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When we add salt to water the volume of the water expands. Why is that

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    $\begingroup$ When you add salt to water, there is no longer such thing as volume of water. Instead, there is volume of (salt+water). $\endgroup$ – Ivan Neretin Mar 27 '19 at 14:22
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    $\begingroup$ If one adds potatoes to a cooking pot with soup, the volume of soup expands as well. Any surprise? $\endgroup$ – Poutnik Mar 27 '19 at 16:10
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Said in easy words: The salt will dissolve into ions, and in this particular case these need space between the water molecules, so the volume will expand.

However this is not always the case! As @JohnCuster points out:

There is no requirement that the volume expands. Things that hydrate (MgSO4 is one example) may actually reduce the volume under appropriate conditions.

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    $\begingroup$ There is no requirement that the volume expands. Things that hydrate (MgSO4 is one example) may actually reduce the volume under appropriate conditions. $\endgroup$ – Jon Custer Mar 27 '19 at 14:55

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