Does carbonic acid, H2CO3, spontaneously decompose?
Absolutely pure carbonic acid does not spontaneously decompose.
In the absence of water, the half-life for the decomposition is 180 000 years.
But even a single molecule of water catalyzes the rate of decomposition by a factor of fifty billion times.
Since we always prepare carbonic acid in the presence of water, it decomposes immediately into water and carbon dioxide:
H₂CO₃ → H₂O + CO₂
Even in pressurized soft drinks, the concentration of carbon dioxide is about 600 times the concentration of carbonic acid.
H₂CO₃(aq) ⇌ H₂O(l) + CO₂(aq)
When you release the pressure, the CO₂ escapes and the carbonic acid decomposes even more.