Which of the following is NOT a dissolved substance typically found in water?

The correct choice is metallic lead, as it is not a dissolved substance typically found in water. Unlike the other options listed, which are elements and compounds found in gaseous form in water, metallic lead is a solid metal that does not dissolve in water under normal conditions.

Hydrogen, oxygen, and carbon dioxide are all components of water in various forms; hydrogen and oxygen are the building blocks of water itself (H2O), while carbon dioxide can dissolve in water to form carbonic acid, particularly in natural bodies of water. These substances are crucial for various biochemical processes and contribute to the chemical characteristics and quality of water.

In contrast, metallic lead, while it can be found in water sources due to pollution or contamination, does not exist as a dissolved entity like the other substances. In its metallic form, lead can form particles or sediments rather than dissolving. This distinction is important in understanding water quality and the impact of heavy metals on aquatic life and human health.