Question

why does an increase in temperature change a solid substance into the liquid state?

Answer 1

A solid changes to liquid on heating! When we heat a solid then the heat energy changes to kinetic energy of particles of solid! They start vibrating on a faster rate! Due to heat enegy the secondary bonds between the particles break down. After that the particles start seperating away due to which the fluidity increases and the. The state of the material start changing! So on heating the solid changes to liquid!

Answer 2

By increasing temperature kinetic energy of the gas solid molecules increases ...
Solid do not have space between particles while by increasing temperature molecules start colliding with each other ... and solid changes to liquid....