|Learning about freezing point can lead to a tasty reward!|
When trying to devise cool activities for my kids, I generally stick with either a culinary or a scientific theme. This is mostly because cooking and science are what I know best. But when the opportunity arises to combine the two in a fun-filled, hands-on, and awesomely secret educational activity, I do my best to keep from micturating in my undergarments.
In my experience, the best example of culinary science fusion involves a lesson on the concept of freezing point, mainly because the end product of this stealth lesson can be topped with whipped cream and a cherry. That’s right, I’m talking about teaching science while making ice cream.
To fully appreciate the culinary chemistry behind this frozen delight, it’s important to understand the concept of freezing point and melting point. Simply put, the freezing point is the temperature at which a liquid freezes and the melting point is the temperature at which a solid melts. For most substances, the melting point and the freezing point are the same.
Let’s use water as an example. If water is cooled below 0°C (32°F), it will transform into ice. Therefore, the freezing point of water is 0°C. However, if the temperature of ice is raised above 0°C, it will melt. Therefore, the melting point of water is also 0°C.
HOWEVER, these points can be manipulated. For anyone who lives in an area where winter happens, you’ve probably seen the massive seasonal salt inventory at The Home Depot. The idea is that by putting salt on walkways, usually in the form of either rock salt (NaCl) or calcium chloride (CaCl2), you will prevent the build up of ice and snow and thus prevent any nasty slips. This is because salt will lower the freezing point of water, thereby helping to keep it from turning into an icy mess, even when temperatures are below freezing. But, it will only work if the walkway is warmer than -9°C (or 15°F).
How does this relate back to ice cream? Well, we can use the same idea of lowering the freezing point of water and apply it to making cream freeze. The set-up involves two resealable plastic bags (one quart-sized and one gallon-sized), ice, lots of salt, cream, milk, sugar, vanilla, and your flavoring of choice.
Into the quart-sized resealable plastic bag, combine the cream, milk, sugar, vanilla, and flavoring (follow the recipe below and ensure that the bag is fully sealed). Fill the second resealable plastic bag about halfway with crushed ice and all of the salt. Place the cream-filled bag into the ice-filled bag and seal. Then…shake!
When salt is added to the ice water, the temperature of the mixture drops. Because the temperature of the ice-salt mixture is lower than the cream mixture, a temperature gradient is created and the cream mixture easily freezes. After about five to ten minutes of shaking, you will have yourself some fresh-churned sciencey deliciousness! Enjoy!
Science Experiment Ice Cream:
½ Cup Heavy Cream
½ Cup Milk
¼ Cup Sugar
¼ Tsp. Real Vanilla Extract
Crushed or Shaved Ice
1 Cup of Table Salt or Sea Salt
1 Quart-Sized Ziploc Bag
1 Gallon-Sized Ziploc Bag
Here is a video of my and a few of my pals doing this experiment with my daughter. We did this last year and my daughter STILL talks about it!
Shakin' it up:
The big reveal:
For more information check these out:
General Chemistry Online Why does salt melt ice?
Sciencebase Salt lowers the freezing point of water
Jeanne Garbarino, Double X Science Editor