Why does water expand upon freezing?

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My mother always asks me not to fill the ice cube tray with water to the brim because it’ll expand and break the box. But doesn’t matter shrink in volume when heat is dissipated, so why does the ice cube expand?

Sorry in advanced, if I word it incorrectly, English is not my first language. Thank you.

In: Physics

5 Answers

Anonymous 0 Comments

While heat tends to favor things expanding, it’s really about stability and (relatedly) free energy, really. While the general trend is that if you heat something up, it will expand, the state transition for water is different here.

Basically, water in solid form has a crystal structure. This structure is less dense than its liquid form. When all the molecules are free to move and slide around each other, they actually compact slightly better than if they were rigidly held in place.

Relevant image: https://chem.libretexts.org/@api/deki/files/78717/CK12_Screenshot_15-2-1.png?revision=1&size=bestfit&width=508&height=249

Anonymous 0 Comments

Water is a highly polar molecule and regularly forms what are called hydrogen bonds. These are fairly strong interactions that have a set distance from each other. They are weak enough to be broken by heat energy in liquid water, and when they’re breaking/reforming in liquid, the molecules of water can actually get closer than the distance of a hydrogen bond.

At the freezing temperature of water, the hydrogen bonds essentially stop breaking, so all the water molecules form a crystal lattice and are held at a set distance from each other that is farther apart than in liquid water.

This is the crystal structure that water forms and gives it some unique properties such as solid water being less dense than liquid water, and thus floating, and causing water to expand when freezing.

If you’re ever curious about what makes water do all the weird things it does, the answer is literally always “Hydrogen Bonds”

Anonymous 0 Comments

The first answer here is right on, but if you really want to blow your mind, think about how the world would work if this wasn’t the case. For instance, nothing could live in a lake ina temperate climate if the ice sank, as the surface would continually cool, freeze, and sink, eventually freezing the entire lake. Instead, the already frozen ice stays next to the cold air and insulates the water underneath.

Anonymous 0 Comments

Pretty sure the H-O-H bond is close to 118°. Standard molecules would be around 120. The hydrogen bonding phenomenon is what holds the angle tighter. At lower temps this bonding is “relaxed” a bit and the angle increases; therefore, becomes less dense

Anonymous 0 Comments

Take a box of nails and dump it on the ground. All jumbled together, they don’t take up much space. But if I were to lay out each nail end to end they would take up more space.

It’s the same way with water. Water is a polar molecule, which means one end is positively charged and one end is negatively charged. The negatively charged side of one molecule wants to be next to the positively charged side of another molecule. But water has too much energy for that to happen. 

When you freeze it, there isn’t enough energy to keep that from happening and so the molecules align themselves. When they align themselves they take up more space.