While oxygen in the form of gas is colorless, what color is liquid oxygen?
And the answer: blue.
Slightly denser than ice, liquid oxygen has a pale blue color. As a cryogenic liquid, it's extremely cold and will start boiling at any temperature above -147 degrees Celsius.
While oxygen in its gaseous form comprises most all of the life around us in one way or another, liquid oxygen serves unique purposes for scientists and experimentation. Interestingly, though, the two are less separate than you may think: oxygen is often liquified so that it can be more effectively transported and stored in large volumes. The oxygen used by hospitals or other medical practices is frequently stored in a liquid state before being converted back to a gas. Ultimately, liquid storage is less bulky and less costly than the equivalent capacity of high-pressure gaseous storage, and so much of the oxygen stored in medical facilities is in a liquid form.
Liquid oxygen serves several primary purposes. Most significantly, it is used as an oxidant for liquid fuels in the propellant systems of missiles and rockets. Otherwise, it is widely applied in the metal industries in conjunction with other fuel gases for metal cutting, welding, hardening, cleaning, and melting. In medical practices, patients can even receive liquid oxygen therapy for diseases of the lung or other pulmonary systems. The additional oxygen intake can help patients' lungs cope and make up for the loss of breathable O2.
See it in action below.