Why do unexpected precipitates form in precipitation reactions?

[danago]

Hi. If i was to conduct a few precipitation reactions, is there any reason why an unexpected precipitate will form, or any reason an expected precipitate will not be formed? I've though about things such as added ions in tap water (chlorine etc). Any other things?

 

[pavadrin]

Hi. If i was to conduct a few precipitation reactions, is there any reason why an unexpected precipitate will form, or any reason an expected precipitate will not be formed? I've though about things such as added ions in tap water (chlorine etc). Any other things?

Perhaps the test tubes were not cleaned properly and some ions from the previous experiment were left behind. Maybe there was nothing wrong and it was the lab technicans fault for incorrectly labeling the bottles containing the substances.

 

[danago]

Perhaps the test tubes were not cleaned properly and some ions from the previous experiment were left behind. Maybe there was nothing wrong and it was the lab technicans fault for incorrectly labeling the bottles containing the substances.

lol

 

[3trQN]

Some precipitation reactions are very sensative to temperature and some, like the silver mirror experiment, require rough surfaces or some point to start precipitating from (seed).

I have had such test fail or succeed just on the basis of the age of the test tube, with better results in older (scratched) glass tubes than in out-of-the-box tubes that are smooth.

 

[GCT]

Hi. If i was to conduct a few precipitation reactions, is there any reason why an unexpected precipitate will form, or any reason an expected precipitate will not be formed? I've though about things such as added ions in tap water (chlorine etc). Any other things?

Yes, that as well as other factors e.g. coprecipitation etc...precipitation isn't so simple as adding the components. Unless carefully devised the precipitated product may be more or less crude. Since this is probably for general chemistry you won't have to worry about the analytical aspects too much, but in actuality the process is drawn out. In your case, I would consider the solubility of the particular precipitate, so as to account for the volume of solvent needed, you want to be well over the point of saturation to see the solids. Also, consider the temperature, you may need to heat the solution to digest sample and then cool it down to a certain temperature; that is sometimes it's preferable to employ an ice bath.

 

[danago]

Thanks for the help everyone :)