Interesting Question for Randy

bobstir · Jun 10, 2015

Randy,

I've noticed a curiosity when filling up my dosing containers. I dose BRS soda ash and calcium carbonate (dry form). I've observed -- not scientifically by any means -- that when adding the soda ash, the water level in my container does not rise very much, but when I add the same amount of calcium carbonate, it rises substantially more than the soda ash container (e.g. apparent higher increase in volume).

There are no noticeable air bubbles. What causes this? This is just for my own edification.

Thanks!

Bob

Randy Holmes-Farley · Jun 10, 2015

This is an interesting question. Thanks for posting it!

It turns out that when many salts are dissolved, the volume actually declines. Sodium chloride dissolved into water actually causes a small volume decline in the water (2.5% at saturation):

http://www.nuffieldfoundation.org/practical-physics/volume-change-dissolving-salt-water

The primary reason is that salt binds some of the water molecules tightly, essentially pulling them closer together.

I don't know if that happens to that extent with the concentration of the sodium carbonate you used, but the calcium salt is quite different.

You probably used calcium chloride dihydrate (CaCl2 x 2H2O). In that case, you are adding not only calcium chloride, but also additional water, and that water accounts for the increased total volume, even if the calcium chloride is causing a slight decline.

Make sense?