Eutectics

When a solution is cooled, it will freeze into a solid, or a solid will precipitate out of the solution. For many solutions, the freezing point will be lower for the solution than for the pure solvent. The mixture with the lowest freezing point is called a eutectic.
(Pb/Sn phase diagram is based on a figure by Metallos, commons.wikimedia.org/wiki/Fi..., CC BY-SA 3.0.)

[@AbdulHannan-uv6ym]

What happens to enthalpy when a system goes from S to S+L. is the total enthalpy the sum of sensible and latent heat ?

[@PhysicalChemistry]

That's correct.
The enthalpy increases when the temperature of a S increases, at a rate proportional to the heat capacity. (This is sometimes called sensible heat, as you say.)
The enthalpy change associated with the phase S->L phase change is proportional to the enthalpy of fusion. (This is sometimes called the latent heat, as you say.)
If there is additional temperature change within the S+L phase, as could happen for the system depicted here (but not for a single-component system with a sharp melting point), there will be additional contributions from both heat capacity and heat of fusion (sensible and latent heat).

[@Piyushkhurana05]

Do the two solids below eutectic point exist as single phase or seperate phases?

[@PhysicalChemistry]

It depends. If they're miscible solids, or alloy-forming, then it might be a single phase. If they're immiscible, then it might be a mixture of two pure solids, or two nearly-pure solids, or two different compounds. The next video in the series explains this in a little more detail: ruclips.net/video/j4Bn2cubxk8/видео.html

[@akashsunil7464]

Why after attaining the eutectic point of concentration the freezing point increases more

[@PhysicalChemistry]

Do you mean as the concentration changes?
From a mathematical / graphical point of view: The eutectic mixture is the one with the minimum freezing point. The freezing point must increase as the concentration changes in either direction from that minimum.
From a somewhat more physical point of view: the phase boundary represents the conditions where the liquid has the same chemical potential as the solid. This temperature decreases as the solute concentration increases, for reasons described in the freezing point depression video (ruclips.net/video/JdA1GZF_51I/видео.html). But this decrease happens from both sides of the graph. The two decreasing curves have to meet somewhere. This is the eutectic.
You can think of the "other side" of the eutectic as changing the role of solvent and solute. Instead of B being dissolved in A, it begins acting like A being dissolved in B.

[@unessff2707]

is there a molecular visualisation to this phenomenon ?

[@PhysicalChemistry]

That's a great question, but I don't have a good answer. I love to break things down to atomistic, mechanistic explanations when I can, but I don't have a good way to do so for this topic.
It's more of a mathematical, rather than physical phenomenon: if freezing point depression happens from both directions, there must be a point in the middle where the freezing/melting point is the lowest.
And freezing point depression itself doesn't lend itself to atomistic explanations all that well, either, because it's an entropic effect.