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A chemical reaction is a process where substances, known as reactants, are transformed into different substances, known as products. These reactions involve the breaking and forming of chemical bonds, which alter the arrangement of atoms. In our example, the chemical reaction of interest is the decomposition of mercuric oxide. When mercuric oxide is heated, it breaks down into mercury and oxygen. This is a type of chemical reaction known as decomposition, where a single compound breaks down into two or more simpler substances. This transformation is typical in chemical reactions, allowing elements to rearrange into new compounds, often accompanied by observable changes such as emission of gas, temperature change, or color change. Chemical reactions follow certain conservation laws, one of which is the Law of Conservation of Mass, ensuring that the mass of reactants equals the mass of products.

The decomposition of mercuric oxide provides a textbook example of how chemical reactions adhere to conservation laws. Mercuric oxide (\( HgO \)) is a compound composed of mercury and oxygen. Upon heating, it decomposes into two distinct elements: mercury (\( Hg \)) and oxygen (\( O_2 \)). This decomposition reaction can be represented by the equation:\[2 \text{ HgO} \rightarrow 2 \text{ Hg} + \text{ O}_2\] When mercuric oxide is heated, the chemical bonds between mercury and oxygen break, freeing the mercury atoms and releasing oxygen molecules as a gas. In the example given, 2.16 grams of mercuric oxide yield 2.00 grams of mercury and 0.16 grams of oxygen. This measurement demonstrates the practical application of the Law of Conservation of Mass since the total mass of the reactants equals the total mass of the products.

Every chemical reaction involves elements or compounds interacting and transforming into different chemical substances. It is essential to recognize the elements and compounds undergoing change during a reaction. In the decomposition of mercuric oxide, the elements involved are:

• Mercury (Hg): A metallic element that occurs naturally and is commonly found in the compound mercuric oxide.
• Oxygen (O): A highly reactive non-metal essential for the oxidation processes, released as a diatomic gas (\( O_2 \)) in this reaction.

Understanding these elements and how they rearrange provides insight into the reaction itself. By identifying each element's role, we can better understand the process that follows. In this instance, the decomposition reaction results in mercury being freed as a liquid metal, while oxygen forms into oxygen gas, emphasizing the transformation nature of chemical reactions.