I have heard that the anomalous properties of water - e.g the fact its solid form is less dense than its liquid form - is extremely important. In fact, some people have gone so far as to tell me it was crucial for the development of life.

What is the significance of the anomalous properties of water to life?


This is a brief treatment of your question, specifically:

the fact its solid form is less dense than its liquid form - is extremely important. ...crucial for the development of life.

I can't speak to the importance of this property in the development of life (as I don't know the exact conditions of the world when life began, and therefore the role expansion of ice played in it) but life would not continue without it.

Water does not conduct heat very well, therefore water temperature fluctuates more near the surface of a body of water (say a lake, an ocean, or even a river) than in the deeper regions. In the winter, surface water freezes.

If water were denser on freezing, it would sink to the bottom, and liquid water would replace it. That liquid water would freeze, sink, be replaced, etc.

When it became warmer, the frozen water at the bottom would be less affected by surface temperature, i.e. it would be less likely to melt.

Even with summers, because of the failure of deep ice to melt, eventually you would be left with a mostly frozen body of water, and life doesn't carry on well in those conditions (freezing of the cells of plants and animals in the water would cause cell lysis, again, because of the expansion of water within), whereas life in a liquid under a frozen surface carries on much better. Life in the water would die.

If there were no life in water, then life on land would be most difficult. (Consider most of our oxygen comes from phytoplankton in the oceans. Even if the phytoplankton didn't happen to die from being trapped in frozen water, their nutrient supply would be cut off, frozen below.)

That's just the tip of the iceberg, really. The anomalous expansion of water also affects ocean currents and weather in ways that enrich life on earth.

Since you didn't ask about the other properties of water, this answer doesn't address surface tension, etc.

Edited to add: Corrected an error about the thermal conductivity (insulating properties) of ice. Ice does conduct heat more than water. It may be advantageous to the temperature of the water below possibly by holding snow (which does insulate well).

Source Thermal Conductivity Plankton: Ocean Drifters

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    $\begingroup$ Nice. I would say that frozen water "contains more air" though. You probably just mean that there are fewer molecules per volume --- lower concentration. And is this lower concentration really related to insulation? Does solid water really conduct less heat than liquid water? A reference for this? $\endgroup$ – Roland Feb 18 '16 at 18:17
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    $\begingroup$ @Roland - You're right in all your points. The thermal conductivity of ice is greater than the thermal conductivity of water. I've edited my answer. $\endgroup$ – anongoodnurse Feb 18 '16 at 23:12

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