Slurpee Science
Ever wonder why syrupy slurpees don't freeze solid in the freezer, or how Slurpee machines really work? Well, you've come to the right place!
Why don't syrupy Slurpees
freeze?
Why doesn't Slurpee freeze
solid in the Slurpee Machine?
How do Slurpee machines
make Slurpee?
Slurpee Calorimetry: How much
energy does it take to freeze a Slurpee?
Why don't syrupy slurpees freeze (solid)
when you put them in the freezer???
This is a question that I once had. So one day, I set out to find an
explanation to this phenomena...The explanation comes from the sugar
content of the Slurpee. Obviously, a syrupy Slurpee has way more sugar
than, say, a Slurpee with a watery or icy consistency. The sugar actually
reduces the freezing point of the Slurpee, much like salt does, so it
needs to be cooled to a lower-than-normal temperature in order to freeze
solid.
You can test this by yourself with a simple experiment.
Materials
Two identical cups
Water
Sugar (brown sugar works best, but any sugar will do)
Thermometer (not necessary)
Method
1. Fill each cup with 1 cup (250mL) of water. Make sure that the water is
the same temperature .
2. Add sugar by spoonfuls into one of the cups and mix thoroughly. Continue
to add sugar until no more can dissolve in the water (it is now saturated).
3. Place both cups into the freezer for 2 or more hours.
4. Observe what happens. Which one solidifies first?
Why don't Slurpees freeze solid when
they're in the Slurpee machine?
Slurpees are cooled to about -3C in the machine, which may cause you
to wonder how or why the Slurpee inside doesn't freeze into an ice cube.
The answer is quite simple. The constant stirring inside the machine
prevents the Slurpee inside from freezing solid, and allows for the
Slurpee to maintain a drinkable consistency. The Slurpee do freeze;
as the water cools, it crystallizes, but the constant motion prevents
it from becoming one big ice chunk.
This also explains why your Slurpee freezes solid when you put it in your freezer: there's nothing to prevent it from doing so (unless, of course, you somehow mix it constantly while it's in there)
Here are some Slurpee Machine Facts that were generously provided (thanks again!) to me from an anonymous source:
The syrup that is used in a Slurpee machine is the same as the syrup used in the fountain drink machine (ie. the Gulp drink machine), except an emulsifier is added to the syrup of the Slurpee machine. The emulsifier helps prevent the mixture from freezing solid.
The consistency, or 'brix', of the Slurpee is determined by the water to syrup ratio. For regular Slurpee, it is 13 to 1. For Diet Pepsi, it is 6:1, and for Crystal Light is 8:1. This is an important attribute, since it affects the quality of the drink (that is, it's thickness, etc).
The syrup and water are mixed in a Blindinator tank in the machine. The CO2 is added into the tank with the water and syrup. The mixture then goes into the barrel and is mixed with the beater bar and scraper blades as it freezes.
The machine will go into a 'defrost' mode every 3 to 4 hours to allow the water and syrup to remix. You can tell that the machine is in defrost when the Slurpee is not frozen. If it is not defrosted, the water and syrup will, after a while, start to separate.
The newer Slurpee machines (in the US) are Lancer's, (FDB550-2bbl) or (FCB554-4bbl).
They are totally computerized, and they lack a Blindinator tank. Instead,
they bring the syrup, water, and CO2 together through the brixing valves
into a
Header block, and then into the barrel. This machine has a bigger compressor
that allows the product to freeze
faster, and they don't have a water bath (unlike the pinnacles, which do).
Older versions of the machines include the V3+ and the V3R machines.
The syrup for the Slurpee machines are located in the backroom of the store, and is contained in what is called a "Bag in the Box", or BIB for short. They are connected to pumps which are ran by CO2. When Slurpee is poured from the taps of the machine, it lowers the level in the Blindinator tank. A float inside the tank drops and more syrup is then pumped from the back room into the tank. On the Lancer's there is a pressure transducer that tells the computer when to add more syrup.
When the BIB is empty, the pressure drops and sends the unit into a syrup sold out condition (you know, when the little red light is flashing, and you cry because it is the flavour that you wanted...).
* A little note on that front. I have found out that if the little red light is flashing (and the Slurpee is all runny), the clerk at the store may be able to fix it for you so that you can get some yummy Slurpee of your preferred flavour. So long as the store isn't too busy, the clerk(s) on duty know how, and if you ask nicely, they will fix the problem for you. After adding the syrup, it will likely take about 5 minutes or so for the the Slurpee to be fixed and ready to pour (when the light is no longer flashing).
How much energy does it take to create a Slurpee? Hmmmm....
The equations:
Q = mcT AND Q = mL
Where Q is heat energy, c is the specific heat capacity, m is mass, T is
temperature, and L is the latent heat.
To solve
Q = mL + mcT [you change states and cool to -3C]
Let m = 1kg (or 1 litre), c (ice) = 2000 J/(kg C°), T (final) = -3C, L (fusion) = 335000 J/Kg
Thus,
Q = (1 Kg) • (335000 J/Kg) + (1 Kg) • (2000 J/(kg C°)) •
(0C- -3C)
Q = 335000J + (-6000J)
Q = 329000J = 329 KJ = 3.29*10^2 KJ of energy to freeze 1 litre of Slurpee
Notes:
The heat of capacity used in this equation is not equal to the heat
capacity of Slurpee. This is because Slurpee is a mixture of sugar AND
water, which would actually make the heat capacity less than that of
water (sugar alters the structure of the H2O molecule, making the freezing
point lower).
Also, I assumed that the water used was already at the 0C. That is, this is how much energy it takes to freeze 1 litre of water from 0C to -3C. If you assume that the water is warmer when added, then the amount of energy needed will increase.
I hope that I did not make any errors in my calculation. If you see any errors, please email me and let me know :)
More to come....
Last Updated: November 5, 2007
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