Carbon dioxide (CO) is a chemical compound and is acidic. It is a gas at room temperature. It is made of one carbon and two oxygen atoms. People and animals release carbon dioxide when they breathe out. Also, every time something organic is burnt (or a fire is made), it makes carbon dioxide. Plants use carbon dioxide to make food. This process is called photosynthesis. The properties of carbon dioxide were studied by the Scottish scientist Joseph Black in the 1750s.