* Impurities: Even though distillation removes most impurities, it's impossible to completely eliminate *all* traces of other substances. There might be tiny amounts of dissolved gases like oxygen and carbon dioxide, or even trace minerals that escaped the process.
* Equilibrium: Distilled water will inevitably absorb gases from the air it's exposed to. This means it will always contain some dissolved gases in equilibrium with the atmosphere.
* Containers: The container holding distilled water can also contribute to the presence of other substances. For example, plastic containers might leach tiny amounts of chemicals into the water.
Therefore, distilled water is not strictly "pure" in the absolute sense, but it's considered the closest thing to pure water we can practically achieve.
Let me know if you have more questions!
