The ph decreases. When you blow out, you exhale carbon dioxide gas, which is soluble in water. The dissolved CO2 (known as carbonic acid) is acidic, and therefore the pH will fall, but not significantly. You will never be able to measurably determine that there is a difference....at least not with pH testing strips. However, if you put a very small amount (just a few grains) of sodium carbonate in about 100 mL of water and add a drop of phenolthalein, the solution will turn pink, indicating that it is a base (a very weak one, about pH 8) Then, if you blow bubbles into the water with a straw for about 2 minutes, the solution will go from pink to clear, indicating that it is no longer a base. Your breath has neutralized the base.
