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An element is the simplest form of matter. It consists of only one type of atom and cannot naturally be broken down into simpler substances. For example, oxygen, hydrogen, and gold are all elements. Each of these retains its own distinct properties and is a building block for other substances.

A compound, on the other hand, is formed through the chemical combination of two or more different elements. This means compounds have more complex structures and properties compared to their individual component elements. Water (H extsubscript{2}O) and carbon dioxide (CO extsubscript{2}) are examples of compounds. They consist of specific ratios of individual elements bonded together.

When a compound forms, the properties of its constituent elements change. A classic example is sodium chloride (NaCl). Sodium is a reactive metal, and chlorine is a toxic gas. Yet, combined, they form table salt, a substance crucial to life. Unlike elements, compounds can be broken down into simpler substances, typically through chemical reactions such as heating or electrolysis.

Chemical reactions are processes where substances interact to form new compounds. During a chemical reaction, the atoms in substances are rearranged, leading to a change in their physical and chemical properties. The substances we start with are called reactants, and the new substances formed are called products.

In the context of electrolysis, a form of chemical reaction, an electric current is used to drive a non-spontaneous reaction. This process involves splitting a compound into its elements or simpler compounds. Electrolysis is widely used in industries, for example, to obtain pure elements from their ores or separate water into hydrogen and oxygen gases.

A notable aspect of chemical reactions is the conservation of mass, meaning the total mass of reactants equals the mass of the products. For instance, consider the breakdown of water by electrolysis: \[2H_2O \rightarrow 2H_2 + O_2\]This equation shows how water molecules decompose into hydrogen and oxygen gases, maintaining the balance of atoms on each side of the equation.

A pure substance has a constant composition and distinct chemical properties. It can be an element, like gold, or a compound, like salt. Pure substances are homogenous, meaning they are uniform throughout and have fixed boiling and melting points.

Substances like elements do not change in composition. They are notable for being unable to be broken into simpler substances by chemical means. However, compounds, while also pure, have a chemical structure that allows them to be divided into simpler elements or other compounds through processes like electrolysis.

Take, for example, substance X from our exercise. It is initially a pure substance, but upon undergoing electrolysis, it breaks down into two different solids, indicating it is a compound. This decomposition is a tell-tale characteristic of compounds as pure substances that can transform under specific conditions.