why do pools need chemicals
Pure water usually has a pH around 7. Lemon juice is acid: dissolved baking soda is alkaline. (Properly speaking, dry materials don't have a pH.) Balance chemicals help you to avoid damage to your pool, either in the form of corrosion or scale deposits. Actually, it's a bit more specific than that: balance chemicals allow you avoid problems that result from having too much or too little limestone in the water (calcium carbonate).