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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: spiderman1441 on November 30, 2006, 06:22:02 AM

Title: Two questions about Al & Zn cations?
Post by: spiderman1441 on November 30, 2006, 06:22:02 AM
1-In Third grpup cations , why Al+3 & Zn+2 are colorless , and why they form a white percipitate , i.e why they don't form colored percipitates ?

2- What,s the reagent of Al ?
Title: Re: Two questions about Al & Zn cations?
Post by: Dan on November 30, 2006, 09:40:28 AM
1. What is it that gives transition metal compounds their colour? Why might you not see this in Zn(II) and Al(III) compounds?

oh, also Zn is not group III

2. Please rephrase the question, it doesn't make sense.
Title: Re: Two questions about Al & Zn cations?
Post by: Albert on November 30, 2006, 01:35:03 PM
Quote from: spiderman1441 on November 30, 2006, 06:22:02 AM
2- What,s the reagent of Al ?

What reagent is generally used to precipitate aluminium?

Ammonia and ammonium chloride, at pH= 8.5 - 9
Title: Re: Two questions about Al & Zn cations?
Post by: mdlhvn on December 01, 2006, 09:45:13 AM
Quote from: spiderman1441 on November 30, 2006, 06:22:02 AM
1-In Third grpup cations , why Al+3 & Zn+2 are colorless , and why they form a white percipitate , i.e why they don't form colored percipitates ?


1. Why does matter have colours or not? Because they can absorb visible light whose wavelengths range from about 400nm to 800nm.

2. Then why Al3+ and Zn2+ can not absorb visible light. Remenber that in solution, they exist under hydrate forms, that is Men+(H2O)n and can be considered as  complexes.

   There are some theories explaining the structure of complex including the explanation of their light-absorbed ability.