In basic chemistry, which one of the following statements is not a characteristic property of a chemical compound as opposed to a mixture?

Introduction / Context:
This general chemistry question checks whether you can distinguish the properties of a pure chemical compound from those of a mixture. Many learners confuse compounds and mixtures, especially when talking about composition and methods of separation. Knowing the correct characteristic properties of compounds is important for understanding classification of matter in basic chemistry.

Given Data / Assumptions:

• We are comparing the properties of a compound with those of mixtures.
• Several statements about composition and particle nature are given.
• Exactly one statement is not a correct characteristic of a compound.
• Assume simple school level definitions of compounds and mixtures.

Concept / Approach:
A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. All particles of a given compound are of the same type of molecule or ion pair, for example only H2O molecules in pure water. The composition of a compound is fixed, and its components can only be separated by chemical methods. In contrast, mixtures have variable composition, and their components can often be separated by physical methods such as filtration or distillation. The wrong statement here will typically describe a mixture rather than a compound.

Step-by-Step Solution:

Verification / Alternative check:
A good way to verify is to take a specific example such as water. The composition of pure water is always hydrogen and oxygen in the mass ratio 1 to 8, not variable. All particles are H2O molecules. You cannot separate hydrogen and oxygen from water by simple filtration or decantation; you need electrolysis or chemical reactions. Every correct statement about compounds must hold for water. The only statement that clearly fails that test is the idea of variable composition. Therefore it must be the option that is not a characteristic of compounds.

Why Other Options Are Wrong:

• All particles are of one type of molecule: This is true for a pure compound and is therefore not the requested non characteristic statement.
• Particles contain two or more elements chemically combined: This is the basic definition of a compound, so it is a correct characteristic.
• Constituents cannot be separated by simple physical methods: Compounds require chemical methods to break down, so this is a correct property.
• Formed by chemical combination of elements: This again is a correct statement about compounds.

Common Pitfalls:
A regular source of confusion is mixing up the ideas of compounds and mixtures. Learners sometimes think that any sample whose composition can vary slightly in the real world must be a mixture. However, for a pure compound at the microscopic level, the ratio of atoms in each molecule is fixed. Another common mistake is assuming that any separation process is chemical; in fact many everyday separations such as sieving and filtration are physical and work for mixtures, not for compounds. Always focus on fixed composition and need for chemical change as key identifiers of a compound.

Final Answer:
The statement that is not a characteristic of a compound is Composition is variable from sample to sample.