Carbon Dioxide, CO2

Carbon dioxide (CO2) normally exists in a gaseous state as a linear molecule, and is a colourless, odourless gas which is denser than air. Carbon dioxide is one of the normal products of combustion of carbon or a carbon containing compound when it is burned in a plentiful supply of oxygen or air.
Carbon dioxide is a product of respiration in animals - and combines with haemoglobin in the red blood cells and is carried to the lungs where it is "breathed" out and exchanged for oxygen. Green vegetation uses chlorophyll to combine carbon dioxide with hydrogen (from water) to form carbohydrates in a process known as photosynthesis - see below:

Glucose and other simple carbohydrates can be converted to more complicated carbohydrates such as starch and cellulose.
Carbon dioxide is the fourth most abundant gas in the atmosphere, but on the planets Venus and Mars it is the most abundant. The gas traps infrared radiation emitted from the warm earth surface. Carbon dioxide is transparent to sunlight, the light warms the surface, as infrared radiation is emmited back, it is trapped by the molecules and the atmosphere is warmed in a process known as the greenhouse effect.

In the laboratory, the test for carbon dioxide is to bubble it through dilute calcium hydroxide solution (limewater). This results in the formation of insoluble calcium carbonate which makes the solution turn cloudy.