ElI5 what exactly happens in the freezing process that makes ice less dense than liquid water?
In: Physics
Water molecules are slightly bent in a 105° angle, rather than the straight 180° angles of most other diatomic compounds. When most compunds freeze they settle into a compacted structure that rests completely stacked into itself (imagine a stack of uniform plates) which makes them more dense than their liquid form. Water molecules, however, form a tessalated structure that has large empty gaps in the middle between the different molecules that decreases the density of the solid form.
Ice is a special case. Water does a thing called hydrogen bonding. This is a weak intermolecular bond formed between hydrogen atoms in one water molecule and oxygen atoms in the adjacent molecule. This occurs because of dipoles within the water molecules – water is a covalent molecule, which means it’s atoms bond together by sharing electrons. Oxygen atoms are more attractive to electrons than hydrogen atoms, so the electrons float around closer to the oxygen than the hydrogen, which means effectively the centre of the molecule is slightly more negatively charged and the ends are slightly more positively charged. This makes them attract to each other like magnets.
Anyway, those hydrogen bonds become stronger and more numerous the more solid the water becomes. As a gas, there are no hydrogen bonds. In a liquid, there are a few. If allowed to freeze normally, hydrogen bonds in ice form a crystal lattice structure that creates a larger distance between molecules than there would be if water didn’t hydrogen bond. More empty space in a structure means it’s less dense, and so the structure has a higher total volume .
Same amount of water takes more space due to expansion of ice crystals. Same water with more volume gives lower density.
The crystal structure of frozen water *expands* which makes it less dense than liquid water
Water Molecules have this triangular shape, this triangular shape is also polarized. This configuration makes it so that when water freezes the least energetic configuration for the molecules(which is in a nutshell what cooling is: removing energy) is a solid structure that takes up more space than it would when free flowing and the molecules can jostle each other a squeeze in a better fit.
Think a LEGO building, it takes up more space when assembled and is less dense.
It’s a result of something called hydrogen bonding. Effectively as the water loses energy and freezes it will fall into the most stable state. The extra volume matters less than favorable crystal formation. In liquid water it has enough energy to overcome this tendency to spaced out crystals and the hydrogen bonding is less relevant to how the molecules behave. This they can fall closer together.
Liquid water is disorganised. Ice is a crystal, which has a specific (hexagonal) structure. The expansion is when the liquid starts to assemble itself in to this regular structure. It takes up more space to be ordered than crammed in however.
The reason for this structure is called hydrogen bonds. The hydrogens on one molecule like to be close to the oxygens on others.
It should also be noted, that water does actually condense as it gets colder, up until about 4 degrees Celsius, where it starts to align like everyone is talking about, and expands again. I can’t remember if there is a specific term for that point or not.
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