The liquid used to test for carbon dioxide is limewater, also known as calcium hydroxide solution (Ca(OH)₂).

Here's how it works:

* Reaction: When carbon dioxide (CO₂) is bubbled through limewater, it reacts with the calcium hydroxide to form calcium carbonate (CaCO₃).

* Observation: Calcium carbonate is a white, insoluble solid. This causes the limewater to turn cloudy or milky.

* Conclusion: The appearance of cloudiness in the limewater confirms the presence of carbon dioxide.