Bottle of Pepsi Max frozen solid as a brick; tonic water still liquid - Why?

My balcony is the temporary refrigerator during the winter months. This past week we have had a nice cold snap. Yesterday I saw that the half-full 2L bottle of Pepsi Max (Pepsi w/ Nutrasweet I think) was frozen solid as a brick. Right next to it was the 1/4 full 2L bottle of Schwepps Tonic water - but it was fully liquid, not even a single ice particle.

Why?

-Tcat

Semi informed WAG: dissolved salts in tonic water lower freezing point.

More likely the sugar.
Especially given that the Pepsi was ‘light’, and the tonic water regular strength sugar-water.

What is “Pepsi Max”? We don’t have that here in NE Ohio…

Freezing point depression is 1.86 degrees C per mole, IIRC. Even if I misremember the number wrong, the principle stands: freezing point depression is only dependent on the concentration in molality, not the nature of the substance dissolved (there may be rare exceptions). Most of Pepsi’s solutes are moderately large molecules, compared to common salts [mol wt of NaCl: 58.5; mol wt of Nutrasweet: 294] so many many grams of dissolves compounds doesn’t equal very many moles, and less freezing point depression than one might think. Carbon dioxide is a small molecule (mol wt: 44), but though it is considered “highly soluble in water” (300 vols/vol of water at STP, IIRC), 300 vols of a gas doesn’t weigh much (few grams, hence, few moles) per mole of liquid water.

Tonic water, on the other hand, is less strongly flavored and sweetened, and may have fewer moles per liter of solute.

Under identical conditions, one might expect the tonic water to freeze sooner.

But there is one other factor: the Pepsi is (by my experience) more heavily carbonated. If I read th phase diagram correctly, the effect of pressure on the freezing point is rather small – a few degrees in the pressure regime of a soda bottle.

I would guess that:
a) since Pepsi Max uses complex organic (larger molecules) flavorants and colorants and Nutrasweet is significantly sweeter per mole than sugar, the total of solutes is maybe a few hundred millimoles, for possibly under 1 degree C of freezing point depression.

b) the pressure in the bottle might raise the freezing point of water more than the dissolved materials lower it, for a net increase in freezing point

c) Pepsi is a mixture. As it freezes, crystals of near-pure ice form first, excluding the gas and solute-laden “pepsi syrup” which have a much lower freezing point. Similarly, the syrupy component will thaw first, absorbing a great deal of heat, while the near-pure ice remains solid. It takes much more energy to convert ice to liquid than to simply change the temperature. For example: it takes only 4.1 kJ/mole to warm a kg of water by one degree C, but the latent heat of melting for water ice is 330 kj/kg – so it takes over 75 times more energy to melt ice than to raise it one degree above the melting point

The Pepsi may have felt “frozen solid”, because the water ice was tightly packed (it expands as it freezes), and the bottle was taut with gas pressure, but the syrup may have been only partly frozen or melting, and acted as a significant thermal reservoir.

Which leads to my final hypothesis:

d) both the tonic and Pepsi froze, since their freezing points would be within a degree or two, but the Pepsi was more heavily carbonated (consistent with my experience), so as they froze, and the ice excluded most of the dissolved CO2. more pressure was created in the Pepsi airspace than the Schweppes, especially after the freezing ice expanded and almost completely filled the former airspace.

Moreover Pepsi is commonly sold in 2L bottles, while Schweppes (locally at least) tends to be sold in 1L or smaller bottles. This would mean twice as much excluded CO2 gas in the Pepsi, even if they were equally carbonated (and I doubt they are) yet the airspace in the Pepsi bottle isn’t proportionately larger (especially after the freezing ice expands into that space), resulting in a much higher pressure in the Pepsi. Since water is almost completely incompressible, only the tiny remaining airspace determines the final pressure, not the overall size of the bottle.

This higher post-freezing pressure caused a more significant elevation in the melting point. Note that this is an asymmetric effect: carbonated beverages would melt at a significantly higher temperature (due to the high pressure) than they freeze at.

When you discovered the bottles, the tonic had melted, but the soda had not. The Pepsi would have a higher post-freezing pressure (higher melting point); a 2L bottle would have a higher thermal mass and smaller surface/volume ration than a 1L; and the "syrupy component of the Pepsi acts as a surprisingly significant thermal reservoir as it melt. If the overnight temperature was (e.g.) -2C , it would take longer for the Pepsi to reach the melting point of its pressurized water-ice.

You presumed that only the Pepsi had frozen. The thawing of the tonic water left no evidence.

The pressure might indeed have something to do with it. When I opened the Pepsi bottle, a lot of gas escaped. The tonic was flat.

Both are 2L bottles and Tonic water is surprisingly sweet, I think they might use corn syrup. Just cuz it’s clear don’t mean it ain’t got stuff in it…

I cut open the Pepsi bottle and hit the block with a food mallet. Solid through and through. Then I put scientific thoughts away and ate a 1 liter Pepsi popsicle. :smiley: The middle and bottom were indeed more concentrated with syrup, but they were completely frozen.

The tonic was not frozen at all and was not again this morning when I looked and last night was definitely below freezing. And no, the gin was not pre-mixed!

-Tcat

I presume that’s for some particular amount of solvent? Surely, if I dumped a bottle of Pepsi into Lake Superior, the freezing point of the lake would not change significantly, despite having the same number of moles of solute as the bottle of pop.

But I’m having a hard time seeing how the freezing point depression could be independant of the solute. Suppose I had two different liquids, with similar freezing points: If I dissolved one in the other, would the solution really have a freezing point lower than either separately? Or is that just a rule of thumb for water with solid solutes?

Some liquids don’t freeze even less.

Maybe the Quinine acts as an antifreeze?

I think the sugar is more likely to play a role in this. A couple of times, the thermostats on the soda coolers at my workplace have broken–the canned diet soda froze, but the canned regular soda stayed liquid.

The sugar is an antifreeze.

I very much doubt it was just the sugar that caused it to not freeze, with long experience in making sorbets, you need a LOT of sugar just to form a semi-liquid slush. Making something that was completely liquid would make it unpleasantly sweet.

You presume correctly. He’s talking about the molarity of the solution, i.e. the number of moles of solute per kg of solvent.

Works for pretty much anything. For example (along with a good explanation) see this page about the freezing points of lead/tin solutions:

http://www.chemguide.co.uk/physical/phaseeqia/snpb.html

Note that lead freezes at 327 [sup]o[/sup]C, and tin freezes at 232 [sup]o[/sup]C. But any mixture containing more that about 50% tin will have a freezing point lower than pure tin. A mixture containing 62% tin will freeze at 183 [sup]o[/sup]C.

Sorry, I missed this point earlier.

For solids dissolved in liquids, the rate of depression depends on the solvent. KP’s figure of -1.86 [sup]o[/sup]C/molal is the correct figure for water. Other solvents have different constants, e.g. benzene = -5.12 [sup]o[/sup]C/molal.

This also applies to liquid/liquid solutions, such as the molten lead/tin example I gave above. At some point, you’ll be below the freezing point of one or the other, or both, and and one of them will start to solidify. If there’s less than 62% tin, the lead will start to solidify first, so you can regard it as a solution of lead in tin (i.e. lead is the solute, tin is the solvent). If there’s more than 62% tin, the tin will solidify first, even though this happens at a temperature way below the freezing point of pure lead; you can regard this as a solution of tin in lead. (There’s a hiccup at the crossover point. With 62% tin, the lead and the tin both solidify at the same temperature, 183 [sup]o[/sup]C. This particular mixture is called the “eutectic”; it’s the one with the lowest freezing point for any two mutually soluble liquids).

Er, molality.

OK, that still sounds weird, but I suppose it’s allowed to be weird. I’ll take your word for it.

Which, IMHO, is an accurate description of commercial sodas (or, as the case may be, commercial tonic water).