Which one of the following is not a mixture?

Analysis

The question asks us to identify which of the given substances is not a mixture. In chemistry, a mixture consists of two or more substances that are physically combined but not chemically bonded. A substance that is not a mixture must be either an element or a compound (a pure substance).

Concept Explanation

• Mixture: A material made up of two or more different substances which are mixed but are not combined chemically. They can be separated by physical means (e.g., filtration, distillation, or evaporation).
• Pure Substance: A substance that has a constant composition and distinct chemical properties. It cannot be separated into simpler substances by physical means. Pure substances are either elements (like Mercury) or compounds (like Water or Sodium Chloride).

Step-by-Step Solution

1. Analyze Option A (Air): Air is a mixture of various gases, including nitrogen, oxygen, argon, carbon dioxide, and water vapor. Since it contains multiple substances physically mixed together, it is a mixture.
2. Analyze Option B (Mercury): Mercury is a chemical element with the symbol Hg and atomic number 80. It consists of only one type of atom. Because it is a pure element, it is not a mixture.
3. Analyze Option C (Milk): Milk is a complex mixture (specifically a colloid) containing water, fats, proteins (casein), lactose, and minerals. It is not a pure substance.
4. Analyze Option D (Cement): Cement is a mixture of various compounds, including calcium silicates, aluminates, and other additives. It is a heterogeneous mixture.

Conclusion

Since Mercury is a pure element, it does not fit the definition of a mixture.

Exam Tip

When you see a question asking for "what is not a mixture," look for the pure substance. If you see a metal (like Iron, Copper, or Mercury) or a simple chemical formula (like $H_2O$ or $NaCl$), that is almost certainly your answer!