The Unique Properties of Water

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Water is the most abundant molecule on Earth's surface. It is essential for life on Earth. The unique properties of water makes life possible on earth.
One property of water is surface tension. Water has a surface tension 2 – 4 times the surface tension of most organic liquids. Surface tension is highest for pure water. This enables insects called neuston to walk and live on the surface of water in low-nutrient water bodies, where they would starve if immersed in the water. This anomaly permits life when the water nutrients are too low to support life. This helps add nutrients to the subsurface water so that it will support life and insects and so that fish can then live below the surface.

Heat capacity is another property of water. The specific heat capacity of a compound is a measure of how difficult it is to get the molecules of the compound to vibrate. Because after all temperature is just a measure of molecular motion, the more the molecules of a compound vibrate, the higher the temperature of the compound. Because of the hydrogen bonding of water molecules it is very difficult to get them to vibrate compared to the molecules of any other common substance. Water is therefore said to have a high specific heat capacity or specific heat. Since water can absorb a lot of heat with only a small increase in temperature, the temperatures of large standing bodies of water remain relatively constant. This thermal buffering protects life on Earth from otherwise possibly lethal temperature fluctuations.

Lastly, density because of the unique bonding and structure of the water molecule solid H2O (ice) is less dense than liquid water. Therefore, when water freezes the ice floats on top of the denser liquid water instead of sinking to the bottom. This is different from most compounds which are denser in the solid state than in the liquid state. Freshwater lakes in mid-latitudes don't freeze solid. Instead ice floats on top and like a blanket, insulates the rest...
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