Which statement about impurities and reactivity hazards is NOT true?

• A. Impurities can catalyze decomposition, alter kinetics, or change thermal properties, increasing runaway risk.
• B. Impurities can decrease the risk of runaway in all cases.
• C. Impurities can alter thermal properties.
• D. Impurities can catalyze decomposition.

Answer: (B)

Impurities can significantly affect reactivity hazards by changing how heat is generated and managed in a system. They can catalyze decomposition by providing surfaces or species that lower the activation energy or open faster, more exothermic pathways. They can alter the kinetics of the reaction, changing rate constants or even the mechanism, which can either speed up or slow down heat production depending on the impurity and system. They can also modify thermal properties such as heat capacity, the heat of reaction, melting point, and thermal conductivity, which in turn affects how easily heat is removed or accumulated.

Because impurities can both increase or decrease the likelihood of runaway depending on the specific impurity and conditions, a blanket statement that impurities decrease runaway risk in all cases is not true. The other effects—catalysis of decomposition, changes to kinetics, and changes to thermal properties—are all real ways impurities influence reactivity hazards.