Carbonic Acid

We know that pH is a measure of H+ ions (for acids) 

or OH- ions (for bases).

So how does adding CO2 to a water-based liquid, manage to change its pH?

What is carbonic acid, and what does it do?

1. Carbonic Acid: How does it work?
2. Bozeman Science: Ocean Acidification video
3. Wikipedia: Carbonic Acid (way too much detail, but a good overview)

What happens when acids react with calcium carbonates?

1. Naked Scientist Website