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Science of: Slurpees, or Sugary Science

Nine million. That’s how many icy-cold, sugary-sweet Slurpees the convenience store 7-Eleven will give away to US customers today. The annual Free Slurpee Day tradition began in 2002, a brilliant marketing move that has made July 11th (7-11 in US notation) the store’s busiest day of the year. In honor of the brain-freezing drink, today we’re highlighting some of the science behind this treat.

As the story goes, Slurpees are so named because of the slurping sound consumers make while eagerly sucking up the drink through a straw. But its name is surprisingly similar to the technical name for a mixture of water and ice crystals—ice slurry. So, let’s start there.

Ice slurry, also known as slurry ice, is the subject of a lot of research. It comes up frequently in the context of refrigeration, forming the basis of energy efficient technologies found in grocery store meat displays and air conditioners. It also has medical applications. Scientists at Argonne National Laboratory developed a machine that can rapidly deliver ice slurry to targeted organs in the human body, protectively cooling them in response to brain swelling, heart attacks, or severe trauma. In a study more closely connected to the drink at hand, scientists recently found that drinking ice slurry may cool the body and brain more efficiently than drinking water alone.

7-Eleven GIF - Slurpee 7Eleven Dispenser GIFs
The likely speed at which 7-Eleven Slurpees will be distributed during Free Slurpee Day.

What’s so special about a slurry? As any Slurpee enthusiast will tell you, they are cold! If you replace the straw in a newly poured Slurpee with a thermometer, you’ll see that the temperature is several degrees below the freezing point of water (32°F). But how is that possible? Shouldn’t water that cold just be ice? To fully answer that question, it helps to go back in time to the Slurpee’s humble beginnings.

It was the late 1950s and the soda machine in Omar Knedlik’s Dairy Queen franchise was broken. As a quick fix, he stashed some bottles of soda in the freezer. When he took them out and popped the tops for his customers, the liquid soda instantly transformed into a delicious slushy drink that everyone loved. It was a lucky, supercooled accident.

When a liquid reaches its freezing temperature, it doesn’t instantaneously turn into a block of ice. Ice crystals form first at nucleation sites—impurities, irregularities, or disturbances in the liquid—and they grow outward from there. In a pure liquid with few nucleation sites, it’s possible (although tricky) to supercool the liquid below its freezing point. In this case, once a nucleation site is finally introduced (for example, by popping the top of the container or slamming the bottle down), the liquid turns to slush or ice, depending on the conditions, right before your eyes. Don’t believe it? Try it yourself.

To get a similar result but in a controlled, consistent way, Knedlik invented a machine that mixed and chilled water, crushed ice, cola-flavored syrup, and carbon dioxide under pressure. The result was the Icee—a frosty treat still sold today in fast food restaurants, theaters, and stores. A few years later, 7-Eleven bought some Icee machines, introduced their own flavors, and rebranded the drink as today’s honored Slurpee.

A human enjoying a Slurpee during a hot supernatural day

Achieving the arctic temperatures of Icees and Slurpees without supercooling requires a bit of chemistry and physics. There are two key components.
  1. The sugar: Sugar is a tasty form of anti-freeze. When dissolved in water, sugar molecules block the growth of ice crystals that would otherwise become ice chunks and, eventually, a solid block of ice. In technical terms, sugar “depresses freezing” by lowering the temperature at which water turns to ice.
  2. Constant mixing: Slurpee ingredients are mixed—churned, really—inside of a chilled barrel. Continual motion and side-scraping prevent ice crystals from building up, keeping the mixture light and frosty right up until you fill your cup.
To fully experience the science behind the Slurpee, you really need to taste one. You can get yours for free today between 11am and 7pm at any US 7-Eleven. I hear they also have hotdogs for $1, but I’ll let someone else write that “science of” story.

Kendra Redmond

Kendra Redmond is a freelance science writer and editor. After earning a master’s degree in physics, she's worked for years in science education and communication, regularly contributing to Physics Buzz and other science news outlets, which you can find on her Facebook and LinkedIn. Kendra lives in Bloomington, MN with her husband and three kids.

Further Reading

The Brain-Freezing Science of the Slurpee via

How to Supercool Water via SciShow


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