What did the second tank of the chemical wagon contain to create carbon dioxide gas?

The correct answer is that the second tank of the chemical wagon contained soda and water, which can create carbon dioxide gas through a chemical reaction. When baking soda (sodium bicarbonate) is mixed with water, it can produce carbon dioxide gas, especially when combined with a mild acid, which is often part of the chemical firefighting process. The creation of carbon dioxide is significant in fire protection, as it helps to displace oxygen in the vicinity of the fire, thereby suffocating it.

In contrast, other options do not lead to the production of carbon dioxide gas in the same manner. Water on its own does not create carbon dioxide without the addition of a reactant. Salt and vinegar create a different kind of reaction and primarily produce acetic acid and sodium acetate, rather than carbon dioxide gas. Alcohol does not react in a way that generates carbon dioxide when mixed solely with water. Therefore, the combination of soda and water is the most effective and scientifically relevant choice for generating carbon dioxide in this context.