a solid is a kind of substance & iron is a kind of solid ->  iron is a kind of substance  iron is a kind of substance & melting means a substance changes from a solid into a liquid by increasing heat energy ->  iron melting is when iron changes from a solic into a liquid by increasing heat energy  iron melting is when iron changes from a solic into a liquid by increasing heat energy & temperature is a measure of heat energy ->  the temperature of iron is increased when iron is melting  the temperature of iron is increased when iron is melting & as temperature increases , the speed of molecules will increase -> the speed of particles in iron increases