Carbon dioxide (CO2) is the name of the linear molecule O=C=O. At normal temperature and pressure (NTP) it is a colorless, almost odorless gas, which is 50% heavier than air. The gas is formed by the combustion of carbon-containing fuels, such as coal, wood, and petroleum. Carbon dioxide itself neither burns nor supports combustion. The gas is very soluble in water (1.45 g/liter at NTP). The solution is weakly acidic—carbonic acid is formed—and therefore CO2 is sometimes called carbonic acid gas. It is the gas that gives the sparkle to many soft drinks, some wines, and beer. The carbon dioxide gas freezes at −78.5 °C (−109.3 °F) and the frozen form is known as dry ice. Lately CO2 has become notorious as a greenhouse gas.