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The Wonderful Weirdness of Water

Updated: Feb 2

 

22nd November 2024 | Ow Yong Ju-lia

Dihydrogen monoxide is a tasteless, colourless and odourless substance, and it’s found everywhere in our lives. It plays a big role in acid rain, contributes to the greenhouse effect, accelerates metal rusting and even causes serious burns. Despite the apparent danger, we use it all the time - from cooling down power plant turbines to preparing beverages.


Wait, beverages?


Dihydrogen monoxide means this compound has 2 hydrogen atoms and 1 oxygen atom. So, if you didn’t already realise it, dihydrogen monoxide is none other than the scientific name for water!


Water covers 71% of our world and makes up 60% of our bodies; however, despite how commonly it’s found, H2O itself, as you’ll see in this article, isn’t a very common substance at all.


Why are water’s melting and boiling points so weird?

Oxygen is in Group 16 of the periodic table, along with sulphur, selenium and tellurium. When these elements react with hydrogen, they form hydrides (eg. H2O, H2S, H2Se and so on).


The Group 16 elements: oxygen, sulphur, selenium, tellurium, polonium and livermorium.


Group 16 hydrides: their melting and boiling points plotted on a graph.


The melting and boiling points of H2S, H2Se, H2Te are all negative, increasing as you go down the group. This indicates that the intermolecular forces (i.e. the forces holding the compounds’ molecules together) increase in strength, so they need increasing amounts of energy to overcome (i.e. break apart) so the compounds can melt or boil.


However, as you can see, H2O’s melting and boiling points are much higher than even H2Te. 


You might probably pause here and think, "Huh, that’s weird." As you should! After all, if it were to follow the trend shown by the other three compounds, water should be boiling at -70ºC or lower - not 100ºC.


It turns out that water has a special kind of intermolecular force holding its molecules together - the hydrogen bond. As one of the strongest intermolecular forces, you’ll need a lot more energy to overcome this than the regular intermolecular forces in H2Te. This means that the heat supplied to the substance needs to be a lot higher.


Why is the density of ice (solid water) so weird?

When you freeze water, you get ice. If you’ve ever had an iced beverage before, you’d probably notice that the ice cubes inside float to the top - which means that ice is less dense than water itself.


Doesn’t this strike you as a little strange? Take sulphur, for example. As a solid, its atoms are stationary, and are packed closely together without much space; as a liquid, its atoms have more gaps between each other. Consequently, the same number of sulphur atoms occupy a larger volume as a liquid than as a solid. Density is inversely proportional to volume, so liquids are generally less dense than solids.


However, this is the very opposite in ice. When water is frozen, its molecules stop moving around. This makes it easier to form hydrogen bonds, which are rigid and hold the molecules far apart in a hexagonal, crystalline structure.




Ice: its molecular structure, which is shown to be hexagonal.


When ice melts, this hexagonal structure collapses into itself. Molecules move around in water, so it’s difficult to form as many hydrogen bonds as in ice. Water molecules are thus closer together, meaning that the same number of water molecules occupy a smaller volume as water than as ice.


Why is the surface tension of water so weird?

Water striders can run across any surface of water, and are commonly found in ponds, rivers and lakes. However, put them in oil or another liquid and they drown. 


Clearly, there’s something special about water - and that lies in its surface tension.


Surface tension is a force that makes the surface layer of a liquid act like an elastic membrane. The surface tension of water is exceptionally high - more than double that of oil’s. In fact, after mercury, water has the highest surface tension of all liquids.

This is, again, because of its hydrogen bonds. Water molecules are strongly attracted to each other due to its strong hydrogen bonds, which resist stretching and allows water to "bend" more compared to other liquids like oil. When the legs of water striders touch water, the water surface won’t "break" and thus lets the water strider walk on water without sinking.



To conclude, water’s unusually high melting and boiling points, less dense solid state and large surface tension are just some of the many reasons why water is ever so wonderfully weird. Hopefully, from this article, you’ve gained a deeper understanding and greater appreciation of how its properties factor into our everyday lives.


 

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