Page 39 - Curriculum Visions Dynamic Book

in air, how fish can swim in water, and so on. Water’s buoyancy makes it an excellent medium for transporting heavy loads, but its viscosity tends to make this slow.
Sound
Water easily transmits sound energy. When a sound wave moves through water, it causes molecules to move in the direction of the sound. As each molecule moves, it bumps against its neighbour, passing on energy that allows the next molecule to move. Once the extra energy has been passed on, the molecules return to their original places.
This kind of wave depends on the density of the material it moves through. We are used to sound moving quite slowly through air. That is because
air is not a very dense material. Sound moves, on average, three times faster through a liquid like water than through air. (It goes even faster through dense solids.)
Diffusion and stirring
The way that molecules of one substance mix
by themselves with another is called diffusion. Substances diffuse very slowly in any liquid. This is in great contrast to what happens in air or in any other gas where diffusion is rapid (allowing us to smell things, for example).
As a result, when you make a cup of coffee, you normally add the coffee and sugar and then stir because, if you didn’t, the coffee and the sugar would be very slow to mix with the water.
Because diffusion is not a helpful process
in liquids, stirring is widely used to mix them. However, stirring does not have to be by giant spoons or rotors. It can also be achieved by forcing jets of air under high pressure through the liquid. This is used, for example, in sewage plants and
in factories that need to separate metals from powdered rock.
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