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Analysis of Unknown Aqueous Solutions

2003

I am currently studying As level chemistry, and as part of our syllabus I am required to perform a practical examination - an analysis of some unknown aqueous solutions. I am given five bottles of aqueous solutions, with no labels and I have to determine which is which: the five solutions include:

• - aqueous calcium hydroxide
• - ethanoic acid (approx. 1 mol dm-3)
• - nitric acid (approx. 1 mol dm-3)
• - aqueous potassium bromide
• - aqueous sodium chloride

I am not permitted to use indicators and must have a positive test for each solution Thank you for your time

Miss Leah Rachel M.
- Gloucester, Gloucestershire, England

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2003

Leah,
The idea here should be to describe what difficulty you are having in doing your homework, what stumbling blocks you are encountering. The idea is not to just reprint your homework question and ask someone to do it for you. So please present a couple of pieces of knowledge that you've acquired that you are unsure of exactly how to apply to this problem. Thanks.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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First of two simultaneous responses --

Well, it depends on what a positive test is. Ethanoic acid is acetic acid ⇦this on eBay or Amazon [affil links] which is vinegar ⇦in bulk on eBay or Amazon [affil links] . Smell will certainly isolate this. Next, smell will tell you which of the remaining 4 is nitric. Calcium hydroxide is the only base, so add one drop or sulfuric acid to one drop of each of the three remaining solutions. The one with a strong reaction is the hydroxide. Adding sulfuric acid to the bromide should release a reddish brown bromine gas. The remaining one is table salt. For really positive tests, get a microqualitative analysis book and find out how to do a test for a chloride, a bromide, a nitrate, an acetate and calcium. Piece of cake.

James Watts
- Navarre, Florida
2003

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Second of two simultaneous responses --

A bit of analytical chemistry! Clearly we will not do your homework for you, but perhaps we can offer a few tips. For a start, many cations (metals) have a particular colour when put into a non-luminous flame, so try dipping a clean platinum wire into the solutions and doing a flame test. Secondly, halides produce different coloured silver salts, so test for the halides with silver nitrate ⇦this on eBay or Amazon [affil links] and look for the immediate colour. Thirdly, nitrates are usually detected with the "brown ring test" - this involves putting about 15 mls of conc sulfuric acid into a test tube, carefully followed by about 3 mls of the test solution. Allow this to cool and then add the test solution. A nitrate will form a brown ring at the interface between the test solution and the acid. Fourthly, ethanoic acid (acetic acid ) has a characteristic smell to it, but it also forms a blood red colour when mixed with freshly prepared neutral ferric chloride ⇦this on eBay or Amazon [affil links] solution. Just to show you are not lazy and were in real need of help, let us know what the results of your chemical analysis are - the colours of the silver halides and the flame tests, for example.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2003

Can someone send me a method on how to test for the presence of nitric acid ?

Matthew Coleman
as-level chemistry student - Swansea, G.B, G.B
2003

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For nitric acid, read nitrate and refer to above answers. A test for nitric acid will give the same answer as for nitrates - now tell us why.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2003

I am doing the very same practical exam as a practice and I still do not understand how you test for nitric acid. I have searched on other sites and they say that nitric acid is a product of the test! Can you help explain this problem for me.

Steve Pearsall
Student - Worcester, Worcestershire, England
2005

I believe the test for nitric acid is fairly simple. All you need to do is add Mg or Zn to give a hydrogen gas. This obviously makes the famous popping smell when it comes into contact with a lighted splint.

Sean Cameron
- Winchester
2007

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I have been asked to identify six colouless unknown liquids:- water, ethanoic acid, bromoethane, butan-1-ol, 2-methylpropan-2-ol and cyclohexene. I have worked out a test for each but was just wondering if using anhydrous copper sulphate ⇦this on eBay or Amazon [affil links] to test for water is ok because I read in a chemistry textbook that weak acids also hydrolyse water. can anyone please help? many thanks.

William B.
student AS level chemistry - Washington, England
2007