Class 4
Disappearing Salt
Review last week's measurement procedures, and repeat liquid measurements, so each group is ready with 3.0 g NaHCO3 (or NaCl, or MgSO4), and 50 ml H2O.
Perhaps 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 one point for each ml left over (but point out results differ for different salts)
Materials
- paper & pencils - don't even proceed unless all students have paper with proper header information
- 1 weighing cup and one dissolving cup per group (or per student)
- scales (triple beam balances)
- graduated cylinders
- 3.0 g NaHCO3 (or other salt)
- water
- ice
- hotpot
- camera, microscope, TV; VCR for TV experiments
- 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!)
- (if thermometers available) take temperature of water - each sudent takes a turn, writes down exact time and temperature observed
- 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!)
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- 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, crystal shape of NaCl
- Leave some of salt water from experiments in evaporation trays, for daily observation and description in diaries (write & draw!)
- Leave some brine shrimp eggs in closed container of salt water, & observe, record
Leave time for cleanup!!