Which of the following is a physical property?

A physical property is a characteristic of a substance that can be observed or measured without changing the substance's chemical identity. The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This property is particularly important in identifying and characterizing materials, as it provides insight into the interactions between molecules.

While both the burning point and flammability involve the substance undergoing a chemical change (combination with oxygen, releasing heat and producing new substances), boiling point does not lead to any change in chemical composition, making it distinctly a physical property.

In contrast, reactivity with acid, while indicative of the substance's behavior under specific chemical conditions, also does not define a physical attribute of the substance itself. Thus, boiling point stands out as a fundamental physical property that can be used in practical applications such as distillation and purification processes in chemistry.