Science fair project: use baking soda solution and purple cabbage indicator to compare candy acidity. Purple cabbage indicator (made by boiling, or soaking, purple cabbage in water) changes color based on pH. If you add a base, it turns blue, and if you add acid, it turns pink. Here's the original color.
First, we dissolved various candies in indicator. The brightest pink are the most acidic.
Then we added baking soda water to each sample to bring it to neutral, and compared how much soda we used for each sample to arrive at an acidity comparison.
One fun little thing: to dissolve the maximum amount of soda in water, we warmed the water up. As it cooled, the soda started to crystallize again. Apparently, our initial solution was a little supersaturated.