by Goldenzenith » Sun Nov 11, 2012 3:38 pm
Think about what stays, well, constant for the duration of the experiment. For both saltwater and plain water, do you keep the volume and temperature the same? Is each piece of potato going into each cup of water the same in size and shape? Are you sure the water content of the pieces are completely uniform before you put them in the cups? Ask yourself what has to stay the same. Then ask yourself what varies; then you'll have your independent and dependent variables. It's simpler than you think!
As for your hypothesis, you'll have to explain why you think the saltwater will cause the potato to dehydrate, unlike the plain water. This is where your research comes in. If you have any questions, post them and I'll be happy to work with you further!
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http://www.sciencebuddies.org/science-fair-projects/project_ideas.shtml
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Enjoy!

-RM, Expert