Which of the following substances is a mixture?

Analysis

The question asks us to identify which of the listed substances is a mixture. In chemistry, a mixture consists of two or more substances that are physically combined but not chemically bonded. This means the components can be separated by physical means (like filtration, evaporation, or distillation).

Concept Explanation

• Pure Substances: These have a fixed chemical composition and distinct properties. They include elements (like Iron) and compounds (like Sodium chloride and Sugar).
• Mixtures: These have variable compositions. They can be homogeneous (uniform throughout, like saltwater) or heterogeneous (non-uniform). • Granulated sugar (Sucrose): This is a pure compound with the chemical formula $C_{12}H_{22}O_{11}$. • Seawater: This is a solution containing water, dissolved salts (like sodium chloride, magnesium chloride), gases, and organic matter. Because it contains various substances physically mixed together, it is classified as a mixture. • Sodium chloride (Table salt): This is a pure ionic compound with the chemical formula $NaCl$. • Iron filings: This is a pure element (Fe).

Step-by-Step Solution

1. Evaluate A (Granulated sugar): Sugar is a compound. It has a constant chemical formula and properties. It is not a mixture.
2. Evaluate B (Seawater): Seawater is a complex solution. It contains water ($H_2O$) and various dissolved minerals. Since these components are not chemically bonded to each other, it fits the definition of a mixture.
3. Evaluate C (Sodium chloride): This is a chemical compound formed by the reaction of sodium and chlorine. It is a pure substance.
4. Evaluate D (Iron filings): Iron is an element found on the periodic table. It is a pure substance.

Conclusion

Based on the definitions above, seawater is the only substance on the list that is a physical combination of multiple components.

Exam Tip

When you see a question about mixtures, look for substances that are "solutions" or "natural occurrences." If it has a specific chemical formula (like $NaCl$ or $C_{12}H_{22}O_{11}$), it is a compound, not a mixture!