What is a hypergolic substance?

A hypergolic substance is defined as a chemical compound that ignites spontaneously upon contact with another substance, typically a fuel and an oxidizer. This characteristic means that hypergolic materials do not require an external ignition source, such as a spark or flame, to ignite. Instead, they ignite immediately when mixed together, which is a crucial property for applications such as rocket propulsion.

In rocket systems, hypergolic propellants are preferred for their reliability and simplicity, as they can be stored for long periods and do not require extensive ignition systems. This contrasts with substances that require an ignition source to burn, which is not a characteristic of hypergolic materials.

The other options, while related to combustible materials, do not accurately define hypergolic substances. A material that explodes on contact with air or is a highly flammable liquid does not capture the essence of the hypergolic reaction, as these descriptions refer to different types of explosive or flammable behavior.