A hydrochloric acid solution was added to the sodium carbonate solution. What connections will be obtained.

Na2CO3 + 2HCl = 2NaCl + H2CO3.
The reaction produces sodium chloride and carbonic acid. But carbonic acid is unstable and it breaks down into carbon dioxide and water:
Na2CO3 + 2HCl = 2NaCl + H2O + CO2.
Why is carbonic acid unstable? Because the carbonic acid solution is easy to compose and easily decomposes into the original components.
Temperatures that make the acid stable (strong) under normal conditions are not typical. Hence, we conclude that carbonic acid is a weak acid.

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