Examining Freezing Points of Solutions

Investigate how changing a mixture alters freezing point characteristics.

Background

Ice cream preferences extend far beyond just craving chocolate or vanilla.  Ice cream connoisseurs and marketing teams have collected loads of data on how fat, air, and serving temperature all affect consumer enjoyment.  After all, they want you picking their tubs over all the other choices in the supermarket aisle!

Besides the ice cream recipe itself, much of the finesse of a delicious ice cream comes down to how it freezes! A faster freeze creates smaller crystals for a smoother ice cream.  Current technology uses anything from liquid nitrogen to giant machines which aerate and freeze in a very particular way.  But ice cream got its start with some humble science, using a mixture of ice and salt as the freezing agent.  

But why the combination of salt and water? Why not just use ice and turn the water inside the ice cream into ice itself in an “equilibrium” type way?  The problem is the sugar — when mixed with the water (inside the cream) it now doesn’t freeze at 0*C like your ice cubes do — it needs to be colder.   Check out the data below:

It has been found that the perfect serving temperature is somewhere around -5.5*C.  For our purposes, we need to weigh the following factors for how to create our perfect ice cream:

Goals for the ideal ice cream:

• Freeze as fast as possible

• Get as close to the ideal freezing temperature as possible

Examine the following data, and suggest a method for creating the best ice cream based on these goals!

Dataset

This is a simulated dataset based on real data collected in classrooms.

Three different water solutions were evaluated for their freezing points. Each solution was poured into a test tube with digital thermometer inserted, and placed in a freezer set to -25℃.  The test tubes were given a little stir every few seconds to ensure the thermometer was reading a temperature representative of the solution.  The solutions include distilled water, salt solution with molality of 2.25 mol/kg (mixture 1), and salt solution with molality of 3.76 mol/kg (mixture 2).

Variables

Time (min) - this numerical variable describes the length of time the experiment has been conducted. Time = 0 is when energy begins to be removed to get the solutions to freezing.  Measured in minutes.

Temperature - this numerical variable describes the approximate temperature of the solution. Measured in degrees celsius. 

Mixture - this categorical variable indicates which solution is being explored. In our dataset it can have three different values (water, Mixture 2, Mixture 1).

Activity

1. Make a graph of Time on the x-axis and Temperature on the y-axis.  Select Mixture for the z-axis and paste your graph below: 

Extension 1: Any real data graph will have a dip like this (though often when viewing a phase change graph this has been removed).  What do you think is physically happening during this dip?  Why would the temperature suddenly increase again?

2. Using your graph, estimate the freezing temperatures for each mixture.

3. Do any of the solutions seem to reach their freezing point most quickly? Be sure to back up your claim with evidence from the graph.

4. Which mixture freezes completely, first?  Be sure to back up your claim with evidence from the graph.

5. Which mixture would you suggest to ice cream makers to use to freeze their ice creams?  Remember that we have the following goals:

Want an Answer Key? Fill out the form below.