Precipitation

No, it's not raining or snowing (at the moment). Last week we did two (of four) small demonstrations from The Joy of Chemistry to illustrate how different conditions affect whether a solid will go into a liquid in solution or fall back (precipitate) out. Here, in my non-official lab report style, are our results:

Demo 1: Baking Soda
Effect of Temperature and Amount

Materials

Baking soda
Water
Glass measuring cup
Plastic spoon

Directions

1. Add a pinch of baking soda to 2 cups of water. Stir to dissolve.
2. Keep adding pinches (about 10-15) until solution starts to become cloudy and baking soda fall to the bottom.
3. Microwave glass for 30 seconds, until warm.
4. Is solution clear? Does precipitate dissolve with stirring?

Observations

It took 45 "pinches" of baking soda to get enough precipitate on the bottom of the cup to notice. The water was only barely cloudy. Measuring out the baking soda in 1/4 or 1/2 teaspoon amounts -- or using less water -- would make the process go faster.

Heating the cup in the microwave even a small amount did allow all the baking soda to go into solution.

As a demo for kids, less than impressive.

Demo 2: Chalk in Water and Vinegar

Effect of Condition of Salt and of Solution

Materials

Prang* white chalk
4 plastic cups
Water
Vinegar

NOTE: Prang brand chalk worked great! (Not like our first attempts with other chalk.) Still haven't found it for sale locally, though -- I found some in a school I visited and borrowed a piece.

Directions

1. Break chalk into 4 pieces.
2. Crush 2 teaspoons of chalk.
3. Fill 2 drinking cups with 1 cup of water. Fill other 2 cups with 1 cup of vinegar.
4. Place 1 whole piece in first cup of water. What happens?
5. Place 1 teaspoon crushed chalk in water and observe. Stir. What happens?
6. Repeat with chalk and vinegar. What happens?

Observations

Whole piece in water: Nothing
Powdered chalk in water: Water got very cloudy
Whole piece in vinegar: Chunks of chalk broke off in layers and floated to the top!
Powdered chalk in vinegar: First it foamed up. After a few minutes, chunks started bobbing up and down!

(Took some lovely video of the bobbing effect which came out sideways. When I can convert the MOV file to AVI so I can rotate it in Windows Movie Maker -- or get more chalk and re-shoot it -- I will post it!)