In the diagram shown, what is occurring at the section marked 4 ? Temperature
:: ا
4
2
Heat Energy
O Melting
Freezing
O Condensation
O Vaporization

Vaporization

Since the question does not specify what molecule is being acted upon by the increment in temperature, I'll assume it's water.

When first taken out of the fridge, water is in the form of ice, and it has not been affected by a change in temperature yet, so it's at the origin.

(origin = ice)

As you raise the temperature, however, the ice starts to melt, and melting occur during phase 2. You have to keep the temperature constant for the process to properly occur.

(phase 2 = melting)

After it finishes melting, the ice is now in it's liquid state, which is water. The temperature continues to rise in order to proceed to the next phase.

(2nd slope = water)

Lastly, Water is being vaporized during phase 4. Notice, the temperature is kept constant in order to allow the process to properly occur.

(phase 4 = vaporization)