In basic chemistry classification, a pure substance can be which of the following?

Introduction / Context:
This question checks your understanding of fundamental definitions in chemistry, especially the distinction between pure substances and mixtures. Classifying matter correctly is the starting point for more advanced topics like solutions, chemical reactions and stoichiometry. A pure substance has a fixed composition and distinct properties, and this question asks you to identify which categories of matter qualify as pure substances.

Given Data / Assumptions:

• You are given four options describing different types of matter.
• Option A suggests that only compounds are pure substances.
• Option B suggests that only elements are pure substances.
• Option C suggests that both elements and compounds can be pure substances.
• Option D suggests that a heterogeneous mixture can be a pure substance.
• Standard school level definitions of pure substances and mixtures apply.

Concept / Approach:
A pure substance is a form of matter that has a uniform and definite composition and consistent properties throughout the sample. It cannot be separated into other kinds of matter by simple physical processes. Elements are pure substances made up of only one kind of atom, such as oxygen gas or copper metal. Compounds are also pure substances, but they contain two or more elements chemically combined in fixed proportions, such as water or sodium chloride. Mixtures, whether homogeneous or heterogeneous, contain more than one substance and their compositions can vary. Therefore, both elements and compounds are pure, while mixtures are not.

Step-by-Step Solution:
Step 1: Recall that an element is defined as a pure substance that cannot be broken down into simpler substances by ordinary chemical means. Step 2: A compound is defined as a pure substance composed of two or more elements chemically combined in a fixed ratio. Step 3: Because both elements and compounds have fixed compositions and uniform properties, they are both classified as pure substances. Step 4: Heterogeneous mixtures have non uniform composition and can be separated by physical methods, so they are not pure substances. Step 5: Option A is too restrictive because it excludes elements. Step 6: Option B is also too restrictive because it excludes compounds. Step 7: Therefore, the correct description is that a pure substance can be an element or a compound.

Verification / Alternative check:
Think of examples commonly used in textbooks. Pure copper is an element and is certainly considered a pure substance. Distilled water is a compound made only of H2O molecules and is also a pure substance. In contrast, sea water contains dissolved salts and gases in varying amounts, so its composition is not fixed and it is a mixture. Similarly, soil, air and alloys like brass are mixtures. This everyday reasoning backs up the classification that both elements and compounds belong to the category of pure substances, while mixtures do not.

Why Other Options Are Wrong:
Option A claims that only compounds are pure substances, which wrongly excludes elements like hydrogen, oxygen and gold that clearly meet the definition of a pure substance. Option B claims that only elements are pure, ignoring compounds like water that also have fixed composition and homogeneous properties. Option D suggests that a heterogeneous mixture is a pure substance, which contradicts the very definition of heterogeneity, where different parts of the sample have different compositions or properties. Therefore, these options are inconsistent with standard chemical classification.

Common Pitfalls:
Students sometimes confuse the term pure in everyday language with its scientific meaning. In daily life, people may call tap water pure even though it contains dissolved minerals and gases, so scientifically it is a mixture. Another pitfall is to think that any substance containing more than one element cannot be pure, but compounds like sodium chloride have fixed ratios and thus are pure substances. Remember that purity in chemistry is linked to uniform composition and not to the number of elements present in the substance.

Final Answer:
A pure substance can be an element or a compound.