Why is ice less dense than liquid water?

Kya_x2

I'm confused :confused:

and revision guides didn't really help...

Reply 1

Profesh

Molecular structure.

Reply 2

Excalibur

It's because of hydrogen bonding - when it solidifies, the geometry of ice is quite loose and there's a lot of 'empty' space. When it melts, the molecules take up this extra space so that density increases (until about 4ºC, from which the density decreases like any other liquid would).

Reply 3

Kya_x2

but why don't the water molecules fill up the spaces when it's crystalline (ice)? why does it have that certain length of hydrogen bonding?

AHHHH the chemistry exams are in two weeks and i still don't know. am i dead? :frown:

Reply 4

arpeggio

Ice has what's known as an "open crystal structure" due to hydrogen bonding - when ice melts the hydrogen bonds break, which is why the space that was between the molecules gets filled up and liquid water is more dense than ice.

It might help if you imagine the hydrogen bonds holding the molecules "at arm's length" - when the hydrogen bonds break down there's now nothing to keep them apart and the molecules can fill in the gaps. That's my non-scientific way of looking at it :biggrin:

Reply 5

theworm

Imagine a glass of water, in this glass the are molecules of H20 constantly moving around as they are always in constant motion, the Hydrogen bonds between them are constantly breaking and re-forming so the hydrogen bonds are relatively weak. Now when water freezes the molecules can't move freely and hydrogen bonds form permanently resulting in a crystal lattice and so water molecules are held in place by these h-bonds. The water molecules can no longer move close to each other like they can in a liquid making the solid less dense. As it is less dense the solid floats ontop of the liquid.

Reply 6

Candy_Shop

Probally just going to repeat what's already been said but, this is my revision too!

Ice has an open structure caused by hydrogen bonding. The hydrogen bonding keeps the molecules quite far apart, and therefore there is a lot of space in ice. Water doesn't have this open structure. On melting the water molecules fall into the spaces, making liquid water less dense than ice.

Thats the sort of thing I would say in an exam. And ok fair enough you may want to know 'why' this is the case but for the purposes of the exam just know that it does!

Reply 7

silent ninja

There are more hydrogen bonds in ice than water and the molecules are fixed in a diamond-like lattice. This occupies more space (a greater volume) than in the liquid state, which has fewer hydrogen bonds and so is more dense.

Reply 8

AWZC

Then why are there less hydrogen bonds in water then ice?

Reply 9

samsonlcy

because the H2O molecules in water are constantly moving around so there is no chance for hydrogen bonding.

Ice is a solid. So the molecules are moving less therefore more chance for hydrogen bonds forming.

Reply 10

silent ninja

In water hydrogen bonds are constantly forming and breaking.

Reply 11

lil one

This is because, when liquid water freezes, the hydrogen bonds take up a lot of space. i.e. the same mass occupies a larger volume, due to the open lattice structure. This makes the denisty of ice less than water.

Reply 12

Wangers

The O-H hydrogen bond in ice is longer then the OH bond in water hence ice is forced apart ontop of the water, therefore it is less dense and so floats.