I freeze my coke in a freezer completely to solid and then keep it out to melt and as it melts portion by portion I starts to drink, initially It will be very sweet and later it wont be sweet at all. why does this happen? We know that all sugar in coke will be dissolved (Solid solution) but how a major portion of this melts faster than water in the juice their by getting more sweet? How these sucrose can escape through the crystals formed by the ice and join in the water. I measured the sweetness (Brix) and found it varies.
Substances in solution have the effect of decreasing the temperature of the freezing point of the liquid they are dissolved in. This is called freezing-point depression. This is one of the reasons why adding salt to ice helps it melt.
Your coke is a complicated solution + colloid and sugar is one of the main substances dissolved in it.
During freezing: What happens is that during the freezing process, as the coke cools a lot of sugar is pushed out of solution which allows the less-saturated water to freeze first. The last bit of liquid to freeze has much more sugar in it and takes a while to freeze because it is a concentrated solution and the freezing point has been lowered a lot.
During melting: The last portion of the coke to freeze has the bulk of the sugar and the lowest freezing point and will melt first when warmed. When you allow the coke to start melting the most saturated portions melt the fastest and you consume most of the sugar in this stage. Later when the drink continues to warm the rest of the water starts melting with much less sugar in it, thereby making the remaining portion less sweet.
More information about sugar solubility: One common way to grow sugar crystals is by slowly cooling the solution to push the sugar out of solution. There is some information about this here and they provide a nice sugar (sucrose) solubility versus temperature graph too:
As you can see the temperature dependence for solubility is dramatic. I haven't been able to find a curve for fructose (the primary sugar in coke in the United States) but I suspect the curve is very similar.
Have you ever looked at a peanut and said, "gee, what an interesting liquid"? Most people would answer in the negative. Peanuts are about 25% liquid fat by weight, but the liquid is held together in a solid form by the proteins and carbohydrate solid matrix which comprises the body of the nut. When this solid matrix is fragmented by mechanical grinding, however, a semi-solid/semi-liquid material is obtained; I have occasionally heard this substance be referred to as "peanut butter".
Likewise, I'm pretty sure that fructose/glucose solutions can retain liquid form even at high concentration and low temperatures. I suspect that there is in fact corn-syrup enriched liquid interspersed within the ice in the frozen Coca-Cola; as the Coca-Cola warms, it dissolves the surrounding ice to maintain thermodynamic equilibrium and equalize the chemical potential and whatnot.
As a result, the first portions of liquid leeched out of the frozen Coke are sugar enriched, and successive fractions which melt off are progressively more dilute. However, even if it is solid all the way through, I'm pretty sure that the first portions which melt off will be solute-enriched. As to how the solution manages to work its way out of the mass of crystals, I'll wait for someone else to explain that.