HELPP - im super confused about the ion tests for alevel chemistry ocr a
hi there,
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
Original post by zynnjah123
hi there,
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
Usually, if you are going to do the tests in succession on the same sample, you would add nitric acid to test for the carbonate ions and then use silver nitrate and then barium nitrate to test for halide and sulphate ions, respectively.
Original post by zynnjah123
hi there,
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
for the testing for ions topic, we are told to do the tests in the following order, carbonates, sulfates and then finally halides, this is because the carbonates react with the bromine chloride to form a white precipitate thus creating a false positive, and the sulfate can react with the silver nitrate also making a white precipitate.
BUT, surely adding hcl for example for the carbonate test would pollute our solution creating a 100% positive result in the halide test due to hcl being used. ok lets say we used nitric acid instead, in the SULFATE test we add barium chloride. again halide ions have been added meaning we will 100% get a positive result for that even if it may have not originally existed.
what do you do in this situation when you are given a question asking about how you figure out the existing ions in an unknown solution.
thank you
I believe the simple thing you're getting yourself confused over is the fact that we don't do all the tests in the same tube.
You get a sample with an unknown negative ion.
In one tube, you add an acid, doesn't much matter one: nitric, hydrochloric or sulfuric. If it bubbles, you know it has a carbonate. You can stop testing. In fact, you probably best had do in case you get a false positive.
If it doesn't bubble, get a new test tube, add you unknown plus some Ba2+ (either nitrate or chloride will do, just not carbonate or hydroxide). If you get a white ppt, you've got yourself a sulfate. Stop testing.
If no ppt, get another tube and add silver nitrate and see what colour ppt you make and then add NH3(aq) of whatever conc. is appropriate.
IF it contains two negative ions, you will need two test tubes.
Into 1 add nitric acid/hydrochloric acid (not sulfuric). If bubbles one ion is carbonate. Then add Ba2+ (again either) if white ppt then sulfate.
If you need to check for halide do that now. BUT if you know there is a carbonate, you first need to add excess nitric acid (which will bubble) before adding Ag+.
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