# Density of Water

by on November 9, 2009

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underwater survival training

The density of water is about 1,000 kg/m3. We’re actually referring to fresh water. The density of seawater is slightly higher, at roughly 1030 kg/m3. In the succeeding discussions, unless we specify otherwise, we’ll be referring to fresh water when using the term ‘water’.

Now, the density of water has a rather peculiar characteristic. You see, most substances having initial temperatures of 0°C continuously expand as you slowly raise those temperatures. Water also behaves that way … but only after 4°C. Between 0°C and 4°C, water contracts.

Thus, the volume of water decreases from 0°C to 4°C, then continuously increases from 4°C onwards. As a result, the density of water, being dependent on volume (which is changing) and mass (which is constant), behaves such that it increases from 0°C to 4°C, then continuously decreases from 4°C onwards.

This behavior of water, which is different from practically all natural substances, plays a vital role in the survival of water-dwelling animals during the cold months.

I invite you to review the concepts mentioned earlier and view it in reverse, i.e. instead of increasing temperatures, try to imagine what happens when temperatures drop.

During winter, the cold air above a lake or pond first cools water at the surface. As the temperature at the surface drops, the density of water there increases. For this reason, water at the surface sinks to the bottom and the body (of water) below it rises to take its place. This body is then also cooled and, like the one before it, eventually sinks.

This process continues until the temperature reaches 4°C. Below this point, the density of water now decreases as temperature continues to drop. As a result, the water at the top stays at the top where it is brought to even cooler temperatures. Once the temperature at the surface reaches zero, water there starts to freeze.

That’s the reason why water starts freezing from top to bottom. As water freezes, it serves as a sort of insulator that slows down the cooling in the deeper regions of the lake or pond. This allows a thin layer of water in liquid form to exist at the bottom. This is where the fishes dwell.

And so here we see how the density of water plays a vital role in the survival of those fishes.

Here are some articles in Universe Today that you might be interested in:
Yes, There’s Water on the Moon
and
Water on the Moon: What Does it Mean?

If those articles got your attention, here are some more from NASA:
NASA Images Suggest Water Still Flows in Brief Spurts on Mars
NASA Spacecraft Confirms Martian Water, Mission Extended

How about listening to some episodes from Astronomy Cast for a change?
Undoing Inflation, Searching for Water, and Seeing Everything a Black Hole’s Ever Eaten
Stellar Roche Limits, Seeing Black Holes, and Water on Mars

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