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Q & A: salt and buoyancy

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Most recent answer: 09/29/2010
Q:
I need help for a science project. Me and my partner are doing a project about bouyancy. I dont understand exactly how salt makes things more or less bouyant. I also need help for project ideas.
- Grace (age 11)
Victoria, canada
A:
An object floats in a liquid if the object weighs less than the liquid it displaces. So the denser the liquid (the more weight for a given volume) the more objects will float in it. Adding table salt (NaCl) to water increases the weight of the water and also increases the volume. However, the volume increase is by a smaller fraction than the weight increase. So the density (weight/volume) increases.

The effect will be different for each different type of salt.  There is at least one type (MgSO4) which causes the volume itself to decrease, so it should have a particularly big effect. Also, there may be some which cause the density to decrease, which would lower buoyancy, but I don't know of any like that.

Mike W.

(published on 09/29/2010)

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