Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?
In: 24
When most substances cool down, their molecules get closer and closer together and move less and less.
As water freezes, its state changes, and the hydrogen bonds create a lattice that makes unique geometric shapes, stopping the water molecules from getting close together.
You know how “every snowflake is a unique shape” this is becausevif this lattice effect.
It’s not a perfect rule that materials expand when heated and contract when cooled, just an observation.
What’s going on is when water freezers it’s molecules arrange themselves into a crystal structure, meaning each molecule enters into a *fixed spot* in relation to the molecules around it. Liquid water has no such structure, each molecule floats freely compared to those around them.
It just happens that with water the arrangement of fixed spots are kind of far from each other, at least farther apart then the liquid water molecules are on average. Thus, as water freezes and the molecules move into their crystal slots, it expands a bit.
The underlying reason for this boils down to the structure of the atoms that make up water molecules, the arrangement of the electrons within the atoms, and the resulting “shape” of the crystals these molecules form. It’s actually pretty complicated but also regular and predictable, if you ever decide to take college level science courses, Material Science is a whole area of study the covers crystalline solids and how they structure themselves.
beyond the phase shift from fluid (water) to solid (ice) water is just all around one of the weirder elements due to how its molecules can be grouped together.
in my eyes the most curious part is that at 4°C, so slightly above freezing, water has the highest density meaning “has contracted the most” compared to both 0°C and 10°C water.
if you heat it up close to the boiling point it will expand (very slightly), so the general trend/rule still applies.
not only water but some other like silicon.
water **also expands** when heated and the energy make the atoms move. when you are referring to ‘water’ I believe you are talking about water at room temp.
what causes water to **also expand when it freezes** is the fact that when this happens the structure of the material (lattice) prevents atom from being as close as at room temp.
Water is a little bit ionic. The atoms are slightly negatively charged on the oxygen side and slightly positively charged on the hydrogen side. You can show this with [a science experiment](https://www.youtube.com/watch?v=HpIJ3cZURpQ) involving static electricity.
When freezing, something interesting happens. This effect causes water, vibrating much less as it cools, to align its molecules with each other. This actually causes ice to expand a bit since the aligned molecules now have limitations on how they want to fit together.
Water molecules are polarized, you can think of them like little magnets in some ways.
Say you put a bunch of rocks in a bag and shook it then let it settle. The rocks are gonna settle in next to each other and fill in a lot of space, it’ll be more compact than before.
Then put a bunch of magnets into a bag and shake it, the magnets aren’t gonna want to bunch in next to each other the wrong way once it settles. They’ll form structures that make the magnets happy. These structures don’t use all of the space efficiently and they’ll be more spread out which increases volume
Not a perfect metaphor but I hope it gets the point across
Water does actually contract when it is cooled. However there is one exception: between about 4 C and 0 C, at atmospheric pressure, water will arrange itself into the lattice that is ice. This lattice is less dense so it expands until it freezes then it starts contracting again. And it’s important to note that the watcler is still “contracting” during that time, kind of, it’s just that the lattice it forms is less dense, so the molecules are getting closer together, but also rearranging themselves so there is more space at the same time.
Water is not the only substance that does this, but it is the only common one on earth that does it. And at different pressures the mechanics change.
There are other materials that do the same. Not many but some. The one that surprised me the most when I had to deal with it was a carbon fiber we were using as a retention sleeve for a very high speed rotor. It would be pressed onto the rotor and for that we had to cool it so it would expand.
Probably worth noting here. We are very lucky this is the case. If a lake froze and ice was denser than water, it would sink down until the lake was frozen solid and all marine life was dead.
Also, water has 19 known solid states! Some of them are actually denser than liquid water. They depend on pressure and temperature conditions for the most part.
Correct, most materials do expand when heated and cool when contracted. So does water: https://www.e-education.psu.edu/earth111/node/842 Water has a maximum density (lowest volume) around 4 degrees C.
Ice (well, the most common kind you find in ordinary pressures) has a hexagonal crystal structure that occupies more volume than liquid water. Basically the molecules can squish together tighter if they’re liquid. https://en.wikipedia.org/wiki/Ice_Ih
They form hexagons because hexagons are bestagons, but more because water molecules are boomerang shaped.
Latest Answers