Science Buddies: "Ask an Expert"

Hi, it's me again.

"In this project, you will do experiments to see what happens when layers of water at different densities are brought together."

That's directly from the experiment page. I have done the experiment and am in the conclusion stage. When liquids at different densities are combined, they separate into layers, with the least dense on top.

However, if it is water, wouldn't it mix until it reached equilibrium? For example, if I had salt water (blue) on top of fresh water (yellow), wouldn't they mix until the salinity of both is equal? If they really did separate into layers, wouldn't salt go down and fresh go up and I would be able to clearly see only one color in each bottle? When I did the experiment, one displaced the other and I got a greenish color. I only waited 3 minutes for each trial, would I have to wait longer?

My thought process is that if you have 2 ten gallon aquariums, and completely filled one with water, and then connected the two with a tube near the bottom, then the empty one would be filled with water until both aquariums had the same amount of water.

I have researched molecular diffusion, diffusion equilibrium, concentration gradient, and osmosis but I still don't understand. For example, for the temperature one, the water is cooling down or warming, so wouldn't they just balance each other out?

Any clarification is greatly appreciated.

Hi,

For reference, I believe the project you are doing is this one:

https://www.sciencebuddies.org/science- ... ?from=Home

Just to get the conversation started, have a look at this site:

http://karismaskids.blogspot.com/2008/0 ... water.html

http://www.exploratorium.edu/science_ex ... trick.html

So your experiments should have shown you that if you very carefully layer a lower density layer on a higher density layer the situation is relatively stable and the two layers mix only slightly at first. In the opposite situation (denser liquid on top) they mix rapidly and often do not separate again.

You ask why they don’t both mix to uniform composition immediately? It is all a question of how fast they mix. Diffusion is relatively slow, while turbulent mixing is rather fast. But in the end, if you wait long enough, the liquids will be uniformly mixed if the two liquids are miscible.

Hope that answers the question. Good luck at the science fair!

Barrett Tomlinson

Yes, those websites help. For the temperature trials, I know both hot and cold will eventually reach room temperature. But for salinity, won't they both eventually mix until they have the same salt concentration? I suppose that's what I've been trying to say. To my understanding, molecular diffusion will lead to diffusion equilibrium. I don't think the denser liquid would just stay below the less dense liquid. Would it? I have no way of measuring the salinity of both bottles. Exactly how long does diffusion take?

Hi,

Diffusion is pretty slow, my gut instinct says it could take several days or more to reach equilibrium if fresh water were floated over salt water. You could test this experimentally by floating fresh water with dye over salt water. The boundary between them should start out sharp if you do it right. If you let the setup sit undisturbed the boundary between the two layers will gradually blur, and if you wait long enough, which could be a really long time, eventually the solution will become a uniform color. The equations which govern this were discovered by Adolf Fick in 1855. For more info. See

http://en.wikipedia.org/wiki/Diffusion_equation

So I think your understanding is basically correct, but the differences in salinity can persist for quite a while or the thermohaline circulation talked about in the experimental writeup would not be possible.

I looked up the diffusion coefficient of 0.1 to 1.0 Molar sodium chloride into pure water at 15 Degrees C in the Handbook of Chemistry and Physics, 40th edition. It is reported as 0.94 square cm per day. That is slow!

Hope this helps.

Barrett Tomlinson

Okay. For my conclusion, I'll just say that when waters at different densities are combined, they separate into layers with the least dense on top. Eventually, molecular diffusion leads to diffusion equilibrium. Short and sweet. Thank you!

The color change of the fluid will probably be instantaneous, but for it to completely reach equilibrium color-wise, it would take some time.

However, the equilibrium of salinity concentrations would probably take longer to reach dynamic equilibrium. Make sure you note that it is dynamic equilibrium, since you are dealing with substances in the liquid state, and molecules are constantly moving and displacing each other.

-Dan