Which of the following is an example of a chemical property of a substance?

Introduction / Context:
This general science question tests your understanding of the difference between physical properties and chemical properties of matter. Correctly distinguishing between these two categories is fundamental for chemistry, material science, and many applied fields such as engineering and environmental science.

Given Data / Assumptions:

• The question asks for a chemical property, not a physical property.
• Options include reactivity, solubility, density, and melting point.
• You are expected to know textbook definitions of physical and chemical properties of substances.

Concept / Approach:
A physical property is a characteristic of a substance that can be observed or measured without changing the chemical identity of the substance. Examples include colour, density, melting point, boiling point, and solubility. A chemical property describes how a substance interacts with other substances and how it can change into new substances with different compositions. Classic examples of chemical properties include flammability, reactivity with acids or bases, tendency to rust or oxidise, and stability in the presence of heat or light. To answer the question, identify which option involves a change in composition rather than just a change in physical state or appearance.

Step-by-Step Solution:

Verification / Alternative check:
As an alternative check, think about whether the substance after the process is the same chemical or a different one. For reactivity, burning magnesium in air produces magnesium oxide, which is a new substance, confirming that reactivity involves chemical change. When measuring density, solubility, or melting point of a pure sample, you still have the same substance, only in different physical forms or arrangements. Introductory chemistry textbooks and classroom definitions will therefore list reactivity as a chemical property and the others as physical properties, supporting this conclusion.

Why Other Options Are Wrong:

Solubility in water is a physical property because dissolving a substance usually does not change its basic chemical composition, and the process can often be reversed by evaporation or crystallisation.

Density at room temperature is a physical property because it describes mass per unit volume without any change in chemical identity.

Melting point is a physical property because it marks the temperature at which a substance changes from solid to liquid, which is a change of state rather than a transformation into a new substance.

Common Pitfalls:
Students sometimes assume that any process that appears dramatic, such as melting or dissolving, must be chemical, but these are usually physical changes. Another common error is to treat solubility as chemical simply because it involves mixing with another substance. The key test is whether the original substance can be recovered unchanged. In contrast, reactivity leads to new substances whose properties differ from those of the reactants, which clearly indicates a chemical property.

Final Answer:
The example of a chemical property among the options is Reactivity with other substances.