What is formed when vent gases from a coal-burning furnace combine with high moisture in a chimney?

When vent gases from a coal-burning furnace combine with high moisture in a chimney, sulfur dioxide, a byproduct of burning sulfur-containing coal, can react with water vapor to form sulfuric acid. This reaction occurs when the sulfur dioxide is released into the atmosphere and comes into contact with moisture, resulting in a chemical process that leads to the production of acid rain.

In the context of a coal-burning furnace, sulfur-containing compounds are often present in the fuel. When combusted, these compounds generate sulfur dioxide gas. When this gas interacts with moisture in the chimney, it can lead to the formation of sulfuric acid, which is a significant environmental concern due to its corrosivity and impact on water quality.

This understanding is critical not only for regulating emissions from coal-fired power plants but also for managing the effects of acid rain on the environment. Therefore, sulfuric acid is the correct answer as it is the product of the reaction between vent gases, particularly sulfur dioxide, and high moisture levels in the chimney.