That chalky ring around the tub isn't (necessarily) a sign that the last person who took a bath was particularly dirty. Even in the most hygienic households, soap scum can strike. Soap scum is the residue that results from body oils and the fats in soap reacting with the mineral salts in water. Bathtubs, showers and sinks are prone to soap scum. Again, hard water aggravates the problem.
Wipe down tubs and sinks after using them to prevent soap scum from forming. If soap scum does show up, sponge it off with a paste of baking soda and dishwashing detergent. Unlike soaps, detergents don't react with salts, so they don't contribute to the build up [source: Ophardt].
To treat stubborn cases, add 1/4 cup (60 milliliters) baking soda, 1/2 cup (120 milliliters) vinegar and 1 cup (235 milliliters) ammonia to 1 gallon (3.8 liters) warm water. Douse the area and rinse it well. Wear rubber gloves and make sure the room is well ventilated when mixing and using this solution: Ammonia is a caustic. It burns tissue on contact and the fumes can damage your lungs.
But porcelain isn't the only target for baking soda. Our next tip makes the point crystal clear.