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.
If heat was added at a constant rate to the solid substance, why did the temperature remain at 16 C for approximately 6 minutes?
The temperature staying constant at 16 degrees Celsius for approximately 6 minutes may be due to a phenomenon known as a phase change. When heat is added to a substance, its temperature typically increases. However, during a phase change, such as from a solid to a liquid or a liquid to a gas, the temperature remains constant even when heat is being added.
In this case, if the solid substance was undergoing a phase change, it would explain why the temperature remained at 16 degrees Celsius. The added heat is being used to break the bonds between the molecules, causing them to transition from a solid state to a liquid state. During this phase change, the temperature does not change until all the solid has fully transitioned to a liquid.
To determine if a phase change is occurring, you can look for other signs such as the substance melting or boiling. If the temperature remains constant while heat is being added, it is a strong indication that a phase change is taking place.
It's important to note that without additional information about the specific substance and conditions, this explanation is a general one. Different substances have different melting and boiling points, and the exact behavior can vary depending on various factors.