Thread: Fluid Density & Density Anomaly Of Water

Hello folks,
I got a question about density.

Okay, as everyone knows, water has a density anomaly, meaning that at 4°C, water has its highest density. That's why water at this temperature would sink to the ground.

Now that is the thing I don't understand. There are two things I don't understand:

1.Water with a temperature of less than 4°C will be above the water having that temperature. Now the kinetic energy of the molecules of the water with 4°C is higher than of water with a lower temperature. Why, then, does it not rise (since Ekin is higher, it can transform more Ekin into potential energy - and due to the Brownian motion, that would probably happen). The same happens with warm air.

2. What's different about the molecules having more density compared to those with less density? They are closer together, but they still have the same mass. Why, then does the "bulk" with higher density sink and the one with less would stay upon? I'm talking about one and the same liquid (or liquids which are soluble).

The same with salt water. Why do not just the ions sink, but the whole water? I've just read about Cold Seeps, that are caused by this phenomenon.


And finally, why is it that in general, all things with lower density than liquid A would flow, and with higher density than A would sink?

Thanks in advance.

the distance between molecule has a relationship with density,but it is not density...

Originally Posted by ~James~

the distance between molecule has a relationship with density,but it is not density...

Err, yes it is. Density = Mass / Volume

The major driver for the water anomaly is the electrostatic forces between the positively charged hydrogen atoms and the negatively charged oxygen atoms. They counter act the microscopic motion due to heat (temperature). At about 4°C, the attraction between the molecules leads to a dense packing. If the heat motion (kinetic energy) is even more reduced, the molecules start to form crystalline lattices. This increases the volume again.

The reason for the changed buoyancy is the increased area and volume per mass unit. The gravitational force produces a pressure which is the force per area unit. If less mass is distributed across a certain area, the effective gravitational down force is less than for denser fluids.

@Dishmaster:

Thanks for your reply.
It's helped me.

But I still have questions:

The major driver for the water anomaly is the electrostatic forces between the positively charged hydrogen atoms and the negatively charged oxygen atoms. They counter act the microscopic motion due to heat (temperature).

Why, then, do other polar liquids not have a density anomaly? Or do they?

So the buoyant force is proportional to the volume, but gravity is not. Okay, I do understand that.
So I really do have to consider the different objects as unities. Since there is no density of single molecules (I suspect).
Still, would, for example (without considering any chemical reactions) a Calcium atom behave differently in water than, let's say, a fluorine atom (due to different atomic mass)?

Thanks in advance.

(ps: There are many thoughts and questions going through my mind at the moment, in respect to fluid buoyancy and behavior of objects in fluids.)