If you’ve been like us and spent the first part of 2014 in the Polar Vortex, then you know how cold it can be. Of course, in our climate’s new normal, we’ve had two rounds of rain since the big freeze. But the Vortex might be returning this week. At least to Vermont.
During the cold days, I’d be making deliveries and people would joke, “is the syrup ever going to freeze?” or I’d get more-serious-like questions about leaving syrup overnight in cars or in unheated garages. The pat response to this is, “syrup doesn’t freeze.” Or because I’m a former high-school teacher, I make something up: “it freezes at -100F” This is all true, sort of.
Syrup certainly can’t freeze in a household freezer or even overnight in a car at -5F.
Maple syrup is a solution made of 66.9% sugar (predominately sucrose). The sucrose molecules get in the way of the water molecules crystallizing into ice and therefore force the solution to freeze at a much lower temperature than water.
But the Polar Vortex got me thinking about the actual freezing point of maple syrup. My obligatory web search came up empty so I was forced to do math. For those of you who remember your chemistry (high-school or otherwise) you may remember there is a freezing point depression equation: ∆T = Kfm.
I dusted it off.
Follow along if you’re so inclined. Otherwise, skip down to the *** to get to the punch line.
∆T = Kfm
∆T is the change of freezing point temperature below that of water.
Kf is the freezing point constant of the solvent which, in our case, is water. Looking this up we can see that it is 1.86C kg/mol.
m is the molality of our solution or moles sucrose/kg water
Our solution is 66.9% sucrose and 33.1% water. One liter of syrup contains 669 mL of sucrose and 331 mL of water.
We then have to convert mL to grams which involves finding the density of sucrose. I look it up and find it is 1.33 g/mL which, multiplied by 669 mL, gives us 889.77 grams sucrose. To find the number of moles of sucrose, we divide by the molar mass of sucrose which is 342 g/mol (the chemical formula of sucrose is C12H22O11) to get 2.6 mol of sucrose in one L of syrup .
To find the molality we need to divide 2.6 mol by the 0.331 kg water to get a molality of 7.85.
Now we plug this into our equation: ∆T = 1.86C kg/mol x 7.85
∆T = 14.6C
***Syrup freezes 14.6C below the freezing point of water. This is -14.6C if water freezes at 0C. -14.6C is also 5.7F.
This puzzled me at first because syrup certainly doesn’t freeze solid at 5.7F. Upon further review (and consultation), 5.7F is when syrup begins to crystallize. It doesn’t actually freeze solid until 85 degrees later at around -80F.
So go ahead, leave that syrup in the car, in the garage, outside: it’s safe. If we lived on Mars we might need to bring it inside. But if we lived on Mars, we’d have bigger problems to consider.