In the context of states of matter, which of the following samples represents a mixture rather than a pure element or compound?

Introduction / Context:
This question tests your understanding of the difference between pure substances and mixtures, and how chemists use physical state symbols such as (s), (l) and (aq) to describe them. Pure substances include both elements and compounds, whereas mixtures contain two or more substances physically combined. By carefully reading the state symbols attached to the formulas, you can decide which option represents a mixture of substances rather than a single pure chemical entity.

Given Data / Assumptions:

• KBr(s) represents solid potassium bromide.
• NaCl(l) represents molten sodium chloride, a liquid ionic compound at high temperature.
• K(s) represents solid potassium element.
• NaCl(aq) represents sodium chloride dissolved in water (an aqueous solution).
• NaCl(s) represents solid sodium chloride.
• We assume standard definitions: elements and compounds are pure substances; solutions are mixtures.

Concept / Approach:
A pure substance has a fixed composition and distinct properties, whether it is an element like potassium or a compound like sodium chloride. When an ionic compound such as NaCl is molten, it is still the same compound, only in the liquid state. However, when a substance is labelled as aqueous, NaCl(aq), that means it is dissolved in water. The solution contains both solute (NaCl) and solvent (water), so it is a mixture. Therefore, to determine which sample is a mixture, you should look for the presence of an aqueous solution rather than a single pure phase of one compound or element.

Step-by-Step Solution:
Step 1: Recognise that KBr(s) is solid potassium bromide, an ionic compound with a fixed formula; it is a pure substance. Step 2: Realise that NaCl(l) represents molten sodium chloride, which is still the same compound NaCl, only melted; it is also a pure substance. Step 3: Note that K(s) is solid potassium, a chemical element with a definite composition and thus a pure substance. Step 4: Understand that NaCl(aq) means sodium chloride dissolved in water, forming an aqueous solution, which contains at least two substances: NaCl and H2O. Step 5: Because NaCl(aq) is a combination of solute and solvent, it is a mixture. Step 6: Solid NaCl(s) is again the pure compound sodium chloride in solid form. Step 7: Therefore the only option that clearly represents a mixture is NaCl(aq).

Verification / Alternative check:
You can verify this classification by thinking about separation methods. Aqueous sodium chloride can be separated into salt and water by physical processes such as evaporation or distillation, which is typical of mixtures. In contrast, separating the elements from solid NaCl(s) or KBr(s) requires chemical reactions like electrolysis or redox processes, since they are compounds, not mixtures. Also, pure molten NaCl(l) has a sharp melting point at a specific temperature, a property of pure substances, whereas the boiling point or freezing range of a solution often changes with concentration. These observations confirm NaCl(aq) as the mixture.

Why Other Options Are Wrong:
Solid potassium bromide, KBr(s), is a single ionic compound with fixed composition; it is not a mixture. Molten NaCl(l) is simply the liquid form of the same compound and remains pure. Solid potassium, K(s), is an element and therefore a pure substance. Solid NaCl(s) is again a pure compound, not a mixture. None of these involve a solvent plus solute as NaCl(aq) does. Thus the other options do not represent mixtures in the standard chemical sense.

Common Pitfalls:
A common mistake is to see the symbol (l) and assume that any liquid is a mixture, which is not true; pure substances can also be liquids, like pure water or molten metals. Another confusion arises when students equate the word solution with any dissolved state, forgetting that the (aq) symbol explicitly indicates an aqueous mixture. To avoid these errors, always read the state symbols carefully: (s) for pure solid, (l) for pure liquid, (g) for gas and (aq) for aqueous solution, which signals a mixture of solute and solvent.

Final Answer:
The sample that represents a mixture is aqueous sodium chloride solution, NaCl(aq).