Nucleation

Experiment Title:

Can sugar and milk make a delicious treat through nucleation?

Objective:

Learn about how and why nucleation works and see if we can use the principles of nucleation to create a tasty treat

Background on Nucleation:

Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Seems like a pretty intense definition that probably makes little sense, but let’s break this idea down. Thermodynamics studies the energy of materials, think about water we commonly see three states of water (solid, liquid and gas). A solid block of water or ice as we call it has little movement in the molecules that make it up, the molecules are fixed in place in a lattice that keep the molecules from moving, to illustrate this think of the basic shape of a water molecule:

It is an Oxygen bound to two hydrogen atoms and makes a v shape. When it is a solid the molecules stack together and form a lattice:

When it is joined together in this way it has less movement and less energy. When water is heated to a liquid state or a gas state it has more energy than in the solid state and the molecules can move all around and get in many different configurations:

So now we understand that a thermodynamic state talks about the movement and energy of the atoms or molecules that are part of that material. 

Now let’s talk about a phase, a phase is a region or area in which all of the material in that area is the same. Think about it this way, if I was to look at a bouncy ball, the ball itself is the region or the area that we are talking about. Now in that bouncy ball we assume that all the material in the bouncy ball is solid and the same (ok a lot of bouncy balls are different colors all over but let’s just assume we have a single colored bouncy ball with all the material being the same and in the same state (i.e. all solid and the same). So we have established that thermodynamic has to do the with energy of the material and phase has to do with a certain area in a material all being the same.

Now when talking about a new structure this could mean a materials going from a liquid to a solid, think about freezing water, the more pure (i.e. does not have stuff like dirt or hairs in it) the longer a material can keep from freezing when at the freezing temperature of that material. Now with nucleation an impurity or contaminant creates a sight in which that material can easily join together with other parts of itself and form a lattice and go from being a liquid to a solid.

Materials

Outer mixture

• 1 gallon-size Ziploc bag
• Ice Cubes
• 1/2 cup Kosher salt

Ice Cream Mixture

• 1 pint-size Ziploc bag
• 1 cup half and half
• 2 tablespoons sugar
• 1/2 teaspoon vanilla extract

Method

Prepare Experiment

1. Mix together half and half, sugar and vanilla extract (this can be made in a stockpot and scaled up for the entire STEM group then divided up into their different plastic bags)
2. Put 1 cup of ice cream mix into the pint size bag and seal it (does not hurt to double bag it but will limit the freezing capability of the ice and if it is a hot day you will have more of a slush than ice cream in the end)

Run Experiment

1. Place the kids ice cream mix into the gallon Ziploc containing ice and salt
2. Seal the bag well
3. Have the kids mix the ice cream for 10-15 minutes

Analyze Data

1. Ask the kids what happened to their ice cream in that time
2. Add toppings and let the kids eat!

Conclusions

Making ice cream is a fun and easy way to demonstrate nucleation. This is also an easy way to show changes in states of matter (i.e. from liquid to solid).

Make It Your Own

To make it your own try using different mediums to make ice cream in this manner. Use soy milk, buttermilk, soda, etc… How do they change? Do all of them freeze to ice cream? Think about what is in some of them and ask yourself what materials might help or hinder nucleation of your modified ice cream mix.

Extension Activities to do at home

Ice cream making is a great way to show nucleation but think about other ways nucleation can happen. One of the most fun things to do is to create a soda fountain. In this experiment mentos cause the CO2 to nucleate out of the solution all at once causing the gas to expand and shoot out of the bottle bring soda with it. Lots of fun and an absolute mess to work this, do this on a tarp or on the grass and stand back as soon as you drop the mentos into the bottle.

Another way to test nucleation that is a little less messy is to add pure vinegar into milk (weak vinegar will not produce a spectacular result so make sure you use the strongest vinegar you can get at the super market). Add milk into the vinegar slowly and stir, what should happen is that the protein, which is in suspension in the milk should precipitate out and form a solid in the vinegar cup, use a spoon or a stir stick to show this to the kids as the vinegar will become cloudy with the milk and needs time to fully settle out.