Materials: 3 jars, skim milk, vinegar, baking soda, paper towels, coffee filter, paper to test glue, stirring rod

Age range: 5-10 (kindergarten - fifth grade)

Introduction

With this experiment you can make surprisingly good glue from common kitchen items. I started by having my daughter draw pictures of insects of her choosing on a piece of paper. I then had her draw a picture of a landscape with flowers on a second piece of paper. I explained to her that we were going to use special glue to put the insects in the garden.

Procedure

1. Pour about 100 ml (roughly 1 cup) of skim milk into one of the jars.
2. Add about 15 ml (roughly 1 tablespoon) of vinegar to the milk.
3. Stir the mixture (optional: heat the mixture slowly).
4. Cover the top of one of the empty jars with the coffee filter.
5. Filter the milk/vinegar mixture by pouring the liquid through the coffee filter.
6. Once the liquid has drained, scrape the solid residue from the filter.
7. Use paper towels to dry the solid as much as you can, and return the solid to a clean jar.
8. Add a small amount of baking soda to the solid. (How much you should use will depend on the amount of solid you have and how well it was dried. Start with about 1/4 teaspoon.)
9. Mix and test the resulting glue.

Leave the remainder of the glue to dry for later observations.

Observations

By adding vinegar to milk we produce a solution containing white solids (precipitation), and by filtering the milk we separate the solution into two substances (called curds and whey). The curds can be dried with a paper towel to produce a cheese-like substance.

The baking soda, when added to the curds, causes them to become a sticky glue. We noted bubbles coming from the solid when the baking soda was added.

The glue dries to become a plastic-like substance. It has different physical properties than the original milk and vinegar individually.

Questions:

• Why does milk, when mixed with vinegar, produce a solid?
• What is the liquid left in the jar after the solution is filtered? What color is it?
• What does the baking soda do? What happened when you added the baking soda to the solid?
• Why do the physical characteristics of the solid change at each stage?
• How does glue work?
• What if you replace the vinegar with another acid? What about lemon juice?

Discussion

Milk contains a variety of substances suspended in water.

Much of the protein found in milk is in the form of casein. Casein consists of long chains of protein molecules that are grouped together into small droplets called micelles.

Full fat milk also contains fat globules. A chemist might describe milk as a ?colloidal dispersion of fat in water.? (Translation: lots of little fat globules suspended in water.) Although most of the fat globules have been removed from fat free skim milk, it still contains most of the casein, which is why it works well for this experiment.

As a side note, these micelles and fat globules are approximately 1 micrometer in size, which is close enough to the wavelength of visible light to cause scattering. This is the reason milk is white.

Altering the acidity of the milk causes the structure of the proteins in the casein to change. Before the acid is added, the proteins are in the form of tiny micelles. After adding the acid, the proteins form larger structures.

By straining the solids (called curds) through the coffee filter, you can remove a majority of the liquids. This tells you something about the size of the porous holes in the coffee filter: they must allow objects that are 1 micrometer in size to pass through (as coffee filters cannot strain casein from milk!)

Next, by drying the casein with a paper towel you can remove most of the remaining liquid. Adding a base (sodium bicarbonate) to neutralize the acid (and produce water) causes the casein to become soluble again, except now the concentration is much higher.

What makes this experiment so cool is that it demonstrates several fundamental chemical processes. An acid is used to precipitate a solid, which is then extracted and purified. Next, a base is then used to reverse the reaction leaving behind a highly concentrated form of the original material.

Here is a question for the curious: If you take ordinary full fat milk and shake it for a long time you produce another type of solid material. How is the production of this solid different from the method used in this experiment?

It is worth noting that casein is used for a variety of purposes, including food additives for use in items such as nutrition bars, salad dressings and many more ? check the ingredient list next time you are at the supermarket. Non-food products also use casein, some examples being adhesives, plastics and cosmetics.

Finally, examine the residual glue after it has dried a few days. It should dry into plastic like material; perhaps you can see why it is used in the production of some buttons!

Our Final Result: A beautiful butterfly garden, butterflies attached courtesy of our home made milk glue!