Tuesday, September 1, 2015
How Cool is This?!
This must be the last gasp of summer-time to make ice cream!
It counts as science of course. There really is no better way to see that phase changes require energy.
We actually used an ice cream mix that someone had given us, so it was just add cream and mix, and put the combination into the inner container. Pina colada flavor, yum!
We packed layers of ice and salt in the outer container, and turned the machine on.
Our machine has a dasher (like a mixing paddle) that stands still while the inner container moves around it. It takes about half an hour to churn and freeze the ice cream.
In the mean time we measured the left over ice: 31 degrees (-1 C). The salt was at 78 degrees (25.5 C)
Then we measured the ice and salt: 2.3 degrees (-16.5 C)!
What the heck?
For the ice to change from a solid to a liquid, it needs to absorb heat, and it's absorbing it mainly from the sides of the metal inner container (the outer container is plastic).
The salt melts the ice faster so it causes more rapid cooling.
Eventually our thermometer registered -4 degrees, but that actually shows that our (cooking) thermometer isn't accurate at extremely low temperatures.
Zero was chosen as the set point for Fahrenheit because it is the coldest point that saturated saline (fully salted water) can get before freezing.
Which is why I prefer Celsius, frankly! Everyone knows how cold water is when it freezes, it's something we experience all of the time. Saturated saline is...less common.
But it makes great ice cream!