In basic chemistry, which one of the following substances is not a mixture but instead consists of a single pure substance?

Introduction / Context:
Chemistry distinguishes between pure substances and mixtures. Pure substances have uniform composition and fixed properties, whereas mixtures contain two or more substances physically combined and can be separated by physical methods. This question asks you to identify which among the listed examples is not a mixture and instead represents a pure substance. It tests your understanding of everyday examples like air, milk, cement, and metallic elements such as mercury.

Given Data / Assumptions:
- Four options are provided: air, mercury, milk, and cement. - We must decide which one is not a mixture under normal conditions. - Pure substance here means a single element or compound with uniform composition. - Mixtures can be homogeneous or heterogeneous combinations of different substances.

Concept / Approach:
Air is a mixture of gases, primarily nitrogen and oxygen, along with small amounts of other gases. Milk is an emulsion and colloidal mixture of water, fats, proteins, and minerals. Cement is a mixture of different powdered compounds like calcium silicates and aluminates. Mercury, on the other hand, is a chemical element, symbol Hg, and is a pure liquid metal under normal conditions. The logical approach is to identify the one option that refers to a single element rather than to a combination of substances.

Step-by-Step Solution:
Step 1: Consider air. It contains nitrogen, oxygen, carbon dioxide, argon, and traces of many other gases. This clearly makes it a mixture of gases. Step 2: Consider milk. It contains water, fat globules, proteins like casein, lactose, and minerals, forming a heterogeneous mixture or emulsion. Step 3: Consider cement. It is produced by grinding together various compounds such as clinkers of calcium silicates, calcium aluminates, and sometimes other additives, so it is also a mixture. Step 4: Consider mercury. It is an element with atomic number 80 and is present as a single type of atom in metallic form, not a blend of different substances under ordinary conditions. Step 5: Since mercury is a single pure substance, and the others are combinations, the correct choice is mercury.

Verification / Alternative check:
You can verify this by looking at the periodic table and standard descriptions in chemistry books. Mercury is listed as an element and is often used as the textbook example of a metal that is liquid at room temperature. Air, milk, and cement are routinely listed as examples of mixtures when explaining homogeneous and heterogeneous systems. This confirms that mercury is the only option that qualifies as a pure substance rather than a mixture.

Why Other Options Are Wrong:
Air (option a) is a homogeneous mixture of gases, even though it may appear uniform to our senses. Milk (option c) is a complex colloidal mixture and can be separated into cream, whey, and other components by physical methods such as centrifugation. Cement (option d) is a heterogeneous mixture of several compounds and does not have a single fixed chemical formula. Thus, all three are mixtures and cannot be the correct answer to a question asking which one is not a mixture.

Common Pitfalls:
Students sometimes think that because air and milk look uniform at first glance, they must be pure substances. This confusion arises from not distinguishing between homogeneous mixtures and true pure substances. Another pitfall is assuming that any industrial material like cement must be a compound; in reality, it is a mixture of several substances. Remembering that elements on the periodic table, such as mercury, represent pure substances will help you avoid these errors.

Final Answer:
The substance that is not a mixture but a pure substance is mercury, a pure elemental metal.