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This is an really good and original idea for a science project. For a third grader, you want to emphasize how fun it is to do a science project and discover something new.
This is a basic study on density, and here is an explanation of the science of the volume of freezing water. http://www.elmhurst.edu/~chm/vchembook/ ... tyice.html
Also, the Wikipedia article contains good information about the chemistry of water and includes lots of useful information. Encourage your child to become an expert on water chemistry.http://en.wikipedia.org/wiki/Water
I would recommend reading this information and explaining it to your child to see if your child can understand that ice has a lower density compared to water due to the arrangements of the water molecules. You can add some ice cubes to water and see if your child can explain why the ice floats on the water.
After you have reviewed all of the information together, your child will know that water always expands as it freezes, so might predict that any rigid container completely full of water might break when the water freezes. If you have a variety of small containers with tight lids you can test this and see what happens. Do avoid using any glass containers because they will break.
One problem with this experiment is that there is nothing that is measurable. You could just take before and after pictures (or make drawings by hand) of the containers to show the results. In my opinion, this is perfectly acceptable for a 3rd grade project, however I have noticed that some 3rd grade teachers are now requiring projects with results that can be measured. If this is the case with your child’s project, then use different sized containers of the same basic shape and use a ruler to measure the increase in the height of the ice cube compared to the water. This should give you a number that can be graphed. The volume of the ice cube should be proportional to the volume of water.
Here is the information from this website on the display board for a science project. Be sure and include all of the sections.http://www.sciencebuddies.org/science-f ... oard.shtml
Hopefully this will help you survive this year’s science project assignment. Please post again in this topic if you have any more questions.