What observable phenomenon occurs when hydrogen peroxide comes into contact with fresh blood?

When hydrogen peroxide comes into contact with fresh blood, it reacts with an enzyme called catalase, which is present in red blood cells. This reaction results in the decomposition of hydrogen peroxide into water and oxygen gas. The release of oxygen gas leads to the formation of bubbles or foam, which is the observable phenomenon that occurs.

This foaming is not just a visual effect; it is also indicative of the chemical interaction taking place. The bubbles are a direct result of the rapid breaking down of hydrogen peroxide, which is why this answer is the most accurate. In forensic and cleanup contexts, this reaction can be useful for identifying blood at crime scenes, as it provides a visible clue when dealing with potential biological materials.