SC25 Flashcards
SC25a
1) Recall flame test colours for some metal ions (lithium, sodium, potassium, calcium, copper)
2) Describe how to carry out flame tests.
3) Describe the advantages of instrumental methods of analysis.
1) The lithium flame test colour is red. The sodium flame colour is yellow. The potassium flame test colour is lilac. The calcium flame test colour is orange-red. The copper flame test colour is blue-green.
2) Flame tests are used to identify metal ions in substances. To carry out a flame test:
i) Light a Bunsen burner and open the air hole to give a hot blue flame.
ii) Pick up a small sample of the test substance using a wire loop.
iii) Hold the sample in the edge of the flame and observe the flame colour.
The wire loop is cleaned in hydrochloric acid before testing each sample. Platinum wire has a high melting point and it is unreactive. It also gives no colour to the flame. However, nichrome alloy is often used instead. Although it produces its own faint orange colour, it is much cheaper.
3) Machines can also be used to analyse substances. Compared to simple laboratory tests such as flame tests, using scientific instruments may improve: sensitivity (they can detect much smaller amounts), accuracy (they give values closer to the true values), and speed.
SC25a
1) Use flame photometer data to determine the concentration of metal ions in solution.
2) Use flame photometer data to identify metal ions.
1) The flame photometer measures the light intensity of the flame colours produced by metal ions. Its data is used to determine the concentration of a metal ion in a dilute solution.
2) The colour of light that we see in a flame test is usually a mixture of different colours. A flame photometer can separate out these colours to produce a spectrum of the light emitted by each metal ion. Different metal ions produce different emission spectra. A metal ion in an unknown solution can be identified by matching its spectrum to the spectrum from a known metal ion.
SC25b
1) Explain why the test for a given ion must be unique to that ion.
2) Recall some metal hydroxide precipitate colours.
1) It is important that the test for any ion is unique. This is because the results of a test must let you determine which ion is present, rather than being uncertain about which ion it is.
2) For iron (II), the precipitate colour is green. For iron (III), the precipitate colour is brown. For copper, the precipitate colour is blue. For calcium, the precipitate colour is white. For aluminium, the precipitate colour is white.
To remember: grass (iron 2) –> dirt (iron 3, brown) –> sky ((brown dirt) copper, blue sky) –> clouds (white, calcium clouds, aluminium)
SC25b
1) Describe how to identify metal ions using sodium hydroxide solution.
2) Describe how to identify ammonium ions and ammonia.
1) A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions, calcium ions and magnesium ions.
2) Dilute sodium hydroxide solution is used to detect ammonium ions, NH4+, in a substance. Ammonia gas, NH3, is produced when the mixture containing ammonium ions is warmed.
A confirmatory test is used to identify ammonia. Ammonia changes the colour of damp red litmus paper to blue.
SC25c
1) Describe how to identify carbonate ions.
2) Describe how to identify carbon dioxide.
3) Describe how to identify sulfate ions in solution.
1) Add dilute hydrochloric acid to the test substance and look for bubbling caused by the production of carbon dioxide. Since the bubbles could be due to a different gas, such as hydrogen, you should carry out a confirmatory test.
2) If the bubbles produced contain carbon dioxide, when the gas is bubbled through limewater, it will turn limewater milky.
3) To test for sulfate ions in a solution, add a few drops of dilute hydrochloric acid. This acidifies the solution and removes carbonate ions that might also give a precipitate in the test. Then, add a few drops of barium chloride solution. A white solution of barium sulfate forms if the sample contains sulfate ions.
SC25c
1) Recall the colours of halide test precipitates.
2) Describe how to identify halide ions in solution.
1) Silver chloride is a white colour. Silver bromide is a cream colour. Silver iodide is a yellow colour.
2) The halogens are the non-metal elements in group 7 of the periodic table. They form compounds that contain halide ions, such as F, Cl, Br and I. Silver fluoride is soluble in water but the other silver halides are insoluble. This is the basis of the laboratory test for chloride, bromide and iodide ions. To detect these ions in a solution, add a few drops of dilute nitric acid. This acidifies the solution and removes carbonate ions that might also give a precipitate in the test. Then add a few drops of silver nitrate solution. Different coloured silver halide precipitates form, depending on the halide ion present.
SC25c - Core Practical
1) What is the aim for the identifying ions core practical?
2) What is the method for the identifying ions core practical (metal cation flame test, metal cation precipitate test, ammonium ions, carbonate ions, halide ions)?
1) Identify the ions in some unknown salts, using the tests for the specified actions and anions
2) Methods
Flame tests for metal cations
A. Light a Bunsen burner and open the air hole to give a hot blue flame.
B. Pick up a small sample of a solid salt using a clean wire loop or a damp wooden splint.
C. Hold the sample in the edge of the flame.
D. Observe and record the flame colour.
Hydroxide precipitate tests for metal cations
E. Dissolve a little solid salt in a test tube using distilled water.
F. Add a few drops of dilute sodium hydroxide solution, one drop at a time.
G. Record the colour of any precipitate formed.
H. If a white precipitate forms, add excess dilute sodium hydroxide solution to see if it will disappear to leave a clear solution.
Testing for ammonium ions
I. Dissolve a little solid salt in a test tube using distilled water.
J. Add dilute sodium hydroxide solution and then warm gently.
K. Remove from the flame. Hold a piece of damp red litmus paper near the mouth of the test tube. Record what happens to its colour.
Testing for carbonate ions
L. Put a little solid salt in a test tube and add a few drops of dilute acid.
M. Record whether any effervescence (bubbling) occurs.
N. Use limewater to check that any bubbles contain carbon dioxide.
Testing for sulfate ions
O. Dissolve a little solid salt in a test tube using distilled water.
P. Add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution. Record whether a white precipitate forms.
Testing for halide ions
Q. Dissolve a little solid salt in a test tube using distilled water.
R. Add a few drops of dilute nitric acid, then a few drops of silver nitrate solution. Record the colour of any precipitate formed.
SC25c - Core Practical
1) What are the independent, dependent and control variables for the identifying ions core practical?
2) What are the safety hazards for the identifying ions core practical?
1) Independent variable: the type of ion being tested
Dependent variable: the solution colour change or the precipitate colour
Control variable: the volume of solution, temperature
2) Wear gloves to avoid skin contact; wear eye protection to avoid eye contact; wash hands afterwards to avoid swallowing barium chloride solution.
SC25c - Core Practical
1) Describe a test to distinguish between calcium hydroxide and aluminium hydroxide
2) Give two reasons why platinum is a suitable metal to use for flame tests
3) Explain why a luminous Bunsen burner flame is not suitable for flame tests
1) Add excess sodium hydroxide solution; only aluminium hydroxide dissolves to form a colourless solution, while the calcium hydroxide precipitate remains white
2) Does not colour the flame; high melting point/ does not melt; does not react with air/ unreactive.
3) The flame is already coloured; the flame test colour is difficult to see; the flame is not hot enough
SC25c - Core Practical
1) Why is dilute nitric acid added when testing for an unknown salt?
2) Explain why dilute hydrochloride acid should not be used instead of dilute nitric acid when testing for an unknown salt?
1) To react with carbonate ions so they are removed; to react with substances that would also form a white precipitate with silver nitrate.
2) Hydrochloric acid contains chloride ions; which would form silver chloride, and give a false positive test for chloride.
