What is Liquid?


There’s a lovely museum near to us.  In the museum courtyard there is a pond.  At the moment it has tadpoles in it.  Kam loves it, he catches the tadpoles, watches them for a couple of seconds and puts them back into the water.

He began to ask about liquids, why he said liquids and not water I don’t know.  Then, he saw an insect land on the water and stand on it.  Mum, my bees die when they land in water, but those flies don’t, why?

Tell me about liquids.

Liquids are one of the four states of matter.  The other three matters are:

  • solids
  • gases
  • plasmas.

Liquid is a fluid.  Liquid flows to match and fill the shape of a container it is inside.  For example,

  • If we pour water into a cup it stays inside the cup, as long as we don’t overfill!
  • When water flows along a river it stays inside the river banks and river bed, unless there is a deluge of rain when it can flow over or break the banks – which from experience is a dreadful, dreadful situation.
  • If it is not in a container it will free flow – following the laws of gravity.

We call the tiny particles that form a liquid atoms and molecules.  In a liquid atoms and molecules have freedom of movement.  (In a solid they do not.)

Liquid particles are bound firmly and compressed tightly, but not rigidly.  The forces which bind or keep the liquid molecules together are temporary.  This is what lets liquids flow.

Liquid examples:

  • water at room temperature
  • oil at room temperature
  • alcohol at room temperature.

Where Gravity is present, liquids such as water droplets, get flattened by that wonderful favourite of ours – gravity.   We’ve written a little about this below.

Surface Tension

At a given point a liquid meets a gas – such as when water meets air.  At this point, the surface of the liquid has contact with the gas. The liquid surface forms a boundary and behaves like a fine stretchy sheet or film coating the liquid’s surface.  We call this behaviour surface tension.

Note:  If two liquids meet – for example, oil and water – this is interface tension, not surface tension

imageMolecules in the interior of a liquid attract equally in all directions.

Molecules at the surface experience a net inward force.  This force:

  • pulls molecules from the liquid’s surface towards the liquid’s interior, which lessens the surface area.
  • makes molecules at the surface pack closely together, – which causes the fine elastic sheet or imagine it as a fine skin.

imageThis effect lets some objects float on the water’s surface and for some insects to walk or stand on water.  (Yet, their density is more than water.)

Surface Tension History

Thomas Young
Thomas Young
Pierre-Simon Laplace
Pierre-Simon Laplace
Carl Friedrich Gauss
Carl Friedrich Gauss

In 1805, Thomas Young developed the theory of surface tension – as an imaginary membrane stretching over the liquid’s surface.  A year later, Pierre Simon Laplace described it mathematically.

In 1830, Carl Friedrich Gauss consolidated & extended both their works.  Interestingly or at least I think so, Young was British, LaPlace French and Gauss German.

Things for us to try

  • A liquid pours easily, but how easy is it to hold.
  • can you squeeze a liquid in a container, try squeezing the same liquid tied inside a plastic bag.
  • Can we find liquids with different density.
  • What happens when we heat water – water evaporates, yes, but where to, what does it change into? Is this a different state of matter? – gains kinetic energy.
  • What if we pour a liquid into a tall container and the same liquid into a flatter wider container – reference the fox and stork fable – is the volume of liquid still the same?
  • What happens when we freeze water? Does this change its state? – loses kinetic energy.

A Note for You and Me

I’m glad Kam asked this question.  I’ve found it really interesting and we have lots of things we can try.

Thank you for reading and we hope you enjoyed our article.  We’d love you to take a look at our blog.

If you have any comments we’d love to hear from you.  As always, if I’ve missed the plot please let me know.


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