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School-college Physics Notes: Thermal energy 5.7 Density

Density & particle theory: 5.7 Comparing the relative density of the liquid and solid state and the curious case of water and its biological significance -and graphs of density versus temperature explained

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INDEX physics notes: Density, particle models, factors affecting density


5.7 The relative density of the liquid and solid state and the curious case of water

Before discussing the structure of ice we need to look at the anomalous density behaviour of water.

Left graph - density versus temperature for a typical liquid:

(1) Increase in temperature of solid, increase in thermal vibration, molecules move increasing a little more apart, density falls.

(2) Melting occurs when intermolecular forces weakened and increased freedom of movement moves the molecules a little bit apart decreasing the density.

(3) Increasing the temperature increases the KE of the liquid molecules, more energetic collisions, increasing with increase in temperature, steadily lowers the density of the liquid as the molecules bash each other a bit further apart as the intermolecular forces weaken more.

All of this is normal expected behaviour.

Right graph -density versus temperature for  water:

You also need to refer to the diagrams of ice structure below, as well as the graph above.

GCSE physics students do NOT have to know the details of hydrogen bonding mentioned on the ice diagrams.

BUT know its the strongest type of intermolecular force, which you do have to know about in chemistry.

Its the same intermolecular forces that holds the double helix together in DNA and RNA, and partly responsible for holding together the specific 3D protein shape of enzymes - you know more about hydrogen bonding than you think!

(1) Increase in temperature of solid, increase in thermal vibration, molecules move increasing a little more apart, density of ice falls - normal behaviour.

(2) Ice melts, but instead of an expected decrease in density, you get an increase in density of liquid water compared to ice - anomalous behaviour.

This is due to the partial breakdown of the open crystal structure of ice and the liquid molecules, despite their greater KE of movement, they actually have the freedom to get closer together (about 10% less volume) and so the density increases - you need to use a little imagination when looking at the ice diagrams - imagine when the open crystal structure breaks down, the water molecules can actually get closer together.

(3a-b) From 0oC to 100oC there is a continuous breakdown of the 'ice-like' structures in liquid water.

YES! the ice structure does not completely break down at 0oC on melting.

Clumps of water molecules persist and gradually get broken down with increase in temperature - increase in KE of molecules.

At the same time normal thermal expansion is going on! From 0oC to 4oC the effect of ice structure breakdown outweighs the normal thermal expansion, so you get a 2nd anomaly of the maximum density at 4oC (just over 1.0 g/cm3 or 1000 kg/m3).

(3b) The maximum density at 4oC is because the breakdown of ice-like structures in liquid water is exactly balanced by the effect of normal thermal expansion.

(3c) From 4oC the increasing KE of the molecules and more energetic collisions outweighs the break down of the clumps of water molecules and normal thermal expansion takes place.

ice diagram (i)

ice diagram (ii)

An important 'biological' consequence of the curious density behaviour of water

The anomalous density behaviour of ice has really important implications for aquatic life.

Because ice forms and floats on the surface of water, life can go on as normal in the liquid water below the ice.

The ice actually provides some insulation from the cold atmosphere, and in deeper ponds, rivers and lakes, most aquatic life can go on as normal.


Keywords, phrases and learning objectives for density

Comparing relative density of liquid and solid states biological significance curious case of water graphs of density versus temperature explained


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