Water Content of Food -  Weigh an apple on a scale. Cut the apple into small pieces. Put the pieces of apple on a paper towel on a tray. Place the tray in sunlight on a windowsill. Leave it there for several days. Weigh the dried-out pieces of apple. Calculate the weight of water lost by the apple.
Now do the same with a potato. Did the potato have more water content than the apple? You can also do this with grapes, pears, etc.

Acid Content of Food -  By using purple "cabbage water" as a test indicator, you can test different foods to find out if they contain acid. Red cabbage can be used to make a solution that is used to test for acid. It's very easy to make red cabbage indicator. First, chop (or grate) a red cabbage into thin slices. Place the chopped cabbage in a bowl and add two cups of boiling water. Stir the mixture thoroughly and let it stand for 30 minutes. Pour off the purple cabbage water and use it as your acid indicator. By placing a spoonful of indicator in a clean cup and then adding a few pieces of the food you want to test, you can watch to see if the indicator changes color. If the indicator turns red, the food contains an acid. (Note: if the indicator turns blue/green, the food contains a base.) Some food won't change the indicator a different color. If this happens, it is "neutral". Two foods you can test: vinegar and baking soda.

Starch Content of Food -  Get samples of foods such as potato, tomato, carrot, apple, banana, bread, sugar, etc. Prepare some dilute iodine solution: add 2 medicine droppers full of tincture of iodine to half a glass of cold water. Put a small piece of each food to be tested into a porcelain or glass dish (as this will stain plastic). With a medicine dropper, add a few drops of the iodine solution to the food. Which foods contain starch? How do you know?

Fat Content of Food - You will need a piece of brown paper, a dropper, some fat (such as butter), and some water. Put a drop of water on a piece of brown paper. Next to it put a piece of butter the size of a drop. After a few minutes, what happens to each? Does the water drop disappear? Does the butter drop remain shiny?
You can test some foods to see if they contain fats. If they leave a shiny spot on brown paper, the foods contain lots of fat or oil.