Question

Why does it generally take more enthalpy to ignite a solid than a gas or liquid?

Answer 1

The molecules in the solid are packed tightly together , whereas the molecules of liquid are bit apart , and that of gas are very fat apart.

So to ignite the molecules of the solid substance , more energy is given to it , so that its molecules take up the energy , their kinetic energy increases and they loosen up a little.

The molecules in liquid and gas substances are already at a higher energy , so they get ignite faster as compared to the solid substances.