Which of the following samples represents a pure substance rather than a mixture, assuming each sample is chemically pure?

Introduction / Context:
In basic chemistry, it is important to distinguish between pure substances and mixtures. Pure substances have a uniform and definite composition, while mixtures contain two or more substances physically combined. Pure substances can be elements or compounds. This question asks you to evaluate several familiar materials and decide which ones qualify as pure substances, assuming that each sample is not contaminated and contains only the named compound.

Given Data / Assumptions:

• Sample A is table salt consisting only of sodium chloride, NaCl.
• Sample B is pure water consisting only of H2O molecules.
• Sample C is zinc oxide, ZnO, in a test tube with no other substances present.
• We assume that no impurities or additives are mixed with these substances.
• We use the standard definition of a pure substance as having a fixed, definite composition and uniform properties.

Concept / Approach:
A pure substance can be an element (such as oxygen gas or copper metal) or a compound (such as sodium chloride, water or zinc oxide). In each case, its composition is fixed and it has characteristic physical and chemical properties. A mixture, in contrast, contains more than one substance and can have variable composition. The approach is to check whether each sample is a single chemical compound or element. If all three are single compounds with fixed formulas, then each one is a pure substance, and the correct answer will state that all are pure subsances under the given assumption.

Step-by-Step Solution:
Step 1: Table salt, when chemically pure, is sodium chloride with the fixed formula NaCl, making it a compound and therefore a pure substance. Step 2: Pure water, H2O, has a constant composition with two hydrogen atoms and one oxygen atom in every molecule, so it is also a pure compound. Step 3: Zinc oxide, ZnO, is another compound with fixed stoichiometry, containing zinc and oxygen in a 1:1 ratio, so a test tube of only ZnO is a pure substance. Step 4: None of these samples involve more than one chemical substance mixed together if we assume they are chemically pure. Step 5: Therefore, all three samples represent pure substances rather than mixtures.

Verification / Alternative check:
If you look up these substances in a chemistry textbook or database, you will find that NaCl, H2O and ZnO each have well defined formulas and properties such as melting point, boiling point and density. Impure table salt used in cooking may contain additives, but the chemical substance sodium chloride is pure. Similarly, distilled water is pure H2O. Laboratory grade zinc oxide is a pure compound. In contrast, mixtures like sea water, air or alloys such as brass involve multiple substances where composition can vary. This confirms that each of the named substances, when pure, qualifies as a pure substance.

Why Other Options Are Wrong:
Option A suggests only table salt is a pure substance, ignoring that pure water and pure zinc oxide also meet the definition. Option B, if it existed alone, would wrongly limit purity to water only. Option C focuses only on zinc oxide, leaving out the others. Option E claims that none of the samples are pure substances, which contradicts the standard definitions. Only option D, stating that all of the above samples are pure substances, matches the correct classification when each sample contains just a single chemical compound with fixed composition.

Common Pitfalls:
Students sometimes confuse everyday impure samples with the underlying chemical substances. For example, tap water and cooking salt often contain additives and impurities, so they are mixtures in practice. However, at the chemical level, water and sodium chloride are pure substances when isolated. Another pitfall is to think that only elements are pure substances and that compounds must be mixtures, which is not correct. Remember that both elements and compounds can be pure substances; mixtures are combinations of these in variable proportions.

Final Answer:
Assuming each sample contains only the named compound, All of the above samples are pure substances, including sodium chloride, pure water and zinc oxide.