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SCSP |
Mixtures and Solutions
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Items Needed:
The skim milk can also be diluted with water for a 50/50 composition.
What to do:
1. Pour out about 50 mL of skim milk into a cup.
2. Add about 2 grams of citric acid or 10 mL of vinegar to the milk and stir.
3. Filter the mixture through the coffee filter.
Or with the hot plate -
1. Pour out about 50 mL of skim milk into a cup.
2. Add about 2 grams of citric acid or 10 mL of vinegar to the milk and stir.
3. Heat milk on the hot plate until it is warm for 3 minutes - do not let it boil!
4. Filter the mixture through the coffee filter.
What should be observed:
With the addition of the citric acid or vinegar, you should observe small white particles in the milk. These are large enough to filter out. With heat, the particles should be larger and easier to filter.
What is happening:
The added citric acid (or almost any acid, like acetic acid found in vinegar) acts the same way as when bacteria convert the lactose to lactic acid in the cultured dairy products. Lactic acid, or in this case citric acid, promotes coagulation of the casein particles. These are the visible white clumps that can be filtered out. The addition of heat actually helps promote polymerization of the protein. The differences between the heated and the unheated particles should be larger particles for the heated milk mixture.
Other things to try: This experiment can also be performed with whole milk. The clumps in this case may be slightly bigger than with skim milk. More noticeably, though, the whole-milk particles will feel smoother and creamier, resembling something like softened cream cheese. This is due to the higher fat content in the whole milk.
Items Needed:
What to do:
Pour a small amount of each kind of milk into separate bowls. When the milk is steady and not moving, add one drop of food coloring to each bowl - the bowl and milk must be perfectly still. Watch how the color spreads.
What should be observed:
The food coloring in the skim milk should spread quickly and become faint in color. The coloring in the cream will not spread as much or as fast as in the skim milk. The whole milk should behave somewhere in between.
What is happening:
The food coloring is water-based and will travel and diffuse better through the aqueous (water) medium than through the fat. So in skim milk, which has a very low fat content and is predominantly water, the food coloring spreads rapidly, whereas in cream the food coloring will take longer to disperse since there is less water for it to travel through.
Other things to try: After the food coloring is added, dab a cotton swab with soap or detergent and dab the cotton swab in the center of the food coloring. This should cause the food coloring to disperse rapidly into the milk. The soap is a surfactant that changes the water/fat interactions and allows for rapid diffusion of the food coloring into the milk.
Needed:
What to do:
In the larger plastic baggie, fill about 1/3 with ice. Add several tablespoons (about 4-6) of salt. Shake to mix the salt and ice together.
In the smaller plastic baggie, put about 4 ounces of milk, 1 tablespoon of sugar, and 1/4 teaspoon of vanilla (or other flavoring). Seal the baggie, getting rid of as much air as possible. Shake the bag until the sugar is dissolved and the flavoring is well mixed.
Place the small baggie inside the large baggie, seal the large baggie getting rid of as much air as possible. Shake the baggies for about 5 minutes.
Remove the small baggie - the milk mixture should now be colder and edible as ice cream.
What is happening:
The salt added to the ice depresses the melting point of the ice. The lower temperature allows for the physical change from a liquid milk state to a more frozen milk mixture. The salt/ice mixture demonstrates a more physical use of mixtures and solutions. The milk itself is a mixture while the sugar is actually dissolved in the milk. For more on this phenomenon, go HERE
Other things to try: The temperature of the ice can be taken before and after the salt is added to show that the temperature has changed. Other types of milk products can be used as well.
