Coffee and Eggshells: Exploring the Browning Reaction

If an eggshell is soaked in coffee, it will become brown. This is because the eggshell is made of calcium carbonate, which is a white mineral. However, when the eggshell is soaked in coffee, the coffee stains the eggshell brown. The stain is caused by the pigments in the coffee, such as tannins and melanoidins. These pigments bind to the calcium carbonate in the eggshell, causing it to turn brown.