Final answer:
Mercury(II) oxide decomposes into liquid mercury and oxygen gas when heated, characterized by a color change from red to silver (option 1).
Step-by-step explanation:
When Mercury(II) oxide is heated, it breaks down into two elements: liquid mercury and oxygen gas. This chemical reaction is an example of a decomposition reaction, a type of reaction in which a compound is broken down into simpler substances.
The process can be represented by the chemical equation:
2HgO (s) → 2 Hg (l) + O₂ (g), where 's' denotes a solid, 'l' denotes a liquid, and 'g' denotes a gas. When Mercury(II) oxide, which is a red solid, is heated to temperatures above 500°C, it decomposes into silver-colored liquid mercury and invisible oxygen gas. Observing the color change from red to silver can indicate that the reaction has occurred.
Hence, the answer is option 1.