# Temperature of minimum density

1. Apr 26, 2011

### zorro

What is the temperature at which water possesses minimum density?

Last edited: Apr 27, 2011
2. Apr 27, 2011

### Staff: Mentor

This is tricky, for several reasons. But you are here long enough to know you should show your attempts first, and you know enough to see why it is tricky.

3. Apr 27, 2011

### zorro

One reason I can think of is that usually the question is about 'maximum density', which is at 4 C.
haha what attempts should I make for a question like this? Random guesses 50C 60C 100C ? I think its better to keep quiet than to talk something silly.

4. Apr 27, 2011

### Staff: Mentor

These don't have to be random - just find a density vs temperature table.

However, what about 200 deg C?

5. Apr 28, 2011

### zorro

Why would I post the question here if I had already found that table?
I searched on the internet but could not find it.

So 200 deg C is the answer?

6. Apr 28, 2011

### Staff: Mentor

7. Apr 28, 2011

### zorro

Yeah anyone would say that after reading my post. Actually my mind was wandering somewhere else while writing that. I did find some graphs/tables but they were only for a short range of temperatures like 0-30 C, 0-80 C from which I could not deduce anything as the graph decreased continuously. In short I could not find a minima of the curve.

I think there is no such temperature....the density decreases continuously with increase in temperature and attains a constant value. So we cannot mention only one temperature for minimum density

8. Apr 28, 2011

### Staff: Mentor

Constant?

9. Apr 28, 2011

### zorro

It cannot decrease infinitely and become negative so there should be a limiting or constant value.

10. Apr 28, 2011

### Staff: Mentor

Sigh.

You have a constant mass, when heating volume goes up. Density is a ratio of these values. Idea that it can get negative is out of this world.

What happens to water when heated?

11. Apr 28, 2011

### zorro

Its starts evaporating at 100 oC and changes its phase above this temperature.
d ∝ 1/V, can we indefinitely increase the volume by heating?

12. Apr 28, 2011

### Staff: Mentor

Good question. In the realm of ideal gases - yes. But water is not ideal. What may happen at 200 deg C? 2000 deg C? 20000 deg C? What happens to all compounds at high temperatures?

13. Apr 29, 2011

### zorro

The final phase of a compound is a gas which is attained above its boiling point/sublimation point. I don't know what happens to it if we keep on heating.

14. Apr 29, 2011

### Staff: Mentor

You probably know, you just don't realize it is universal. Have you heard about thermal decomposition?

15. Apr 29, 2011

### zorro

Oh yes I know about it. But never heard it being used for water. So at some high temperature, water dissociates into hydrogen gas and oxygen gas. I think this temperature must be the temperature of minimum density because volume becomes maximum.

16. Apr 29, 2011

### Staff: Mentor

You are on the right track - wasn't that hard, eh?

Trick is, there is no simple single temperature at which you can say "water decomposes here". It is an equilibrium process, so all we can do is to calculate percentage of water that is decomposed at given temperature - no idea about exact numbers, according to wikipedia at 2200 °C about 3% of water decomposes, at 3000 °C above 50% and so on. So there is no simple answer to the question as asked.

17. Apr 29, 2011

### zorro

Its very easy but I did not think about it that way .

Thank you Mr.Ph!! I understood it now.