How do we turn oxygen into carbon dioxide?

The process of respiration. As long as you know about digestion and transportation in human beings, you'll know how glucose gets to human body parts. If you do, skip to the third paragraph, otherwise read on. read more

Next, blood carrying oxygen gets to the body needing energy, where the glucose is burnt in the presence of oxygen (dissolved) to provide energy in the form of heat, decomposing glucose into water and carbon dioxide. read more

Thus two oxygen atoms and 1 carbon atom form a double bond between them so more energy is required to break this bond. In this way $\ce{CO2}$ is formed. For converting $\ce{CO2}$ back into carbon and oxygen atoms, $\ce{CO2}$ should be heated at almost 298K so C becomes gaseous and oxygen becomes part of air. read more

The carbon dioxide in Earth's early atmosphere could have been turned into oxygen by intense UV light as well as by photosynthesis. Credit: NASA Earth's atmosphere wasn't always full of life-giving oxygen — it was once a choking mixture of carbon dioxide and other gases, more like the atmosphere of Mars or Venus. read more

You don't just turn oxygen into carbon dioxide (not without nuclear transmutation anyway). You combine oxygen with carbon (or a compound that contains carbon) in a combustion process and get carbon dioxide. read more

It allows plants, bacteria, and algae to take carbon dioxide and, with the help of a little sunlight, turn it into the oxygen we all breathe. But now scientists have taken photosynthesis out of the equation and have managed to make oxygen (O2) by zapping carbon dioxide (CO2) with a laser. read more