Oxygen gas is colourless and odourless and makes up 20.8 % of the atmosphere. Solid oxygen forms at a normal atmospheric pressure at -218.79 degrees Celsius. Both solid and liquid oxygen is clear with a light sky-blue colour caused by the absorption of the red part in the light spectrum. At very high pressure, solid oxygen can change into an insulating metallic state; at very low temperatures, it may transform into a superconducting state. Oxygen in liquid state is a cryogenic liquid which means that they are liquified gases with a normal boiling point below -90 degrees Celsius.