SOURCE Lesson Plans Detail

The Science Behind Ice Cream

Topic Chemistry
Program Brown Science Prep
Developed by Sandra Chang, Frank Chung, Alfonsina Simo
Developer Type High school students

Overview / Purpose / Essential Questions

Performance / Lesson Objective(s)

  • To provide an introduction to basic chemical properties, phase changes, and Ostwald Ripening

  • To apply this knowledge to the production of ice cream

Lesson Materials

-Ice, lots

-Half and Half

-Salt, lots

-Sugar

-gallon ziploc

-pint ziploc (preferably with the zipper seal)

-vanilla extract

-Maybe some gloves (something to keep hands warm so it's easier to keep working the ice cream)

-Liquid Nitrogen

-Some big clean container

-syringes

-melted ice cream


Lesson Motivation

  1. Introduction:


Today we’re going to be talking a lot about energy… and of course ice cream!


Question: What is your favorite thing about Ice Cream? (hopefully one of the students respond talking about the texture ? if not, a mentor can mention the texture)


Question: This texture is actually a by product of the physical phase of ice cream. What physical phase do you think ice cream is? (After discussing, show picture below)


Answer:

http://iopscience.iop.org/0031-9120/38/3/308/pdf/0031-9120_38_3_308.pdf

This picture shows that ice cream actually has all three phases of matter.The air bubbles displayed represent the gas phase, the sugar solution is the liquid phase, and the ice crystals are in the solid phase. Notice how the ice crystals are tiny. The scale bar below is 100 microns, which is about the thickness of a piece of paper, which is why the ice cream is smooth!

Fun fact: ½ the volume of the ice cream you purchase is actually AIR!





  1. Phase Changes:

First let us look at what atoms look like in the three different phases (Draw different phases on board w/o characteristics or labels).


Now can anyone guess which one of these boxes with atoms inside represents the solid phase? (let the students answer!-if no one knows give it away) Go over characteristics with students (the ones under the pictures). Then do the same with liquid and gas phases.


Q1: So what do we think will happen to these atoms in the solid and liquid phase if we increase the temperature?

A1: In the solid state the atoms will start to move, as a result of added energy due to the temperature increase, all this moving will let them go into the liquid phase. The same goes for atoms in the liquid phase, except these atoms will go from the liquid phase to the gas phase.   

Q2: What do you think will happen to the atoms in the liquid and gas phase if we decrease the temperature?

A2: In the liquid phase, the atoms will have a slowing of movement and change into the solid phase (known as freezing). In the gas phase, the atoms will also slow down changing into the liquid state (known as condensation) or the solid state (known as deposition).

 

  1. Phase Diagram:

Draw diagram on board without phase changes and see if students can name all the phase changes.


Now let us look at how these phases interact with each other via another graph! Show/draw graph:


This graph is specifically for water and it shows us a more quantitative version of the other graph, in the sense that we get to see how the pressure and temperature can affect the phase changes. For example, this graph shows us that water freezes at 0 degrees C and boils at 100 degrees C at 1 atm.

-The triple point on this graph is where the temperature and pressure at which the three phases coexist in equilibrium.

-The critical point or critical state is the point at which two phases of a substance initially become indistinguishable from one another.

-The lines separating the phases represent the different phases which we first identified. The line between ice and water has a negative slope. This is because water in the solid state, or ice, is less dense than water in the liquid phase.

-For other substances, this line would have a positive slope revealing that the solid phase for that substance is more dense than the substance in the liquid phase, which is what we know intuitively.


Q3: So this is the phase diagram for a pure substance. However when we are making ice cream, we are not dealing with a pure substance. In fact, we are dealing with a salt and water solution. Does anyone have any ideas about what will happen because of this?  

A3: Since we are not dealing with a pure substance the melting points and freezing points will change. This can be shown in this graph:

https://www.youtube.com/watch?v=izRMgYSW4Lk

(video that talks about concepts in graph)


Because the red lines move below the pure substance lines, the substance requires a lower temperature and lower pressure in order for phase changes to occur.


Optional if time permits:

https://www.youtube.com/watch?v=ph8xusY3GTM

or (same link)

http://bit.ly/1t9efxT

Video explaining and showing supercooled water.



  1. Ostwald Ripening:

We covered one thing that makes ice cream really tasty is its texture. Part of that is that we have a nice mixture of all three phases of matter in the ice cream. We’re going to focus on the solids.

Q1: What is the solid, liquid, gas in the picture?

A1: Ice= solid, sugar solution = liquid, air bubbles = gas

Notice how small the ice crystals are. They are smaller than the 100 micron scale bar, which is about the thickness of the sheet of paper. So these ice crystals are TINY!

Q2: Has anyone ever left ice cream in the freezer for a long time? What happens?

A2: One thing is that the ice crystals grow really big, the ice cream becomes exceptionally crunchy

Q3: Does anyone know why this happens? Or what this effect is called?

A3: It is called Ostwald Ripening!

Ostwald Ripening explains why crystals get larger over time; small crystals get smaller and big crystals get bigger! This is driven by thermodynamic processes, so the energetics. Most things “want” to be as low energy as possible; imagine that most crystals are lazy and do not want to be high energy. In a crystal, atoms or molecules on the surface have higher energy (it’s because in part due to things called dangling bonds, but that’s beyond the scope of this lesson) whereas the atoms or molecules on the inside of crystals have lower energy. If you have a small crystal, a larger percentage of the molecules or atoms are touching the surface so it’s actually higher energy. As a crystal increases in size, the percentage of surface atoms or molecules decreases, so a large crystal is lower in energy than dozens of small crystals (more surface area = more high energy species). This is something that happens to ice cream over long periods of time, formation of nanoparticles, and formation of certain geologic rocks.

Q4: How does this affect what we’re doing today (making ice cream)?

A4: Smaller ice crystals mean a smoother ice cream, which is generally desirable. The faster you can cool something off, the smaller the ice crystals will be and the smoother the ice cream.

Q5: What do you think will have smaller ice crystals: this bag ice cream or the liquid nitrogen ice cream?
A5: LN2 ice cream since it’s so cold (about -321F !)


Lesson Activities

  1. Instructions (Traditional Ice Cream):

-One bag for every TWO students

-place ½ C Milk, 1 Tbsp sugar, ¼ tsp vanilla extract into smaller ziploc bag— seal the bag well!

-shake bag carefully to mix ingredients, dissolve the sugar, remove as much air from the bag as possible

-place ice and salt (~6Tbsp) into the larger bag

-place the small ice cream solution ziploc into the larger ice bag, seal large ziploc

(you should now have a bag inside a bag, both sealed well)

-shake bag, move it around, play with it, just carefully avoid breaking either bag


  1. Instructions (Dippin dots)

-(mentors) fill a small styrofoam bowl with LN2 (for each student)

-use syringes or transfer pipettes to draw from the bowls of melted ice cream flavors available.

-slowly drip the droplets of ice cream from the syringe, making tiny dots of ice cream!


Wrap up / Conclusion

Discussion about Liquid Nitrogen

Q: Why is liquid nitrogen dangerous?

A: It’s cold! But breathing it in is not actually harmful. Most of the air we breath already is nitrogen! It’s also dangerous because it can quickly displace oxygen we need. One liter of liquid nitrogen has about 810.6g of N2. If that were to be at room temperature in its gaseous form, that would actually be 648 liters of gas! That’s almost like 324 bottles of soda worth of gas!


Why dots are spheres?

It is the most energetically favorable shape for them.


Leidenfrost effect explained and demonstrated (if time permits, or background)

DO NOT let students attempt to replicate this

https://www.youtube.com/watch?v=wV7g8L633Sg

(Youtube: Hand vs. Liquid Nitrogen - Revisited)


Alignment Info

Audience(s) High school students
STEM Area(s) Chemistry
Standard(s)
Physical Sciences (RI GSE) PS1.K-2.1a
Students demonstrate an understanding of characteristic properties of matter by … identifying, comparing, and sorting objects by similar or different physical properties (e.g., size, shape, color, texture, smell, weight).
Activity Type(s) Hands-on
Lecture
Grade Level(s) High School
Version 1
Created 01/13/2015 05:55 PM
Updated 12/20/2018 11:58 AM