Many cleaning products, such as soap and oven cleaner, are bases. Bases neutralize (cancel out) acids. Alkalis are bases that dissolve in water. Strong bases, such bleach, are corrosive and burn skin. Bases contain particles called hydroxide ions. The more hydroxide ions a base contains, the stronger it is.
Limestone is an important base that is dug from the earth in quarries. It comes from calcium carbonate, which formed millions of years ago from the compressed remains of sea-shells and other marine life. Once quarried, limestone is crushed and used to make cement, fertilizers, paints, and ceramics.