Carbon reacts with water, air, acids and halogens. Each of these things, when combined with carbon, leads to a different reaction.
The most well-known reaction of carbon, to anything, is a diamond. Diamonds are the result of carbon reacting with air, pressure and heat, over long periods of time.
When the air surrounding carbon is restricted during combustion, carbon monoxide is formed. When carbon reacts with water under pressure, water gas is formed. Water gas is a mixture of carbon monoxide, carbon dioxide, nitrogen and other elements.
Carbon reacts with halogen at high temperatures. Together, the two form carbon tetrafluoride. Graphite, a form of carbon, also reacts with certain acids, such as nitric acid, to form mellitic acid.