IS 220 Class 4 - Disappearing Salt
Review last week's measurement procedures, and repeat liquid measurements, so each group is ready with 2.0 g NaHCO3 and 50 ml H2O.
Mention CONTEST AMONG GROUPS: try to use least amount of water for your salt! Remind them of Griffendore vs Slitheran, and VOW OF SILENCE with demerits. Award 50 points for least water, 40 points for 2nd least, 30, 20, 15, 10, 5.
Materials
- paper & pencils - don't even proceed unless all students have paper with proper header information
- 1 dissolving cup per group
- one scale (triple beam balance) if available, for reference - or instructor weighs out exact amounts for class' use
- graduated cylinder(s)
- 2.0 g NaHCO3
- water
- ice
- vinegar
- NaCl
- hotpot
- camera, microscope, TV
- bromthymol blue (for demo at end, if time)
- pH paper
Procedure
- each group will put a little ice in water (cold, and hot), and describe/record what happens (in writing, on data sheets! temperature of water if thermometers available)
- each group will carefully measure out 50 ml H2O, & record this step as part of procedure
- SLOWLY add water from cylinder into beaker with salt sample, pause, stir (REMEMBER CONTEST!)
- STOP when salt is all dissolved, and figure out how much water was used (how? - arithmetic using 50 minus amount left in cylinder) - RECORD RESULT!
- break in "vow of silence" for inter-group comparisons
Discussion and Analysis
- Try to explain what happens when ice melts and when salt dissolves (Hypothesis); similarities, and differences
- Scientific method - develop a system to test guesses/hypotheses
- Solicit hypotheses re disappearing ice, disappearing salt, crystal shape of NaCl
- are the 2 processes the same? (they both result in similar appearances)
- have class devise ways to tell the difference (one glass is pure water, one has something in the water)
- hold up vinegar as a hint
- put some yellowed bromthymol blue in each (melted ice water, bicarb solution) & see a difference (?)
- blow through a straw into blue solution of bromthymol blue; add seltzer to blue
- Leave some of salt water from experiments in evaporation trays, for daily observation and description in diaries (write & draw!)
- one tray of melted ice water (LABEL IT!)
- one tray of students' bicarb solution (LABEL IT!)
- one tray of NaCl solution (LABEL IT!)
- keep watch on them, and draw, write about what appears (Note: NaCl crystals are easiest to see before too many form, so look every day)
Leave time for cleanup!!