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I am reading through this High School science lab about how dissolving salt changes the volume of water:

  • Place 300 to 400 g of salt in the flask (1L)

  • Pour in enough water to cover the dry salt, and swirl the water around in the flask to wet the salt and let air bubbles float up to the top. (This will not be enough water to dissolve more than a little of the salt; students will still see a lot of salt crystals.)

  • As soon as the air bubbles seem to have gone, fill the flask to the mark with water.

  • Label the water level clearly, with an OHP pen or some other marker. Point out that most of the salt is still there, as a solid unable to dissolve.

  • Shake the flask to hurry the dissolving until as much salt as will dissolve has done so.

  • Point out the consequent small contraction. Ask students why they think this has happened.

If I remember Chemistry class from ages ago, salt is ionic so it can dissolve in water, which is itself slightly polar: $$\mathrm{NaCl} \stackrel{H_2O}{\longrightarrow} \text{Na}^+ + \text{Cl}^- $$ The water and the salt together can pack more tightly than water by itself.


Reading the lab more closely 300g of salt is just over 1 cup, so you have to add a substantial amount of salt to find this decrease of volume. How much is this rate, and how might one compute it?

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  • $\begingroup$ sorry... room temperature, atmospheric pressure, etc. Salt and water may exhibit other interactions in extraordinary conditions. $\endgroup$ – john mangual Apr 15 '16 at 19:11
  • $\begingroup$ Note that at 20 C the solubility of NaCl is only 36 g/100 g of water. $\endgroup$ – Gert Apr 15 '16 at 21:58
  • $\begingroup$ The volume actually decreases. reddit.com/r/askscience/comments/3snwse/… $\endgroup$ – Farcher Apr 16 '16 at 7:05
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Note that the instructions do not claim that the volume of water is changing measurably over the course of the procedure. The volume of water is not changing noticeably. What this procedure allows one to observe is the fact that a salt+water solution has less volume than the total volume of its ingredients.

Rather than a demonstration of water changing volume, this is a demonstration of water NOT changing volume when a student might intuitively expect the volume of water to increase. It is a fresh take on the rather boring demo of pouring salt slowly into a beaker of water as you stir it and watching the water level not go up. Demos are usually more engaging when something unexpected and visible happens.

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