By fifth grade, most children have developed their critical thinking skills to the point that they are ready to tackle scientific inquiry without too much guidance. They are able to make connections between events and draw conclusions. They are not yet well equipped to handle advanced math topics, however, so their projects should revolve around a qualitative test, rather than trying to handle complex numerical data. This limitation does not keep students from exploring more advanced areas like biology, chemistry, or physics, it just keeps it at a level where they can actually understand what they are doing. (A judge who sees advanced math will realize quite quickly that the student was not working on his or her own.)
One great idea for a science project is to evaluate the claims made in commercials on TV. The average student sees hundreds or even thousands of commercials weekly, so there are plenty of ideas. For example, my sister one year decided to see if Zest (the bath bar) really did clean better than other soaps. To do this, she collected several brands of soap to compare, prepared a few small bowls of tomato soup (as a culture medium) with covers, and made a well mixed filthy germy concoction with dirt and spit and anything gross that came to a kids mind. The procedure was simple, she would dip a finger into the glop, wash for a specified period with one brand of soap, and then dunk the finger into a bowl of soup, which would then be sealed. Once all bowls were so innoculated, she just had to wait to see which grew the least to determine the winners. (Zest performed very well, incidentally.) Judges loved this project because it clearly had real-world significance, and it was obviously something she was able to do on her own. Her project made it to the state-level.
Another great way to initiate a science project is to listen to your child’s own musings. A friend’s daughter once wondered why it was dangerous to hold electronics in the bathtub, and while saying “you can be electrocuted” is simple, it really did nothing to explain that electricity can travel through water which has salts dissolved in it, including the water in our bodies. “It can hurt you” is rarely an adequate answer for a child, especially a near-teen, so she remained curious. We made a science project out of it. We built a simple conductivity tester, using a 12-V battery, some alligator clips, a large blue LED, and a pair of nalis mounted on a wood handle as our electrodes/probe. She then tested various household items to see if they conducted electricity well enough to light the LED, and if they did, how brightly (since a dim light would only mean slightly conductive). She was able to see that tap water does conduct, and distilled does not, and learned that tap water has stuff dissolved in it. She also found that salty items with water in them conduct well, like cheese and yes, even herself. Having tested dozens of items, she was able to make some general conclusions, and predict correctly whether other familiar items would conduct or not. Sadly, this project didn’t win, the prize going instead to an environmental one that the judges failed to recognize as coming straight from a book. Still, the project got an ‘A’, which is an honorable achievement too.
I don’t recommend reproducing either of these experiments – the whole point of a science fair is to creatively explore things to which the students don’t already know the answer. There are always bonus points to be had if the project addresses real-world issues like pollution, alternative energy, recycling, bio-degradability, natural vs synthetic food stuffs, health and aging, etc. I would suggest that at the fifth grade level your goal should not be to win the state science fair. While a victory is nice, the stress involved can turn a child off. More important at this age is to cultivate an interest in science, which is best done by focusing on those projects which interest the child. Save the super-competitive projects for high school, when the prizes include scholarships and open doors to college – which the student will actually have begun to think about by then.