precipitate reactions

Discussion in 'Supporting Chemistry' started by clairelucas, Jul 14, 2017.

  1. I've just had a last minute set up for lead nitrate /KI precipitation reaction.. I only gave 0.01M lead nitrate, (though this was yr10), but thought it gave an instant cloud of yellow precipitate, not the slower forming (only over a few seconds) crystalline precipitate. Were my concentrations or storage of the reagents wrong, or am I misremembering at this dilution? As a backup I gave him CuSO4 and NaOH, which worked fine when trialling before sent out, but left a white/pale blue solid aswell as the usual cloud of precipitate.. Any thoughts?
     
  2. I only use 0.005M lead iodide and it produces and immediate yellow precipitate. In order to get twinkly crystals you have to filter the ppt and dissolve in hot water. As it cools the crystals will come out of solution as they are insoluble at lower temps.
    CuSO4 forms a complex with NaOH and a different complex with XS NaOH. That would give 2 different precipitates. One is darker blue than the other. could that explain it?
     
  3. XS Na OH would definitely explain the CuSO4, as two different precipitates..
    Not sure how I managed to get lovely twinkly crystals in room temp solutions, will play/trial on Monday..
     
    Sarah Ward likes this.
  4. let us know if you have success.
     
  5. Have had fun- The lead nitrate wasn't the problem ( which is nice.. didn't want to have to redo it), but the KI worked fine having added a little of the more concentrated stock to refresh it... Still don't know what the cause was-didn't look yellow as storage in CLEAPSS suggests it might go over time.
    Mystery not solved, even if experiment now ok...
     
    Sarah Ward likes this.
  6. It is possible to demonstrate this in a Petri Dish using only 1-drops of 0.001M of these reagents. See CLEAPPS.:D:D
     
  7. Have suggested the powders with water pipetted onto circles precipitate experiment as per CLEAPSS, though as effectively using powders rather than 0.01M or less solution I'm not sure I wouldn't effectively be giving the students more lead salt instead, though in a different form. Not enthusiastically taken up by teachers...
    I may have missed the petri dish and solutions one.. will look again..
     
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