What is the most common impurity found in tap water?

The most common impurity found in tap water is calcium carbonate. This compound is prevalent in many water sources due to its natural presence in rocks and minerals. When water percolates through soil or rock formations, it can dissolve calcium carbonate, leading to its incorporation into the drinking water supply.

Calcium carbonate can contribute to water hardness, which is a measure of the concentration of calcium and magnesium ions in water. While hard water is generally safe to drink, it may lead to scale buildup in plumbing and appliances.

In comparison, while zinc, lead, and mercury are also considered impurities, they are found in much lower concentrations in tap water. Lead might be found in older plumbing systems, but it's not as ubiquitous as calcium carbonate. Similarly, zinc and mercury are usually only present due to specific industrial discharges or environmental contamination and are typically monitored more closely due to their potential health risks.