# Water at 4 degree Celcius has maximum density

1. Nov 23, 2012

### Outrageous

During winter why not all the water molecules on the water surface that have 4°C go deep under the water?

2. Nov 23, 2012

### K^2

They basically do. Water at the bottom of the frozen-over pond will be at about 4°C. That's also why lakes freeze starting from the top.

3. Nov 23, 2012

### Outrageous

Can I say in a frozen-over pond, the highest temperature of the water will be 4 °C which is at the bottom?

4. Nov 24, 2012

### K^2

In practice, almost always, but it isn't necessarily true under all conditions. Certainly, there cannot be any water warmer than that. If there is, it will rise to the top, until it either meets colder water and cools, or until it gets to the very top and meets ice, in which case it will melt some ice and cool down. So in equilibrium, the warmest water you can have is 4°C.

However, all water in the pond could be colder than 4°C. That really should only happen right before the entire thickness of water freezes through, because while there is still a lot of water, the heat is lost only from the surface, and whatever little heat there is coming from the ground will keep the bottom at these 4°C. But if for whatever reason there is no heat from bellow, it could, in principle, be colder.

But certainly, you could say that there is no water warmer than 4°C, and whatever temperature the warmest water is, it is at the bottom.

5. Nov 24, 2012

### Outrageous

Thank you very much

6. Nov 24, 2012

### Staff: Mentor

Note in the ocean things get slightly different - deep ocean water is colder, usually between 0 and 3°C.

7. Nov 24, 2012

### Outrageous

Is that because there is high concentration of salt? Since 3°C is lower than 4°C , why not lower than 0 ?

8. Nov 24, 2012

### Staff: Mentor

I believe it is a combination of salinity and high pressure that changes water properties. Perhaps there is more to it, I don't know details. Sorry.

9. Nov 24, 2012

### K^2

At high pressure, the maximum density will certainly be at a lower temperature, so it's probably a factor. But I don't know how significant that would be or how the salt will play into it.

10. Nov 24, 2012

### Outrageous

Thank you guys

11. Nov 24, 2012

### Studiot

Do not try to apply the 'anomalous' density curve for pure (fresh) water to sea water.

For sea water the density continues to increase as temperature decreases all the way below zero (C) to the freezing point. There is no anomalous maximum.

Of course the actual value of freezing point depends on the salinity, as Borek said.

The reason icebergs float is tha they contain a large quantity of entrained air.