(1)
At phase A, the water is fully solid, At phase C the water is fully liquid and at phase E the water is fully gas. At this ponts all the substance is in one state other than the fact that its molecules are getting more excited as they gain more energy. This is why there is an increase in temperatures. At the transition points water is changing phase and therefore at any point in between the transition not all water will be in the same phase.
(2)
Point F
This is because the solid water stops rising in temperatures and begins changing phase into liquid water. This is the highest temperatures that the solid water can attain without turning into liquid. You can see from the graph that the temperatures rise at this point levels off between point F and G
(3)
Point H
This is because the liquid water stops rising in temperatures and begins changing phase into gaseous form. This is the highest temperatures that the liquid water can attain without turning into gas/water vapor. You can see from the graph that the temperatures rise at this point levels off between point H and I.
(4)
The energy is increasing
You can see from the graph that energy is being absorbed by the water (due to increases in energy on the x-axis on the graph) but there is no corresponding rise in temperatures in y-axis. This means the energy begin absorbed is being used to change the phase of the water.
(5)
The water is changing phases
Because the energy absorbed is being used to change the phase of the liquid. The energy is being used to break the bonds between the molecules so the molecules become farther apart and causing a change in phase on the water. Therefore the vibrational moments of the molecules (responsible for rising in temperatures) remain the same over this latent phase. This energy absorbed to change phase is called latent energy/heat.
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