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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.

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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:

sugar solubility curve

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.

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    $\begingroup$ Nice writeup, although I disagree with your assessment that during the first part of the cooling a large part of the sugar will crystallize out of solution. According to the solubility curve you posted, the solubility of sugar is 180 g sugar per 100 g water, whereas the sugar content of commercial Coca-Cola is less than 1/20 of this amount. Likewise, fructose is even more prone to forming frozen glasses than sucrose is, and the amount of fructose is less than 1/20 of the saturation limit near the freezing point. Even at -20C (typical freezer temperature), crystallization is not possible. $\endgroup$ – DumpsterDoofus Nov 22 '13 at 14:24
  • $\begingroup$ That said, I definitely agree with your assessment that the last bits of liquid to freeze will be sugar-enriched, but for a slightly different reason; pure ice will be the first thing which crystallizes out of solution, based on the solubility data. As a result, the remainder will be continually sugar-enriched, until it reaches the point where it reaches the saturation limit, whereupon either sugar will crystallize out or the fluid will glassify. Mixed high-solubility organics tend to glassify upon concentration, which is why I suggested liquid interspersion in my answer. $\endgroup$ – DumpsterDoofus Nov 22 '13 at 14:39
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    $\begingroup$ I upvoted your answer anyways though, since it's a lot better formatted than mine was :) $\endgroup$ – DumpsterDoofus Nov 22 '13 at 14:40
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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.

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  • $\begingroup$ Could the high solubility of sugar in water explain why the first portions are sweeter, because on average the amount of sugar will be well below the solubility limit, whereas when melting the sugar will dissolve until this limit? $\endgroup$ – Michiel Nov 22 '13 at 6:43

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