Salty Chalk Soda

This activity will study CO₂ and carbonates (salts containing CO₂), with an emphasis on how living things produce and use them.

Materials:

Chemicals:
- water H₂O - distilled is best
- vinegar - acetic acid
- "sidewalk salt" of a specific kind - calcium chloride CaCl₂
- baking soda - sodium bicarbonate NaHCO₃
- chalk, seashells, limestone, coral, ... - various natural forms of calcium carbonate CaCO₃
- table salt - sodium chloride NaCl (kosher salt dissolves best, but cheap salt has best crystal shapes)
- seltzer - carbonic acid H₂CO₃ = H₂O + CO₂
- (optional) bromthymol blue indicator
Equipment:
- paper, pencils/pens, etc. for WRITING DOWN OBSERVATIONS
- bottle for water porting and waste liquid collection (Note: safe to dispose down drain)
- 3-4 medium (8 oz) clear plastic glasses
- plastic shot glass for each student
- paper towels
- masking tape for labels on shotglasses
- (optional, if available) digital camera and TV to attach it to

Activity

Students will do for themselves a "magic trick" (which the teacher can first perform in front of the class) - "making chalk appear from nowhere". We'll then study the chalk by testing it with acid (vinegar), and compare its reactions to other common substances, especially some made by living things in the ocean (e.g. chalk!).

Step-by-step procedure for the "experiment":

Safety speech: keep away from eyes, faces, mouths, don't splash, no throwing around, especially at each other, NO TASTING!!...
Put bottle of water in view of class, commenting on "main ingredient of ocean"
Distribute shotglasses among class, have paper towels handy, get students to prepare lab sheets for notes and drawings
Put some CaCl₂ in one glass, some NaHCO₃ in another
Add water, stir til dissolved, and discuss how its disappearance is a mystery we've solved earlier...
Half of students in each group put a small piece of tape on their shotglass, as CaCl₂ label
Give each student 1/2 shot of appropriate solution
Have pair of students (w different solutions) mix into UNLABELED shotglasses, observe what happens, let stand a while, write down procedure followed, everything observed, draw if inspired
While precipitate is settling, distribute chalk, rocks and shells, pick a shell (for TV/camers display, if available), have all try to draw it
Ask if anyone knows what will happen if vinegar is added to baking soda? (Bet most do!) Demonstrate to class
Have them carefully decant leftover water in chalk glass into extra vessel, for later crystallization (hope to see NaCl cubes)
Distribute small amounts of vinegar in now-empty shotglasses, have them test chalk, various rocks, shells, finally their precipitates
Last but not least - CLEANUP TIME!!

If still time left, take out bottle of seltzer, get class to describe everyday experiences (e.g. no bubbles visible til opening, opening after dropping, going flat...). Gasses dissolve, too! What gas is in soda? HINT: same as in our experiment - we made "salty chalk soda!"

If microscope available, set up, pass around artemia egg samples for examination, then put under microscope to show how much magnification the microscope provides; do the same for for NaCl crystals (use brand of salt with good crystals! cheap generic is usually fine).