I`ve just crystalised a batch of Strontium Nitrate, but instead of Cubic crystals, I have an equal amount of Hexagonal ones too, and a few are even Rhombic like a pair of square based pyramids stuck together at the base?

 

now on a burn test (the dried filter paper) there didn`t apear to be any color contamination.

 

so it there Any explaination for this?

I was looking for images of the crystals you mentioned. They sounded interesting. I came across this site which has strontium nitrate down twice with two different names :S. sr(NO3)2 and srNO6. This is the page link http://environmentalchemistry.com/yogi/periodic/N-compounds.html . Why two names ?

Are they different ?

Nice link!

 

I think what the one is, is just "parts list" of the molecules involved (NO3)2 is technicaly the same as N2O6, but it`s rare such a way is used nowadays.

(NO3)2 is prefered

Sr(NO3)2 in reality consists of Sr(2+) ions and two NO3(-) ions. These ions really exist independently from each other. This is nicely demonstrated by dissolving it. Separate ions of Sr(2+) and NO3(-) will be floating around in the water, at a ratio of 1 : 2.

 

This even is better demonstrated by dissolving two salts. E.g. if you dissolve one mol of Sr(NO3)2 and you dissolve another mol of BaCl2 in the same solution, then you have ions Sr(2+), Ba(2+), NO3(-) and Cl(-) floating around in ratios 1 : 1 : 2 : 2.

 

Now, if you made a second solution, by dissolving the same molar amounts of Ba(NO3)2 and SrCl2 in the same amount of water, then this solution also contains ions Sr(2+), Ba(2+), NO3(-) and Cl(-) floating around in ratios 1 : 1 : 2 : 2.

 

Given one of these solutions, it is impossible to tell, which salts were used as starting point. This is the best demonstration of the separate ions in the salts.

 

For this reason chemists usually write the ions, of which the salt consists in the correct ratio, but without the charge on the ions, hence Ba(NO3)2. Formally, it would even be nicer of we were writing

 

[math]Ba^{2+}(NO_3^{-})_2[/math], unfortunately, that is not done.

I`ve just crystalised a batch of Strontium Nitrate, but instead of Cubic crystals, I have an equal amount of Hexagonal ones too, and a few are even Rhombic like a pair of square based pyramids stuck together at the base?

 

now on a burn test (the dried filter paper) there didn`t apear to be any color contamination.

 

so it there Any explaination for this?

 

I can give an explanation for the "rhombic" crystals. The key here is that crystal system an crystal morphology are not necessarily the same. Many compounds with cubic unit cells will form the "pair of square based pyramids stuck together" (better known as octahedral crystals) that you descibe. In mineralogy the octahedral shape for cubic minerals is quite common and is observed in species such as fluorite (CaF2), galena (PbS), cuprite (Cu2O) and even that great mineral tetrahedrite (Cu12Sb4S13) As for the hexagonal crystals, I am not sure. Is it possible that you have a hydrate that has a different crystal system?

Strontium nitrate, when crystallized from water ican behydrated: Sr(NO3)2.4H2O. This water of crystallization can be removed by carefully heating the salt. It will "melt" and liquefy and on continued heating the water boils off.

 

Strontium nitrate also can crystallize as Sr(NO3)2.5H2O. I think that all these different modes of crystallization lead to differently shaped crystals.