A "Solution" to the Mystery of the Disappearing Salt
The disappearance of salts when water is added is a Mystery! "Why does this happen?" - figure out Nature's magic tricks.
Distribute samples to each group of students: NaCl and pinch of brine shrimp eggs for naked-eye and (if available) magnifying glass inspection (try to draw what you see!). If possible, set up projector with laptop and attached USB microscope (or recorded video) to show whole class a more magnified view. Then add water to NaCl crystals under the scope and watch patiently - have them describe (in writing, drawings, etc. what they observe).
Distribute a soda bottle with a sugar cube and hexnuts (beebees would be even better, but are nearly impossible to get in NYC and surrounding area). Have them shake the bottle and describe what happens to the sugar cube as it's pummelled by the hexnuts. Does it look the same as the magnified salt dissolving?
Prod class to come up with descriptions of what actually happens when solids dissolve - breaking apart into tiny pieces too small to see ("atoms" and "molecules"). Then start them on a modelling activity, using styrofoam spheres to represent atoms of Na, Cl, H, and O.
Materials
- paper and pencils/pens (don't proceed til everyone ready to record!!)
- cups, NaCl, water
- styrofoam spheres of different colors and sizes: small Na (black), Cl (white), H (blue); large O (white)
- toothpicks to join atoms together
- NaCl (N.B. kosher best for dissolving, Morton or other generic best for cubic crystal shape), other salts
- brine shrimp eggs
- empty soda bottles, hexnuts, and sugar cubes
- 2 beakers
- hotpot
- water (a pint will do)
- (if available) camera, TV, microscope (to show whole class at once the shapes of NaCl crystals, brine shrimp eggs)
Procedure
- Groups observe carefully as salt dissolves, asking questions (get them to list in advance)
- Get discussion going about how water dissolves things, and names of smallest pieces formed ("atoms", "molecules")
- Ask for descriptions of students' experiences with magnets - get them to come up with "opposites attract, likes repel"
- Distribute materials for molecule modelling - each student gets (at least) 1 Na or 1 Cl, maybe each table gets 1 O and 2 H's
- "Puppet show" of atomic interactions to assemble a crystal of NaCl, & have class put all their atoms together
- Hold up own model of assembled crystal, then "puppet show" of water molecule breaking it apart
- Introduce ideas of energy, motion, heat, with more of "puppet show"
- Make sure students write down &/or draw basic ideas (e.g. magnets), and draw what they've made (NaCl crystals, H2O molecules)
Analysis and Discussion
We have now done two steps of a good "mystery-solving" procedure:
- observe (in detail! RECORDED in words, numbers, and drawings - also photos)
- made a guess about how it happens - "atomic" theory of materials, and "kinetic" theory of heat - explains NaCl shape, and disappearance
We've also fixed the ideas in our minds by making "models" representing these hypotheses. Now time for the next steps:
- make predictions based on our hypotheses
- test those predictions
Have class use logic & common sense to predict whether cold or hot water will dissolve some salt faster - refer back to "puppet show" modelling done earlier. Then do a test with equal amounts of salt in 2 beakers, and equal amounts of water, one hot and one cold. RECORD OBSERVATIONS! Also record steps of "scientific method". Finally, talk about other predictions of atomic/kinetic hypotheses, (e.g. melting, boiling, expansion and contraction, gasses dissolving in water; enact with groups of students (if time permits).
Take some of the salt water produced in the demo, and put it on an evaporating tray for examination next time - we'll look at crystal shape.
Also put some brine shrimp eggs into some salt water, have the class observe frequently over the next week.