in the liquid, with a lot of empty space inside it, so the solid is less dense. In most other liquids, cooling lets molecules pack more tightly, so the solid sinks. Scientific American also notes that water expands when cooled below four degrees Celsius. [1] [3] [4]

Since ice stays on the surface, it forms a barrier between the water and the cold air. That layer insulates the water beneath, so a lake or pond is much less likely to freeze solid. Sources describing this note that if ice sank, lakes would freeze from the bottom up, and plants and animals would be crushed or trapped in a block of ice. [3] [5] [6]

Freezing is also harmful to living cells directly. Ice crystals that form inside cells can rupture the delicate membranes they depend on, causing irreversible damage. Cells survive freezing only if their water is temporarily replaced by another liquid such as glycerol. [2]

Life continues beneath the ice, though more slowly. Fish are cold-blooded, so cold water slows their metabolism, respiration and digestion. Some may move to the warmest water at the lake bottom and enter a low-energy state called torpor. Snow or white ice can limit the light that reaches plants, reducing photosynthesis and oxygen production. [6]

Floating ice matters in the oceans too. As sea ice freezes it pushes out salt, leaving the water beneath it very salty and dense. That water sinks and pushes deep water upward, which helps circulate the ocean worldwide. Melting ice in spring absorbs heat from its surroundings, which slows the rise in temperature. [1] [5]