Why does water expand when frozen?


Title says it all, why does H2O expand when frozen? In school I was told that water was weird because it expands when frozen, but was never told why. Does anyone know?

In: Chemistry


It’s molecular. I really can’t use the words right without googling it but I’ll try my best lol

Water starts as a fluid where it’s molecules scared to fill the container they’re in. The water is free to move so the space between the molecules is minimal. As it heats, the molecules spread up and turn to gas where they can move freely throughout the air, usually rising to become a cloud. At lower temperatures the molecules slow down, and i think what happens is air is actually forced between them as they solidify, taking up the space they were free to move in, but causing an expansion effect. It’s confusing because when a gas like oxygen for example, reaches freezing temperature, let’s say in a balloon, it will appear to deflate, because the molecules aren’t moving as freely or quickly.

I butchered this explanation and should’ve google it. Sorry.

Below 4°C, the hydrogen bonds between each water molecule become stronger and cause the matter to expand.

As a liquid, water molecules are constantly moving (forming and breaking hydrogen bonds) resulting in less expansion and the molecules being jumbled up on top of each other.

When frozen, water molecules take a more defined shape and arrange themselves in six-sided crystalline structures. The crystalline arrangement is less dense than that of the molecules in liquid form because the molecules don’t get jumbled up on top of each other it actually creates space between them, which makes the ice less dense than the liquid water.

The hydrogen atoms are positively charged and the oxygen atoms are negative. The angle of the two bonds is 135° which is the same as a hexagon. When the water is cold enough, the molecules line themselves up in hexagon patterns.if it isn’t cold, the molecules shake too much and the hexagon structures fall apart

Picture a bunch of people in a room. They’re each a water molecule. Their body is the oxygen atom and each stretched out hand is a hydrogen atom.

In liquid form they’re moving around some, bouncing into each other. Occasionally one will grab someone else’s shirt for a while. That’s a hydrogen bond. It’s pretty weak but a bunch of them combined can actually be pretty strong. That’s what gives water its surface tension.

As the water cools, they start to slow down. The bounces become less frequent and less energetic, so when someone grabs someone else’s shirt they’re able to hold on longer. As we cross the freezing point these interactions start getting locked in place. So now instead of losing grip on the shirt and bouncing together, they now stay at arm’s length.

They start to chain together forming a 3D crystal structure. Now everybody is nice and organized, spaced apart from each other at arms length. There’s very little bouncing off of other people.

So we went from a relatively dense state with people in close proximity frequently bouncing into each other, to a crystal state where everybody has space between everyone else, a less dense state. That’s how water expands and density drops when frozen 😀