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Q & A: Salt's Effect on Water's Density

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Q:
How does salt in water (salt water) make a difference on the fact salt water is denser than fresh water? What makes it that way?
- Leandra (age 13)
California, USA
A:
Density is defined as:

mass /volume.

So when we say that salt water is more dense than regular water it means that there is more mass in a certain volume of the salt water than there is in the same volume of normal water.

When you add table salt (sodium chloride, NaCl) to water, the salt dissolves into ions, Na+ and Cl-. The volume increases by a small factor, but the mass increases by a bigger factor.  There are two reasons. One is simply that the NaCl is much denser than water to begin with, mainly because its ions have more mass than the oxygen and hydrogen atoms in the water molecules. Also, the ions bind nicely with the water molecules, so that the volume of the saltwater isn't as big as the water volume plus the salt volume.




Jason (mods by mw)

(republished on 07/24/06)

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