Page 25 - Curriculum Visions Dynamic Book

different from a gas or a solid. We could not have kept a gas in an open glass because a gas expands to fill the shape of the container it is in, and if there is no lid, it will simply expand out into the air. If we tilted a glass with solid water in it, the ice blocks might tumble around, but they would not change their shape. And when they were poured out, they would not re-form into a single whole.
Surface tension
The molecules in water can move around even though they are attracted together, and that is why water has no fixed shape. Yet because the molecules are all bound to one another, they form a heavy mass that collectively sinks down to the bottom of any container it is placed in.
Water is held together by forces that are balanced inside the liquid. But at the surface water molecules are more attracted to their neighbours than they are to the molecules in the air. Thus there is a dominant sideways force and an inwards force, but there is no outwards force. This imbalance creates a net inwards force, and it is this force that makes water form a shape that has the smallest possible surface area and thus behave as though it were surrounded by an invisible skin.
We know this surface force as surface tension. We also see it, for example, as a faucet drips or when water globules gather on the side of a cold glass. It is also the force that holds raindrops together. You can see this effect used by tiny water insects such as pond skaters. Their light weight means they do not break the force between the molecules, and so they merely dent the surface where they stand.
The surface tension effect works on all liquids, but it
is particularly strong in the case of water. The strength of the surface tension force explains why water tries to gather into little balls instead of spreading out over the surface it is resting on.
In places where gravity does not work (for example, in spacecraft) water droplets in air are truly spherical. This also shows that surface tension is a property of
the water and is nothing special to the Earth. But in the Earth’s gravity the weight of the liquid causes the shape to
(Below) Surface tension attracts the surface water molecules together, holding them together as though they had an invisible skin.
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