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“You get the great increase in mass because the weight of the oxygen attached to the carbon is more than 10 times the weight of the hydrogen whose place it’s taking.”

Increase in ‘mass’ or density???

Where does the oxygen come from?

What happens to the C02 that goes into our atmosphere? Does it stay C02?

It's actually an increase in mass. Density is mass/volume. As a gas CO₂ has a low density, but the atmosphere is pretty big and even with a low density, there's plenty of mass. The O₂ comes from the atmosphere. Some of the CO₂ in the atmosphere eventually is picked up by water to form H₂CO₃, a.k.a. carbonic acid. That's carried to the surface as the famous acid rain. This can take a while, and having the pH of ponds and lakes and seas and oceans drop is not a great thing for numerous reasons. Some of the CO₂ in the atmosphere is picked up by green plants and incorporated into various organic molecules, liberating O₂ into the atmosphere. All free oxygen in the atmosphere is created by plants. If plants and certain kinds of bacteria did not exist, there would be very little free oxygen in the atmosphere; it would all combine with other chemicals. Mars is a good example; there's a huge amount of oxygen tied up in the red rust that gives the planet it's color.