Carbon can exist in various forms owing to the property of its atoms to combine in different ways.
Carbon is said to exist in a combined state when it is bonded with some other element other than carbon such as oxygen, and calcium in the form of carbonates.
Combined states of carbon are in the form of organic and inorganic compounds.
Organic compounds include; Food nutrients (like proteins, fats and vitamins), Carbohydrates(like glucose, starch and sugar), Hydrocarbons(like methane, ethane), Acids and alcohols, etc.
Inorganic compounds include; Carbon dioxide, Carbon monoxide and salts like (limestone), (washing soda) etc.