The weirdness of water:
molecules and mysteries
Water is weird!
We enjoy the feel of water on our skin and watching the way it flows, and the sound of it can be incredibly soothing. But it also has some strange characteristics.
Why does it get sucked up a straw without you even sucking?
And why does ice float on water?
And have you noticed when you jump into the sea, the way the surface feels hard as you hit it? It’s almost like it’s solid on the top!
Each of these qualities is connected to the unusual characteristics of the water molecule – its shape and the way water molecules are bonded to each other. And these same qualities are the reasons we have life on earth!
The shape of water
The water molecule is triangular, with one oxygen atom and two hydrogen atoms like this:
The way the oxygen and hydrogen atoms are bonded together is by sharing electrons. Each hydrogen atom shares its electron with the oxygen atom. Electrons are negative so this means the negative charge of the electron is pulled over towards the oxygen, leaving the the Hydrogen atom with a positive charge. So the water molecule is positive on one side and negative on the other.
‘The Shape of Water’ by Kate Adams
But what does this mean for water? Why does that make it so weird?
Opposites attract -hydrogen bonding
Because one side of the water molecule is positive and one side negative, water molecules form unusually strong bonds with each other. The positive area of a hydrogen atom is attracted to the negative area on the oxygen in a neighbouring molecule. And they bond.
So water really is attracted to water.
This means water doesn’t evaporate as easily as you might expect. The Hydrogen bonds hold the molecules together, so it takes a lot of heat (100°C) to break them apart. That is why we have oceans, lakes, rivers and rain on the earth instead of water vapour, a gas.
But water molecules don’t just stick to other water molecules. They attract all kinds of other substances, so water can dissolve salts and proteins and carry them around plants, trees, animals, humans taking nutrients to where they are needed.
Capillary action – how water beats gravity!
Because water molecules are attracted to other molecules, they also get pulled towards to other surfaces. So if you have a tiny tube, the water molecules are attracted to the sides and they get pulled along the tube, even travelling against gravity. This is called capillary action and it is part of the reason water can travel up the stem of a plant.
This simple experiment, the Rainbow celery experiment, is an amazing way to see capillary action at work.
Rainbow Celery Experiment, PBS, Creative Commons
Surface water tension and why a belly flop hurts
If you hit the surface of the water with your hand (or any other part of you), it actually hurts! It almost seems solid. But once you get past the surface, it feels lovely – soft and fluid around you. We never really think about it, but that’s definitely weird!
As we know, water molecules are attracted to water molecules. But the water molecules on the surface have air above them so they have fewer water molecule neighbours to bond with. This means they form stronger bonds with the water molecule neighbours they do have. So they are closer together and the surface is more dense. And that is why a bellyflop hurts.
‘Surface’ by Kate Adams.
How ice floats on water
When water freezes, the molecules connect together in hexagons to form ice crystals. That’s why snowflakes are so pretty. They grow outwards from the six corners of a hexagon and form these beautiful stars.
But what do hexagons have to do with floating you might be wondering.
‘Snowflake’ by Michele Morgan, Creative Commons
Well, usually when molecules turn from liquid to solid they get closer together not further apart. But not water! Because when you think about it, there’s rather a lot of space in the middle of a
so when water molecules are in their ice form, they actually take up more space than when they’re liquid. In ice, the molecules are more spread out whereas in liquid, the water molecules are more densely packed. This means if you have the same number of water molecules, they weigh less as ice than water. So ice floats! And that is weird and amazing!’
‘Icebergs’ by Gérald Tapp, creative commons
If you want to learn even more about water, this BBC animation ‘Why water is really really weird’ is lovely. It is narrated by the author of The Water Book, Alok Jha with Animation by Oliver Smyth. And take a look at our page on water and sustainability!