Lemons and other citrus fruits taste sour because they contain citric acid. Citric acid is used to add a tangy taste to food and soft drinks. Citrus fruits also contain another acid, called ascorbic acid or Vitamin C, which we need for healthy skin and gums.

The pock-marked appearance of some statues is caused by acids in rainwater attacking the stone. Rainwater is always slightly acidic because carbon dioxide in the air dissolves in water to form carbonic acid. In addition, industrial areas give off pollutants, such as sulphur dioxide. These pollutants react with water in clouds to form strong acids that react with stone, especially limestone.

This frilled nudibranch sea slug oozes a strong acid called sulphuric acid to protect itself. The sulphuric acid makes the nudibranch poisonous and taste awful, so it does not have many predators. Ants and stinging nettles contain an acid called methanoic acid, which they use to protect themselves.

Strong acids and bases are extremely poisonous, corrosive, and cause bad burns, so their containers are labelled with hazard symbols. Some give information about how to handle the chemicals safely. The symbols are also displayed on the tankers that transport acids and bases, so emergency services know how to handle the substances in the case of an accident or spillage.