Which of the following is an example of a mixture?

1. Analyze the question We need to pick the option that is a mixture, not a pure substance.

2. Explain the concept

• A pure substance (element or compound) has a fixed composition and a definite formula.
• A mixture has two or more substances physically combined, in variable proportions. The components keep their own properties and can be separated by physical methods such as filtration, decantation, evaporation or centrifugation.

3. Evaluate each option

• A. Common salt: sodium chloride (NaCl) is a compound with a fixed composition. Not a mixture. (Table salt sold in shops contains small additives, but in chemistry questions "common salt" means NaCl.)
• B. Blood: made of plasma, red cells, white cells, platelets, dissolved salts, proteins and more. Its proportions vary and the parts can be separated physically, for example by centrifugation. This is a mixture.
• C. Sand: this is the one to be careful about. Sand is mostly silicon dioxide (SiO₂), but natural sand also contains other minerals, shell fragments and so on, so it can loosely be called a mixture. In JAMB-style questions, though, sand is usually treated as the compound silicon(IV) oxide, and blood is the clear-cut example of a mixture.
• D. Washing soda: sodium carbonate decahydrate (Na₂CO₃·10H₂O) is a compound with a fixed formula. Not a mixture.

4. Answer

5. Tip If a substance has a definite chemical formula (NaCl, Na₂CO₃·10H₂O), it is a compound. If it is a "natural or biological material" like blood, milk, air, seawater or soil, think mixture. Also watch for sand: pure sand is SiO₂, but natural sand is impure, so choose the option that is unambiguously a mixture.