What You Need:
- 1/4 cup milk
- 1 tablespoon vinegar
- Sturdy paper towel or coffee filter
- Baking soda
- Measuring cup
- Small jar or cup (or other wide-mouth container; baby food jars work well).
- Craft stick, straw, or plastic spoon for stirring
- Scraps of paper for testing
To Do and Observe:
- Pour ¼ cup milk into the measuring cup. What does it looks like?
- Add 1 tablespoon of vinegar. Stir the liquids with a stick until the mixture stops changing. What does it look like? What does it smell like?
- Place a paper filter (coffee filter or paper towel) over your jar or cup. Without tearing the paper filter, gently push the paper filter into the jar until it forms a bowl or well that dips into the jar. The paper filter should not dip more than halfway into the jar.
- Slowly pour the milk/vinegar mixture though the paper filter into the jar. This step requires some patience. While waiting for the mixture to drip through the filter, carefully watch and note what happens.
- Lift the filter paper and gently squeeze any remaining liquid into the jar. What does it look like? What does it smell like?
- Save the paper filter. Dispose of the liquid from the jar. Dry the jar or cup.
- Using the stirring stick, gently scrape the white clumps from the paper filter. Place the white clumps into an empty, dry jar or cup. Record what it looks like.
- Add a pinch of baking soda (about an 1/8 teaspoon). Watch carefully. What happens? Stir the mixture.
- Try using the mixture to glue scraps of paper. Does it work?
What's Going On:
The major protein in cow’s milk is called casein. Like other proteins, casein has a three-dimensional structure that determines its behavior, characteristics, and properties. Denaturing a protein means changing its shape, which can make proteins look and act differently. In this case, the acetic acid (vinegar) denatures the casein. As a result, the casein turned into a solid white curd. When baking soda (a base) is added to the vinegar (an acid), the acid and base react. The chemical reaction produces new chemicals, including water and carbon dioxide gas. Carbon dioxide (CO2) is the same gas we exhale from our lungs, and the same gas in the bubbles of soft drinks.
Proteins are long molecules made of smaller molecules called 'amino acids.' Large molecules made of repeating units are called 'polymers.' Proteins are naturally occurring polymers. Synthetic polymers are often used by industrial chemists in the development of products such as glues, plastics and fabrics.
Have your child experiment with different substances and/or different formulas to make glue. Have them carefully measure and record their ingredients and processes. What happens if you change the formula (or recipe) for this glue? For example, what if you use less liquid? You might get a glue stick or a useless glob. You might get glue that dries really fast. You might get glue that dries up before it has a chance to stick things together. You might get better glue. You might get glue that doesn’t work, etc.
Encourage your child to think of other things that change proteins. For example, cooking an egg will denature the egg protein, called albumin. The denatured albumin changes from a clear liquid to a solid white substance.