Rock Salt Crystalization

[paula eden]

I placed a small amount of rock salt in the bottom of a glass and covered it with about four ounces of tap water. In a few days the glass is covered with salt above the water line. Why does this happen?

[Ceal Craig]

I placed a small amount of rock salt in the bottom of a glass and covered it with about four ounces of tap water. In a few days the glass is covered with salt above the water line. Why does this happen?

Paula, controls on experiments are important to really know what is happening. Marking the first water line is important. I am assuming (maybe incorrectly) based on what you wrote you did not mark that line.

If a glass of water with salt in it is open to the air, over time the water evaporates. Depending on diffusion, salinity concentrations, etc., certain parts of the volume of water will have a higher salt content thant the other, but that is largely irrelevant here. Bottom line, the water is evaporating and salt crystals are forming on the glass easily. As this situation continues, I imagine you'll see salt crystals on bottom too as water dissipates.

Thus, the water line is sinking, salt crystals are forming on the side and ultimately in the rest of the glass as the water joins the ambient air.

Hope that helps! Let us know if it does not.

Ceal Craig
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[candicebrownelliott2]

Ceal,

As my tag line says, "That's odd"...

Paula,

As well as marking the water line, note the water surface is not flat? This is curved as it meets the glass. This curve is called a meniscus. You may find this explaination helpful:

http://van.hep.uiuc.edu/van/qa/section/ ... 390792.htm

Why is this important for your question? It's because as the water spreads thin as it reaches up to the glass, the surface is under even more tension than the middle of the curve, since it is being "pulled up" by the wetting to the glass and "pulled down" by the weight of the water in the meniscus... this added tension means that the evaporation rate increases in this tiny area. This means that there is water flowing up the thin meniscus to replace the water that evaporates. This added water brings with it more salt... and if the water flowing up the meniscus is traveling faster than the salt can diffuse back, then the water gets saltier (higher concentration). If that concentration gets higher than the water can dissolve at that temperture, etc. then it must crystalize out. Thus there is a tendency for the salt to crystalize out at the top of the meniscus first and faster than the bulk of the water.

I hope this has helped.

Oh... and if you wish to research this phenomena... I would suggest trying putting salty water droplets on glass slides and letting them dry... then determine where the salt crystalized.