Tests for carbonates, halides and sulfates, why the order of reagents matters, and the advantages of instrumental methods such as flame emission spectroscopy. Chemistry only content.
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1.How do you test for a carbonate?
Add dilute acid and bubble any gas produced through limewater.
2.What is a positive result for a carbonate?
The mixture fizzes and the limewater turns cloudy, showing carbon dioxide.
3.How do you test for a halide ion?
Add dilute nitric acid, then silver nitrate solution.
4.What precipitate shows chloride ions?
A white precipitate.
5.What precipitate shows bromide ions?
A cream precipitate.
6.What precipitate shows iodide ions?
A yellow precipitate.
7.Why is dilute nitric acid added before silver nitrate?
To remove any carbonate ions, which would also produce a white precipitate and give a false positive.
8.How do you test for a sulfate ion?
Add dilute hydrochloric acid, then barium chloride solution.
9.What is a positive result for a sulfate?
A white precipitate of barium sulfate forms.
10.Why is dilute hydrochloric acid added before barium chloride?
To remove carbonate ions, which would otherwise form a white precipitate and be mistaken for a sulfate.
11.Why must you use nitric acid for halides but hydrochloric acid for sulfates?
Hydrochloric acid contains chloride ions, which would give a false positive in the halide test.
12.What would happen if you used hydrochloric acid in the halide test?
A white precipitate of silver chloride would form from the acid itself, whatever the sample contained.
13.Give the general pattern of these tests.
Add an acid to remove interfering ions, then add a reagent that forms a coloured precipitate with the ion being tested for.
14.Name three instrumental methods of analysis.
Flame emission spectroscopy, mass spectrometry and gas chromatography.
15.Give three advantages of instrumental methods.
They are more accurate, more sensitive so they work on tiny samples, and much faster than chemical tests.
16.Give a disadvantage of instrumental methods.
The equipment is expensive, requires special training to use, and needs comparison with known results.
17.How does flame emission spectroscopy work?
A sample is put into a flame and the light given out is passed through a spectroscope, producing a line spectrum.
18.What does the line spectrum tell you?
The pattern of lines identifies which metal ions are present, and the intensity of the lines shows their concentration.
19.What advantage does flame emission spectroscopy have over a flame test?
It can identify several different metal ions in the same sample at once, which a flame test cannot.
20.Why are instrumental methods used in industry rather than chemical tests?
They give accurate quantitative results quickly and reliably, which matters when testing large numbers of samples.
21.A sample fizzes with acid and the gas turns limewater cloudy. What does it contain?
A carbonate.
22.A cream precipitate forms with nitric acid and silver nitrate. What is present?
Bromide ions.
23.Why is the precipitate colour the key observation in these tests?
Each halide forms a silver salt of a distinctly different colour, so the colour identifies which one is present.
24.Why is a blank test sometimes carried out?
To check the reagents themselves do not produce a precipitate, so any result must come from the sample.
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