What kind of reaction occurs when substances ignite upon combining?

• A. Exothermic
• B. Hypergolic
• C. Endothermic
• D. Catalytic

Answer: (B)

When substances ignite upon combining, the type of reaction that occurs is classified as hypergolic. Hypergolic reactions are characterized by their ability to ignite spontaneously when the two components come into contact with each other, without the need for an external ignition source like a spark or flame. This property is particularly significant in certain fuels and oxidizers used in rocket propellants, as it ensures reliable ignition and simplifies the handling of the materials.

In contrast, exothermic reactions release energy in the form of heat when reactants combine, but they do not necessarily ignite spontaneously. Endothermic reactions, on the other hand, absorb energy and typically require an external heat source to initiate them. Catalytic reactions involve a catalyst that speeds up the reaction without being consumed in the process, but do not specifically pertain to ignition upon combination.

Understanding these distinctions helps clarify why hypergolic is the correct choice when discussing substances that ignite on contact.