5th grade science experiments are fun because the children are able to work more independently and find answers out for themselves. They don't need as much adult assistance and are expected to shoulder much of the responsibility in thinking up their own topic, question and experiment to find the answer.
There are many possibilities for fifth grade science experiments, including seeing if the color of a light affects how bright it appears in fog or in water, finding out where the best place to store apples is, such as fridge, wicker basket or plastic bowl, or finding out if the temperature of a magnet affects it's magnetic field lines. To find out, the students will need a few magnets, one from the freezer, one kept at room temperature and one heated up. Have the students trace the magnetic field lines of each magnet by putting iron fillings on a sheet of paper over the magnet.
Another experiment students could try is to see if the starting temperature of water affects how long it takes to freeze. All you need to do is get three ice cube trays (or one and label the rows with the temperature of water you started with) and start with three different temperatures of water; hot, cold, and room temperature. When done, the tray is placed back in the freezer. The temperature of the water will need to be monitored to see which one freezes first, second, and third. Be sure to have the students record the results for all experiments!