Caroline's weblog

Qualitative analysis

Crime Scene Investigation, Raleigh
You just learned that your chemistry T.A. has been kidnapped!  All you have to investigate this kidnapping is a little bit of liquid that was left in the chemistry lab.  Is is a clue?  Who left it?  What do you do with it?

Here is a cool forensic website from Court TV.  You can read about techniques scientists use to solve crimes.

Qualitative analysis is a big part of crime forensic scientist's techniques to solve crimes.  And although I have not been kidnapped, you should treat this lab with the same care if you were investigating someone's disapperance.

In this lab you will be seperating out ions and confirming their presence.  What are ions again?  Oh yeah they are cations (girls) and anions (boys)!  In this lab you will be separating three cations. 
Different ions have different solubilities in different solutions.  This means that some ions might precipitate out. 

*** Remember insoluble, a no in the table- means that a precipitate did form, and soluble, a yes in the table -  means that no precipitate formed. ***

The solubility rules are listed in your lab book and also with the webassign questions.  Most ionic compounds are insoluble - they will form a precipitate.  The solubility rules tell you the compounds that are soluble - no precipitate.

You will be adding chemicals - HCl (hydrochloric acid) and NH3 (ammonia) - and using a hot water bath to seperate out the ions.  You will be using a solution of KI (potassium iodide) to confirm the presence of the ions.  (All the different ions that you will be using in this lab react differently when KI is added to them.)

*** Remember that ions MUST come in pairs - for example I can not give you a bottle of just cation Na+, it must be combined with a anion, such as Cl-. (So all girls must have a boy with them!) ***

If you do get a precipitate, you need a way to seperate out the precipitate, and then you will be saving the liquid on top - or supernatant.  To seperate out the precipitate from the supernatant - you will use a centrifuge.  This is a rotor which will spin the samples at high speeds, so that the precipitate will be pushed to the bottom, and the supernatant will float on the top. 

This lab has many parts so it is important that you follow the directions carefully.  Also, you will be using as one of your solutions AgNO3 - this solution, if gotten on the skin will turn it black for about a week, so be careful with it.

Posted at 11:26AM Jul 14, 2006 by CHRISTIAN, CAROLINE in General  |  Comments[3]