Fluorine is an almost colourless gas. In dense concentrations and in the liquid state it is seen to have a pale greenish yellow colour. It has a characteristic odour, similar to a mixture of ozone and chlorine. As the most reactive of all the elements and the most power ful known oxidizing agent, fluorine is able to react with almost all elements and compounds,with the exception of lighter, noble gases and fluorides of the highest valency. Fluorine reacts with many substances even at room temperatures, sometimes explosively, and often accompanied by combusition.