In A Heating And Cooling Curve, What Is Occurring During The Flat Portions?

In a Heating and Cooling Curve, what is occurring during the flat portions of the curve when the temperature is constant, and what is the energy being used to do?
there is no energy being added
it is speeding up the particles
it is changing the phase
it is increasing the kinetic energy
it is decreasing the kinetic energy

Regarding Heating And Air Conditioning?

I had a few concerns regarding heating/AC in your home:
1. Is it normal for smells of food cooking in the kitchen to enter other rooms in the house through the heating/AC ducts/vents?
2. What exactly are you breathing in when you turn on the heat or air? Mold particles? Dust?
3. What happens if you turn on the heat or AC and then BLOCK up a couple of vents in a couple of rooms? Would the air continue to circulate because the temperature would be slow to change in those rooms?

In A Heating And Cooling Curve, What Is Occurring During The Flat Portions Of The Curve?

In a Heating and Cooling Curve, what is occurring during the flat portions of the curve when the temperature is constant, and what is the energy being used to do?
A) there is no energy being added
B) it is speeding up the particles
C) it is changing the phase
D) it is increasing the kinetic energy
E) it is decreasing the kinetic energy
I think it is either A or C???

Questions On Heating, Cooling, Melting And Boiling Points. Why Does The Temperature Remain Constant?

We were doing an experiment of heating and cooling of naphthalene. The temperature remain constant when naphthalene starts to melt, and then rise again when all napthalene becomes liquid. The same thing happened during cooling.
My textbook says that “When solid naphthalene is heated, heat energy is absorbed. This causes the particles to gain kinetic energy and vibrate faster, which causes the temperature to increase.”
“During the melting process, the temperature of naphthalene does not rise, even though heating continues. THE TEMPERATURE REMAIN CONSTANT BECAUSE THE HEAT ENERGY ABSORBED BY THE PARTICLES IS USED TO OVERCOME THE FORCES BETWEEN PARITICLES SO THAT THE SOLID CAN TURN INTO LIQUID.”
Now my question is, why the temperature rise in the first place? Why didn’t it remain constant at the beginning to “overcome the forces between particles”?