Saturday, 27 August 2011

5 Easy steps to identify a compound

I'm writing this post to help students who are getting ready for chemistry exams. When I was doing my higher secondary, some students were not able to identify a compound and found it very tough. But, I can help you in finding a compound only in few steps. You need not to do all steps that you see in the chemistry manual.
Just follow my simple steps and they will guide you through.

First, we are going to take concentrated sulphuric acid (king of chemicals) which will confirm the anion straightaway.

1. Brisk effervescence - Carbonate
2. Rotten egg smell - Sulphide
3. Dense pungent smelling gas (Hydrogen chloride) - Chloride
4. Pungent smelling gas evolves (Nitric acid vapor) and it liberates reddish brown gas (Nitrogen dioxide) on adding Magnesium or Copper turnings. - Nitrate
5. Dark red gas resembles the smell of chlorine to an extent - Bromide
6. Purple vapor - Iodide
7. Pungent smelling gas decolorizing potassium permanganate solution (sulphur dioxide)  - Sulfite
8. Reddish brown gas turns ferrous sulphate paper black - Nitrite

(Caution: Add only few drops of concentrated sulphuric acid to the salt as it reacts with most of the compounds vigorously and will produce toxic gases enormously  in some cases. Also, do not pour water in the concentrated sulphuric acid directly as it has great affinity towards water and the reaction may spit out the acid and also will generate lot of heat. To dilute the sulphuric acid, add the acid slowly through the side of the beaker containing water)

May be we are not allowed to use metal turnings in the school lab. In that case, please add few drops of concentrated hydrochloric acid and heat the mixture (Hcl+salt) gently. If reddish brown gas evolves - Nitrate is confirmed. Please do not heat it strongly as the reaction may spits out the acid.

If none of these happen, It must be sulphate (salt of sulphuric acid). Because, sulphate ions always do form a strong bond with cations and they cannot be decomposed easily by most of the reagents. In that case, add Barium chloride solution and it will give white precipitate (Barium sulphate) - Sulphate is confirmed.

If we find the anion correctly, we are not far away from the success as we have been already provided with the list of salts that we are going to use in exam practicals. For example, if the anion is nitrate, then we can come to the conclusion, it must be one of the nitrates that we are using for practicals. We had used only 2 nitrates i.e. Copper nitrate & Lead nitrate. Copper salts can be identified by its color. So, it is enough to do the test for Lead.

Next, we are going to find the cation. Before that, let's do the flame test which will help you in finding the presence of two cations, Barium & Calcium.

Barium - Green flame
Calcium - Brick red
Strontium & Lithium  - Crimson

If the above flame test is not successful, then add sodium hydroxide to the salt and test the result with the following:

1. White precipitate - Aluminium, Magnesium & Zinc (White precipitate will lead you for the confirmatory test for these three)

If white precipitate not appears, heat the mixture. Pungent smelling gas confirms the presence of ammonium ion.

2. Hydrogen sulphide test: This test can be used to determine the presence of heavy metals as they do precipitate out the colorful sulphides.On passing the hydrogen sulphide through salt solution, the characteristic colors show the presence of the below cations:

1. Copper, Nickel, Lead, Silver & Cobalt - Black (In excess, it will also precipitate out the metal on the surface)
2. Chocolate brown - Bismuth, Stannous, Manganese.
3. Yellow to Orange - Cadmium or Antimony

 You can distinguish the salts of these metals by noticing the color.

1. Copper - Most of the copper compounds are blue in color. There are some exception though. Ex: Copper chloride is green.
2. Nickel - Most of the copper compounds are green in color.
3. Cobalt - Rosy red, blue
4.Tin, Lead & Bismuth - Mostly colorless

Then, you can do the confirmatory test for these metals if needed.

You can find the anion in a single step and for cation, you will not need more than 3 or 4 steps. Hence, you can find a compound no more than 5 steps or may be less.

You can also identify other ions by these steps. As I'm not sure if there is any new ion added in your practicals (new syllabus), I just write these based on the compounds that we used for our practicals.
I hope this is very helpful for your exams. If you have any questions, you can post them below and I will explain you in detail.

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