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.
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
- 2 beakers
- hotpot
- water (a pint will do)
- (optional, if available) camera, TV, microscope (to show whole class at once the shapes of NaCl crystals)
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 & discipline level permit).
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.