Liquid water is denser than ordinary ice by about 9%, which is why ice floats. A liter of water at near-freezing temperature weighs roughly 1,000 grams, while a liter of ice weighs around 917 grams. This seemingly simple fact is actually one of the stranger things in all of chemistry: most substances get denser when they freeze, not less dense. The reasons water breaks this rule, and the consequences that follow from it, reach into everything from why fish survive winter to how scientists once defined the kilogram.
Why Ice Is Less Dense Than Water
When most liquids cool down and solidify, their molecules pack closer together, making the solid heavier per unit of volume. Water does the opposite. As it cools below about 4°C, it actually starts to expand rather than contract, and this expansion accelerates as it freezes into ice.1Energy Conversion and Management. Freezing phenomena in ice–water systems The reason lies in the shape water molecules prefer when they lock into a crystal.
Water molecules are held together by hydrogen bonds, which have a strong directional preference. Each oxygen atom bonds with four neighbors in a roughly tetrahedral arrangement. In liquid water above 4°C, molecules jostle around enough that this geometric preference only loosely governs their positions, and they pack fairly efficiently. But as water freezes, every molecule snaps into a rigid, tetrahedral crystal lattice. That geometry leaves a lot of open space between molecules, like a scaffold with large gaps.2Angewandte Chemie International Edition in English. Anomalies of Liquid Water The result is a solid that takes up more room than the liquid it came from.
This open structure is specific to ordinary ice, called ice Ih (the “h” stands for hexagonal). It is the only form of ice you will ever encounter in daily life, and its hexagonal lattice is what gives snowflakes their six-fold symmetry. That same lattice is also what makes ice about 9% less dense than the water that formed it.
The 4°C Density Peak
Water does not behave like a normal liquid even before it freezes. Instead of getting steadily denser all the way down to its freezing point, water reaches its maximum density at about 3.98°C under normal atmospheric pressure.3Fluid Phase Equilibria. Simulation and theoretical analysis of the origin of the temperature of maximum density of water Below that temperature, water starts getting lighter again as the hydrogen-bond network begins nudging molecules into those roomy, ice-like arrangements even while the water is still liquid.
This density maximum shifts under pressure: it drops by roughly 1°C for every 50 bar of additional pressure.4MURAL – Maynooth University Research Archive Library. A Study of the Change in the Temperature of Maximum Density of Water and Aqueous Solutions as a function of Pressure That detail matters mostly to physicists and oceanographers, but the 4°C anomaly itself has enormous everyday consequences. In a cooling lake during autumn, the densest water (at 4°C) sinks to the bottom while the lighter, colder water stays near the surface. By the time ice forms, it sits on top, insulating the warmer water below. If water behaved like a normal substance, lakes would freeze from the bottom up, killing off most aquatic life each winter.
Why Frozen Pipes Burst
The 9% expansion that accompanies freezing translates into serious mechanical force. When water trapped inside a closed pipe freezes, the expanding ice has nowhere to go, and the resulting pressure can easily split copper or steel plumbing.5European Journal of Physics. How can freezing water burst pipes and containers? The same principle cracks boulders over geological time: water seeps into tiny fissures, freezes, expands, and slowly pries the rock apart.
A common misconception is that the burst happens at the spot where ice forms. In practice, the ice plug often forms in one section of the pipe, then pressurizes the still-liquid water trapped between the plug and a closed faucet. The pipe typically fails at its weakest point in that pressurized zone, which may be well away from the actual ice. This is why leaving a faucet slightly open during a cold snap can prevent damage: the drip gives the pressurized water somewhere to escape.
How Pressure Changes the Rules
Under everyday conditions, ice is always lighter than water. But pressure fundamentally reshapes water’s behavior. Squeezing ice hard enough lowers its melting point, a phenomenon called regelation.6arXiv. Regelation: why does ice melt under pressure? Under compression, the hydrogen bonds that prop open ice’s airy crystal structure get distorted: the weaker long-range bonds shorten while the stronger short-range bonds stretch, and the energy balance tips in favor of the liquid state.7PubMed Central. Pressure-Induced Melting of Confined Ice
Push the pressure even higher and water can freeze into entirely different crystal structures that are denser than liquid water. Researchers have directly observed high-pressure ice phases with densities between roughly 1,200 and 1,260 kilograms per cubic meter, well above liquid water’s 1,000.8PubMed. In situ observations of a high-pressure phase of H2O Ice These exotic forms, known as ice V, ice VI, and others, have tighter crystal geometries in which the hydrogen-bond network has been overridden by the sheer force pushing molecules together. At such pressures, the balance between hydrogen bonds and weaker intermolecular forces shifts dramatically, with those weaker forces gaining influence over which crystal structure wins out.9PubMed. Hydrogen bonds and van der waals forces in ice at ambient and high pressures
So the statement “ice is lighter than water” is only universally true at the pressures you encounter on Earth’s surface. Deep inside icy moons or at the cores of giant planets, water ice could be denser than the liquid oceans above it. At least 20 distinct crystalline phases of ice have been identified in the laboratory, and most of them sink in water rather than float.
How Salt Shifts the Picture
Everything above applies to pure, fresh water. Adding salt changes both the freezing point and the temperature of maximum density, and the two do not shift at the same rate. In freshwater, the density peak at 4°C sits well above the freezing point of 0°C, creating the layering effect that protects lake ecosystems. But as salinity increases, the temperature of maximum density drops faster than the freezing point. Once salinity exceeds about 24 grams per kilogram, the density maximum vanishes entirely: the water simply gets denser and denser as it cools, all the way down to its freezing point.10PubMed Central. The Effect of Ocean Salinity on Climate and Its Implications for Earth’s Habitability
Typical ocean water has a salinity around 35 grams per kilogram and freezes at about −1.9°C. Because there is no density maximum above that freezing point, cold surface water in the ocean just keeps sinking, driving large-scale circulation patterns. Sea ice still floats, because the ice crystals themselves expel most of the salt as they form, leaving relatively fresh, low-density ice sitting on denser brine. But the convective dynamics of a salty ocean differ profoundly from those of a freshwater lake, and both ultimately trace back to how dissolved salt disrupts the hydrogen-bond network that gives pure water its quirky density behavior.
Supercooled Water and the Two-Liquid Debate
Water’s density anomaly is not just a curiosity; it is a clue to something deeper going on at the molecular level. For decades, theorists have proposed that liquid water might actually be a mixture of two subtly different structural forms: a high-density arrangement where molecules pack tightly, and a low-density arrangement where they settle into more ice-like, open configurations. As temperature drops, the balance shifts toward the low-density form, which is why water expands as it approaches freezing.
Testing this idea has been difficult because water tends to crystallize into ice before you can push it into the temperature range where the two forms would separate cleanly. But recent experiments have found ways around this. One group studied a water-rich solution that mimics pure water’s hydrogen-bond structure but resists crystallizing, and observed a sharp, reversible transition between two distinct liquid phases.11PubMed. A liquid-liquid transition in supercooled aqueous solution related to the HDA-LDA transition Another team used ultrafast X-ray pulses to catch pure supercooled water in the act, watching low-density liquid domains appear and grow on a timescale of nanoseconds to microseconds, fast enough to outrun the crystallization process that kicks in after a few microseconds.12PubMed. Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure
Computer simulations point to a critical point for this liquid-liquid transition somewhere around −66°C and 50 megapascals of pressure, where the low-density liquid that emerges has a density only slightly above that of ordinary ice.13PubMed Central. Liquid-liquid transition in supercooled water suggested by microsecond simulations If this picture is correct, the familiar fact that ice is lighter than water is really just the surface manifestation of a competition between two fundamentally different ways water molecules can arrange themselves. The “normal” liquid you drink is dominated by the high-density arrangement. Ice, and near-freezing water, are dominated by the low-density one.
Amorphous Ice Beyond Earth
On Earth, ice almost always means the hexagonal crystal we have been discussing. But most of the ice in the universe is not crystalline at all. In the vast, cold regions of interstellar space and on the surfaces of comets and outer-solar-system moons, water freezes so quickly, or at such low temperatures, that molecules never organize into a regular lattice. Instead they form amorphous ice, a glassy solid with no repeating crystal structure.
Low-density amorphous ice is thought to be one of the most abundant solid materials in the cosmos, yet its precise structure remains under active debate. Recent computational work suggests that what scientists have been calling amorphous ice actually contains embedded grains of crystalline ice, blurring the line between glass and crystal.14Physical Review B. Low-density amorphous ice contains crystalline ice grains Whether amorphous ice should be thought of as a frozen snapshot of liquid water or as a disordered crystal matters for understanding water’s anomalies, because the two interpretations imply different things about the energy landscape that governs how water molecules interact.
The density of amorphous ice varies depending on how it formed. Low-density amorphous ice has a density in the neighborhood of crystalline ice, while high-density amorphous ice, produced by compressing the low-density form, is denser than liquid water. Both forms are studied in laboratories as analogs of the ices found in space, and they connect directly to the two-liquid theory: the low- and high-density amorphous ices map onto the low- and high-density liquid states that researchers are trying to pin down in supercooled water.
Ice Expansion and Living Cells
The expansion of freezing water is not just a plumbing concern. It is one of the central challenges in cryopreservation, the science of freezing biological material for long-term storage. When cells are frozen, ice crystals can form both outside and inside them, and the damage depends heavily on how fast the cooling happens.
During slow freezing, ice forms in the fluid surrounding cells first. As that extracellular water locks into ice crystals and expands, the concentration of dissolved solutes in the remaining liquid spikes, creating an osmotic imbalance that draws water out of the cells and can shrivel them. If cooling is too fast, water inside the cells does not have time to escape and freezes in place, forming intracellular ice crystals that puncture membranes and destroy cell structure.15PubMed Central. Ice Inhibition for Cryopreservation: Materials, Strategies, and Challenges Cryobiologists walk a narrow line between these two failure modes, using controlled cooling rates and cryoprotectant chemicals (essentially antifreeze for cells) to minimize ice damage. Vitrification, an alternative approach, attempts to cool cells so rapidly that water solidifies into a glass-like state without forming crystals at all, sidestepping the expansion problem entirely.
The stakes are significant: organ and tissue banks, fertility clinics, and stem-cell research all depend on reliably freezing and thawing living cells. The root of the difficulty is the same 9% expansion that cracks pipes, just scaled down to a level where it is shredding membranes instead of splitting copper.
How Water’s Density Helped Define the Kilogram
The connection between water’s density and the metric system is older than most people realize. When the French revolutionary government set out to create a rational system of measurement in the 1790s, they wanted the unit of mass to be grounded in a physical property of nature rather than an arbitrary royal decree. The kilogram was originally defined as the mass of one cubic decimeter (one liter) of distilled water at the temperature of its maximum density.16Metrologia. A brief history of the unit of mass: continuity of successive definitions of the kilogram
That temperature, as we have seen, is about 4°C. The choice was elegant: water is universally available, and tying the definition to its maximum density meant the measurement was as insensitive to small temperature errors as possible, since density barely changes near a maximum. In practice, measuring water precisely enough proved difficult, and the kilogram was soon transferred to a physical platinum artifact. That artifact, and later a platinum-iridium cylinder kept in a Paris vault, served as the world’s kilogram standard for over a century. It was only in 2019 that the kilogram was redefined in terms of a fundamental constant of physics. But the link to water’s anomalous density peak persisted in the system’s DNA for more than two hundred years.
When “Heavier” Gets Ambiguous
A source of confusion in casual conversations about water and ice is the word “heavier.” If you take a one-kilogram block of ice and a one-kilogram jug of water, they weigh the same. Mass is mass. What differs is density: how much mass fits into a given volume. The ice block will be noticeably larger than the water jug, because each gram of ice takes up more space. When people ask whether water is heavier than ice, they almost always mean denser, and the answer is yes under normal conditions.
This distinction matters when thinking about floating and sinking. An object floats when it is less dense than the fluid around it, regardless of its total weight. A small ice cube and an iceberg both float for the same reason: their density is lower than the surrounding water’s. About 90% of a floating piece of ice sits below the waterline, and roughly 10% protrudes above it, directly reflecting the roughly 9% density difference between ice and water. That ratio holds whether the ice chunk weighs a gram or a million tons.
The density gap also explains why adding ice to a glass of water and letting it melt does not cause the glass to overflow. The submerged portion of the ice displaces exactly the amount of water equal to the ice’s weight. When the ice melts, the resulting liquid water fills precisely the volume that was being displaced. If ice were denser than water and sat at the bottom of the glass, melting it would actually raise the water level, because the meltwater would occupy more volume than the ice did. The everyday experience of ice in a drink is, in miniature, a demonstration of the same anomaly that keeps lakes from freezing solid and cracks boulders apart on mountainsides.