Why is density of water maximum at 4 degree celsius?
We were having a discussion to this question in class and I could gather the following points.
1) $H_2O$ exists in a cage like structure in ice form, due to extensive hydrogen bonding.
2) density=mass/ volume : We are not changing the mass here, therefore increase in volume will directly correspond to decrease in density and vice versa.
2) This cage like structure can trap gases and result in increase in volume. Therefore, forming of this cage like structure will result in increase in volume and breaking of this structure will result in decrease in volume.
3) At 273K , this cage like structure of ice exists as shown. On heating, we find that molecules gain kinetic energy and this cage like structure breaks. Due to this breaking, the volume decreases. At 277 K or (4 degree celsius), it so happens that the entire cage like structure is broken which results in decrease in volume resulting in maximum density. Therefore, density of water is maximum at this temperature.
4) At this stage, the cage has been entirely broken. Now, the molecules form this cage like structure again due to which volume again starts to increase as a result of which density decreases.
Here's what I couldn't understand:
1) I cannot understand why trapping of gases results in increase of volume. The gases are trapped in the free space inside the cage. Whether that free space is occupied by gases or whether it remains unoccupied shouldn't result in increase of volume. Then why it's said that trapping of gases inside the cage like structure formed by ice results in increase in volume?
2) I am not sure I heard the 4th point right because it clearly doesn't make sense. If at 277K, the cage like structure broke, why it will reform on further heating. On heating, we are giving the molecules more kinetic energy.They should roam about and not get tied down to a cage like structure. Why that cage like structure is being formed again after 4 degree celsius?