posted by amanda .
if heat was added at a constant rate to the solid subsance, why did the temperature remain at 16 degree C for approximately 6 minutes?
You really don't give enough information for us to know what you are doing; however, I'm guessing you were running a melting point curve. The melting point is defined as the point at which BOTH solid and liquid are in equilibrium. Therefore, as long as there is some liquid and some solid present, heat can be added and the temperature will not change (extra solid will melt) or the material can be cooled and the temperature will not change (some liquid will crystallize). So you have a flat line at the melting point and it stays at that temperature until ALL of the solid has melted OR (if going the other way) the temperature stays at the melting point until ALL of the liquid has crystallized.