# Liquids with different densities

#### strid

I have always wondered why liquids of different densities dont mix up if put together with caution. As if you have a glass halffilled with cold water, you can with caution add another layer of warm water above the cold. You will see a clear boundry between the layers.
Is this something with the surface tension or what? I cant see any logic (which there probably is) behind why they would be kept seperate... I would apreciate an answer to this...

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#### Integral

Staff Emeritus
Gold Member
A visible boundary between fluid means that there is a difference in the index of refractions of the liquids.

There are some plastics that have the same index of refraction as mineral oil. A common classroom demonstration is to immerse such a piece of plastic in the oil, it seems to vanish.

A boundary is only visible if there is some reflection of light at that boundary, this can only occur when the index of refraction changes.

#### strid

indeed, it is so... but my question is why the two liquids dont mix-up... which forces keep them apart..?

#### Integral

Staff Emeritus
Gold Member
If there is a density difference then the materials will be separated by gravitational forces. The heavier molecules will displace the lighter, forcing them to the top. Thus warm air or water will tend to rise to the surface.

If the materials layers as you suggest are of close to the same density then diffusion will mix them over time.

#### NewScientist

Integral is this all to do with density currents? I have vague recollections of this!

-newScientist

#### Q_Goest

Homework Helper
Gold Member
I think the easiest way to understand this is to examine why a helium balloon goes up. At school they teach you it's because it's "lighter than air", the force up is due to the difference in masses displaced. If the mass of air displaced by the balloon is MORE than the balloon, the difference is the force upwards.

Yea, that's an easy way of looking at it, but it doesn't tell you where the force is coming from. The force up is actually due to the difference in air pressure on the top and bottom of the balloon which is equal to density*g*h. The very slight difference in atmospheric pressure between the top and bottom of the balloon causes a slight upward force. If that upward force is greater than the balloon's weight (density*Volume) then the balloon will be forced upwards by the air pressure.

Now imagine helium coming out of a valve into atmosphere. The helium atoms bounce off the air molecules like a crowd of people pushing through a doorway into a larger crowd of people. The helium molecules can't disperse into the air instantaneously, so they tend to bunch up as they come out, just as you'd have a hard time pushing your way through a crowd.

So this volume of helium, with perhaps a small amount of air that's gotten into it is less dense than the surrounding air which has only a small amount of dispersed helium in it. Before the helium can disperse in the air, it actually is forced upwards because the air pressure underneath is higher than the pressure above and the weight of the helium is less than this difference, just like the balloon.

The same can be said for liquids. The reason a balloon filled with less dense liquid will float in a higher density liquid is because of the difference in pressure between the top and bottom which results in an upward force.

And the same can be said of a liquid that has no balloon around it. In fact, the collisions are much more numerous and there is a much slower diffusion rate of liquid in liquid. So the reason a lighter liquid can be made to float is because of the difference in pressure. As the molecules start to mix by diffusion, the barrier will eventually go away.

The heavier molecules will displace the lighter, forcing them to the top. Thus warm air or water will tend to rise to the surface.
This isn't exactly true. The only reason warm air or water tends to rise is because of the above, there's a rate at which the warm and cold air can mix by diffusion or convective currents, and untill they get mixed, the warm air or water rises because it is being pushed upwards by a difference in pressure of the colder surrounding fluid. If for example, you have an insulated container, the contents will eventually come to some thermal equilibrium, and there will be no significant density gradiant other than that created by gravity. If we put helium in a room with nitrogen, there would similarly be very little separation between the two. The two gasses would eventually diffuse into each other and be very well mixed. The small difference in concentration between the top and bottom of the room has to do with the velocity of the molecules/atoms of the gas. It's a calculable difference, but relativly slight.

#### strid

that helped alot:)

was wondeirng further, what would happen if there would be two liquids with different salinity instead.. they would although thermal equlibrium not have same density... so would they mix-up by diffusion eventually by time?

#### Q_Goest

Homework Helper
Gold Member
As long as the two liquids are miscible, they will eventually mix. So yes, water with a high salt content will eventually mix completely with fresh water but until that happens, you'll find the fresh water tending to separate out on top.

#### strid

but can the porperties and behavior be predicted by a model or something...? like the wave speed of it, the thickness of it etc...

#### DaveC426913

Gold Member
Actually, wave speeds in separated liquids are a fascinating topic.

Did you know that Lake Champlain (on the NY/Vermont border) is a lake within a lake?

Becasue if it unique structure, the lake stays vertically separated into a cold layer and a warm layer. The boundary between them (a thermocline) is quite distinct (no big deal so far - smaller thermonclines are often observed by divers).

But the really cool thing is that "surface" of the colder depths act just like they were the surface of a body of water. They have waves that travel along, reflect and refract off the underwater obstacles. These waves take 4 days to slosh back and forth underwater, and have wave heights as much as 40 metres.

As a side note, Lake Champlain and its cousin, Loch Ness, are almost unique geographical structures in the world.

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