Aluminum

Aluminum is an unusual metal in that it reacts not only with acids, but with bases as well. Like many active metals, aluminum dissolves in strong acids to evolve hydrogen gas and form salts. In fact, cooking even weakly acidic foods such as tomatoes in an aluminum pot can dissolve enough aluminum to give the dish a "metallic" taste. But aluminum also dissolves in strong bases such as sodium hydroxide, commonly known as lye. Most oven cleaners, which are designed to work on steel and porcelain, contain sodium or potassium hydroxide; the user must take care not to get it on any aluminum parts of the range because it will cause adverse effects. Some commercial drain cleaners contain lye mixed with shavings of aluminum metal; the aluminum dissolves in the sodium hydroxide solution to produce bubbles of hydrogen gas, which add a mechanical clog-breaking action to the grease-dissolving action of the lye.

Hydrated aluminum chloride, AlCl₃•H₂O, also called aluminum chlorohydrate, is used in antiperspirants because, like alum (potassium aluminum sulfate), it has an astringent effect—a tissue-shrinking effect—that closes up the sweat-gland ducts and stops perspiration.

Over one million tons of aluminum sulfate, Al₂(SO₄)₃, are produced in the United States each year by dissolving aluminum oxide in sulfuric acid, H₂SO₄. It is used in water purification because when it reacts with lime (or any base), it forms a sticky precipitate of aluminum hydroxide that sweeps out tiny particles of impurities. Sodium aluminum sulfate, NaAl(SO₄)₂•12H₂O, a kind of alum, is used in "double-acting" baking powders. It acts as an acid, reacting at oven temperatures with the sodium bicarbonate in the powder to form bubbles of carbon dioxide gas.