Why is water a liquid at all at the temperatures found on the surface of this planet? Why does almost everything that falls into the lake dissolve in it? And why, on a cold February morning, does the coldest water in Lake Geneva not simply sink to the bottom?
This lesson answers those questions, then follows one property out into the ocean. The single most important job the ocean does in the carbon cycle depends entirely on the fact that carbon dioxide dissolves more readily in cold water than in warm water.
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4.1.7 Water has unique physical and chemical properties that support and sustain life.
Include polarity, resulting in cohesion (attraction between water molecules due to hydrogen bonding), adhesion (attraction between water and other substances) and solvent properties. Also include transparency, high specific heat capacity, differences in density depending on temperature and differences in gas solubility (oxygen and carbon dioxide) depending on temperature and pressure.
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A water molecule is small and light. Compare it with two molecules of similar size:
On size alone, water should be a gas everywhere on Earth. There should be no lakes, no oceans and no rain. The reason it is not comes down to the shape of the molecule.
In a water molecule the oxygen atom pulls the shared electrons towards itself more strongly than the hydrogen atoms do. The result is an uneven spread of charge: slightly negative at the oxygen end, slightly positive at the hydrogen end. A molecule with this uneven charge is a polar molecule.
Because the molecules are polar, the positive end of one is attracted to the negative end of the next. These attractions are hydrogen bonds. They are much weaker than the bonds inside the molecule, but there are a very large number of them, and holding water molecules together is what makes water behave so unusually.
Two consequences follow directly:
Almost everything in this lesson traces back to polarity.
Solvent properties. Because water is polar, it pulls apart other charged and polar substances and surrounds their particles. Water dissolves more substances than any other common liquid, which is why rivers carry dissolved minerals, why soils carry nutrients to roots, and why seawater is salty.
Transparency. Water lets visible light pass through it. Light penetrating the upper layers of a lake or ocean is what allows photosynthesis to happen below the surface at all.
High specific heat capacity. A large amount of energy is needed to raise the temperature of water by one degree, because that energy first has to overcome the hydrogen bonds. Water therefore warms slowly and cools slowly. Large water bodies are thermally stable places to live, and they moderate the climate of the land around them.
Density changes with temperature. Warm water is less dense than cold water, so warm water floats on cold water. Below about 4 °C the pattern reverses and water becomes less dense again as it cools further, which is why ice floats and a lake freezes from the surface downwards rather than from the bed upwards.
Gas solubility changes with temperature and pressure. Cold water holds more dissolved gas than warm water. Water under higher pressure holds more dissolved gas than water at lower pressure. This applies to oxygen, which aquatic organisms need, and to carbon dioxide, which is where the second half of this lesson goes.
Calculated from Weiss (1970, 1974), fresh water in equilibrium with air at 1 atm.
Complete the table below. For each property, give one consequence for the environment or for living things.
| Property | One environmental consequence |
|---|---|
| Cohesion | |
| Adhesion | |
| Solvent properties | |
| Transparency | |
| High specific heat capacity | |
| Density changes with temperature | |
| Gas solubility changes with temperature and pressure |
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Lake Geneva: density in action, 309 metres down
Lake Geneva mixes from top to bottom once a year, in late winter. It is the only time of year it can.
Through spring and summer, the surface warms. Warm water is less dense, so it floats, and the lake settles into a warm upper layer sitting on a cold deep layer that stays at around 6 °C all year. Wind cannot mix the two, because moving dense water upwards takes energy the wind does not have.
In winter the surface cools. As it cools it becomes denser and begins to sink. If it cools far enough that the surface and the deep water are close to the same temperature, the density difference almost disappears, and a strong enough wind can then stir the whole 309 metre column from top to bottom. Cold, oxygen-rich surface water reaches the bed, and the lake resets.
It is not doing that any more. The last complete mixing was the harsh winter of 2011/12.
| Winter | Depth reached | Lake depth |
|---|---|---|
| 2011/12 | 309 m, complete | 309 m |
| 2021/22 | 130 m | 309 m |
| 2024/25 | 110 m | 309 m |
| 2025/26 | about 140 m | 309 m |
The reason is that the surface no longer cools far enough. In winter 2024/25 the coldest the surface layer got was 7.8 °C, which is 1.5 °C warmer than the 1991 to 2020 average. Across 2025 as a whole, surface waters ran 1.7 °C above the 1991 to 2020 reference, the largest departure since 1991. CIPEL track an index of how strongly the lake resists mixing, and it has risen markedly since 2012.
Notice that the series is not a straight line. Winter 2025/26 mixed deeper than winter 2024/25, because it was colder. Winter weather still decides the depth in any given year. What has changed is the baseline it starts from.
What the failure to mix does. Deep water is only resupplied with oxygen when the surface reaches it. Between 2012 and 2025 the dissolved oxygen concentration between 300 and 309 m fell from 8.4 mg/L to 2.4 mg/L, a drop of about 70%, and well below the 4 mg/L that sensitive species need. The deep water has also warmed by 1.2 °C, because nothing cold has reached it for over a decade.
Has this happened before? Yes. Lake Geneva went through another long spell without complete mixing between 1986 and 1999. The difference now is the baseline: that earlier episode ended when cold winters returned, and CIPEL judge that warming makes a spontaneous return far less likely this time.
Source: CIPEL (International Commission for the Protection of the Waters of Lake Geneva), winter mixing releases 2022, 2025 and 2026.
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