Question

A pure substance can only be (1) a compound (2) an element (3) an element or a compound (4) a heterogeneous mixture

Short answer

An element or a compound

Step-by-step solution

Understand the terms

First, it's important to understand the key terms in the question: a pure substance, a compound, an element, and a heterogeneous mixture.

Define a pure substance

A pure substance has a uniform and definite composition. This means the substance is made up of only one type of particle.

Define a compound

A compound is a substance formed when two or more chemical elements are chemically bonded together. Examples include water (H2O) and carbon dioxide (CO2).

Define an element

An element is a substance made of only one kind of atom. Examples include oxygen (O2) and hydrogen (H2).

Define a heterogeneous mixture

A heterogeneous mixture is a combination of two or more substances where the components can be visibly distinguished and the composition is not uniform throughout. Examples include salad and sand.

Eliminate incorrect options

Since a pure substance must have a uniform and definite composition, it cannot be a heterogeneous mixture. Additionally, pure substances include both elements and compounds.

Choose the correct option

Based on the definitions and elimination, a pure substance can either be an element or a compound.

Key concepts

elements

An element is a fundamental substance that consists of only one type of atom. Elements are the building blocks of all matter and cannot be broken down into simpler substances. Each element is unique and has its own set of properties, which are listed on the periodic table. For example, oxygen (O) is an element that is essential for respiration, and gold (Au) is an element known for its luster and malleability. Elements have a uniform and definite composition.

compounds

A compound is a substance formed when two or more elements are chemically bonded together. These bonds can be covalent or ionic. Compounds have properties that are distinct from the individual elements they are composed of. For instance, water (H2O) is a compound made from hydrogen and oxygen, yet it has properties different from either of these gases. When elements combine to form a compound, they do so in fixed ratios, resulting in a substance with a uniform composition. Therefore, compounds are also considered pure substances.

heterogeneous mixtures

A heterogeneous mixture is a combination of two or more substances where the components can be visibly distinguished. This means that the composition of the mixture is not uniform throughout. Examples of heterogeneous mixtures include a salad, where you can see the individual ingredients, and sand mixed with iron filings, where the different particles remain separate. Unlike pure substances, heterogeneous mixtures do not have a uniform and definite composition. Each component within the mixture retains its own properties and can usually be separated by physical means.

chemical composition

Chemical composition refers to the types and amounts of elements that make up a substance. Understanding the chemical composition is crucial in identifying whether a substance is an element, compound, or mixture. In pure substances, such as elements and compounds, the chemical composition is uniform and consistent. For example, a molecule of carbon dioxide (CO2) always contains one atom of carbon and two atoms of oxygen. This precise arrangement leads to a specific and predictable set of properties. In contrast, the chemical composition of mixtures, especially heterogeneous ones, is variable. Different parts of the mixture may have different compositions and properties. Knowing the chemical composition helps chemists to understand, predict, and manipulate the behavior of substances in reactions and various applications.