Topic 8 - Chemical Analysis

Question 1

1) In chemistry, what’s the definition of a pure substance?

Answer 1

1) A pure substance contains only one compound or element throughout.

Question 2

2) Say whether each of the following substances are pure or impure:
A. sea water C. graphite D. fresh mountain air
B. ethanol

Answer 2

2) A. impure - sea water has lots of other compounds dissolved in it.
B. pure - ethanol contains more than one element, but it’s a single compound.
C. pure - graphite is made of carbon and nothing else.
D. impure - fresh air is a mixture of gases, mostly N₂ and O₂.

Question 3

3) Give three examples of everyday products that are formulations.

Answer 3

3) E.g. paint, cleaning products, fuels, medicines, cosmetics, fertilisers, alloys

Question 4

4) What effect will impurities have on the boiling point of a compound?

Answer 4

4) Impurities increase the boiling point of a substance, and may also result in it boiling over a range of temperatures.

Question 5

5) The lists of ingredients for two different brands of cough medicine are identical.
Explain why this does not necessarily mean they have the same formulation.

Answer 5

5) Formulations contain specific quantities of their components. The medicines have the same ingredients, but if they’re mixed in different ratios, the formulations aren’t the same.

Question 6

6) Sally is testing the purity of a sample of a compound by measuring its melting point.
The solid begins to melt at 68 °C, but does not all melt until it reaches 73 °C. Sally checks a data book, which says the melting point is 75 °C. “A-ha,” says Sally, “I knew when I saw my results that the value in the data book would be higher.” How did she know?

Answer 6

6) The sample melts over a range of temperatures, which suggests it’s impure.
Sally knows that impurities lower the melting point of a compound.

Question 7

1) What’s the formula for calculating the R, value of a spot on a chromatogram?

Answer 7

1) R_f = distance travelled by substance/ distance travelled by solvent

Question 8

2) A mixture is analysed using paper chromatography. One component is insoluble in the solvent used. How will you be able to tell this from the chromatogram?

Answer 8

2) There will be a spot of the insoluble substance remaining on the baseline. It will stay on the baseline because it can’t move up the paper if it doesn’t dissolve in the solvent.

Question 9

68, 3 The chromatogram on the right was made by carrying out paper chromatography on five pure substances, A-E, and two mixtures, X and Y. Use it to answer questions 3-5.

3) Which of A-E could be components of both X and Y?
4) Which of A-E can’t be found in either X or Y?

5) What are the R_f values of substances A and B?

Answer 9

3) A and E. Both X and Y have produced spots at the same height as spot A and spot E.
4) B. Neither X nor Y produced a spot at the same height as spot B, so substance B can’t be in either of the mixtures.
5) R_f of A = 18/20 =0.9,
R_f of B = 15/20 = 0.75

Question 10

6) Explain why a pure chemical might have two different R, values on chromatograms
made using two different solvents, if all other conditions are kept the same.

Answer 10

6) The R, value of sub depends on how easily that substance dissolves the solvent used as the mobile phase. If the chemical is more soluble in one solvent than the other, it will spend more time dissolved in that solvent, and therefore move further up the
chromatogram and have a higher R_f value.

Question 11

1) How do you test for chlorine gas? What’s the result if chlorine is present?

Answer 11

1) Put a piece of damp litmus paper into a sample of the gas. Chlorine will bleach the litmus paper, turning it white.

Question 12

2) Describe the test for hydrogen, and explain what causes the result you would
observe when hydrogen gas is present.

Answer 12

2) Hold a lit splint at the end of a test tube of the gas. If it is hydrogen, you’ll hear a popping sound, as the hydrogen burns rapidly with the oxygen in the air.

Question 13

3) A particular experiment is thought to produce both oxygen and hydrogen gases. A sample of the gas produced in the experiment is collected in a test tube. Suggest why it might be dangerous to test the sample for both oxygen and hydrogen at the same time.

Answer 13

3) The test for oxygen is to put a glowing splint into the test tube and see if it relights. The test for hydrogen is that it burns quickly when a lit splint is held near to the open end of the test tube. So relighting the splint with oxygen could ignite the hydrogen in the enclosed space of the test tube, which could cause an explosion.

Question 14

4) A student is carrying out a reaction that produces a gas. The student believes the gas is carbon dioxide. Describe the steps he could take to collect and test the gas produced to confirm that it is carbon dioxide. State what result he will observe from his test if he is correct.

Answer 14

4) E.g. carry out the reaction in a flask which is sealed with a bung that holds a delivery tube. Put the other end of the delivery tube into a test tube containing calcium hydroxide solution (limewater) so that the gas produced in the flask bubbles through the limewater. If the gas produced is carbon dioxide, the limewater will turn cloudy/milky.

Question 15

1) What gas is produced when you react a carbonate with an acid?

Answer 15

1) Carbon dioxide. The carbonate (CO₂) ions react with the hydrogen ions from the acid to make carbon dioxide and water.

Question 16

2) In the test for halide ions, what colour precipitate do iodide ions produce?

Answer 16

2) Yellow. The halide test produces insoluble silver halide compounds from chloride, bromide or iodide ions. Silver iodide is yellow.

Question 17

3) The test for sulfates involves adding two reagents to the solution being tested.
Name the two reagents and explain why they are used.

Answer 17

3) Hydrochloric acid is added first, to remove any carbonate ions which would interfere with the result of the test. Then barium chloride solution is added, because barium ions react with sulfate ions to form a white precipitate of barium sulfate.

Question 18

4) Describe the test for halides, and suggest why it might be difficult to tell which halide is present when you get a positive result from this test.

Answer 18

4) Add dilute nitric acid to the solution being tested, followed by silver nitrate solution.
A silver halide precipitate will form if halide ions are present. The colours of the precipitates are white (chloride), cream (bromide) and yellow (iodide), so it may be hard
to distinguish between white and cream or cream and yellow.

Question 19

5) Adi is testing a solution to see what ions it contains. He wants to test for both sulfate ions and halide ions. He tests a sample of the solution for sulfate ions first, but gets no result. Explain why Adi needs to use a fresh sample of the solution to test for halides.

Answer 19

5) Adi can’t use the same sample to test for halides after he’s tested it for sulfates, because the test for sulfates involves adding hydrochloric acid and barium chloride solution. He will have added chloride ions to the sample, so he’ll get a positive result from the halide test even if there weren’t any halide ions in the original sample..

Question 20

1) What colour flame do you get when you burn potassium ions?

Answer 20

1) Lilac

Question 21

2) Name a solid metal hydroxide that dissolves in excess sodium hydroxide solution.

Answer 21

2) Aluminium hydroxide/Al(OH)₃. Watch out the question asked for the hydroxide, not just the metal ion, so if you said aluminium(III)/Al³⁺ here, you’re only halfway there.

Question 22

3) How do you clean a wire loop so you can use it to do a flame test?

Answer 22

3) Dip the loop in dilute hydrochloric acid and then hold it in the blue flame of a Bunsen burner until it burns without any colour.

Question 23

4) A fireworks company makes coloured displays by adding compounds of metal cations to their fireworks. They use compounds of Lit, Cu²+, Ca²+ and Na⁺ ions. A client asks for a display of green and yellow fireworks. Which ions will the company use?

Answer 23

4) Cu²⁺ and Na⁺. Copper ions burn with a green flame and sodium ions with a yellow flame.

Question 24

5) A solution containing a single metal compound is tested using a flame test and then using sodium hydroxide. The flame test produces an orange-red flame. Predict the colour and formula of the precipitate formed in the sodium hydroxide test.

Answer 24

5) An orange-red flame suggests Ca²+ ions, so a white precipitate of Ca(OH)₂ will form with sodium hydroxide.

Question 25

6) Jelena mixes iron (III) chloride solution with a few drops of sodium hydroxide, NaOH. Give the full equation for the reaction and describe what Jelena will observe.

Answer 25

6) FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl
Jelena will see a brown precipitate of iron(III) hydroxide. The question asked for the full equation, so you have to figure out the formulas you need you can’t just use the ions.

Question 26

1) What does the term ‘instrumental analysis’ mean?

Answer 26

1) Instrumental analysis means analysing substances using machines.

Question 27

2) What are the advantages of using instrumental analysis instead of chemical tests?

Answer 27

2) E.g. instrumental tests are sensitive, fast, accurate and only use a small sample.

Question 28

72, 3 Four flame emission spectra are shown on the right.
They represent three ions, A, B and C, and a mixture, Z.
Use the spectra to answer questions 3 and 4.

3) Which of ions A, B and C could be present in Z?
Explain your answer.

4) How you can tell that Z must contain at least one other ion that is none of A, B or C?

Answer 28

3) A and B could both be in Z, because all the emission lines on each of their spectra are also present on the spectrum for Z. C can’t be in Z because some of the wavelengths of light it emits are not present in the emission spectrum for Z.
4) Some of the emission lines on the spectrum for Z are not found on any of the spectra for A, B and C, so those lines must be due to another ion present in Z.

Question 29

5) Briefly describe how flame emission spectroscopy works,
and how it can be used to distinguish between different ions.

Answer 29

5) A sample containing ions is placed in a flame, which causes them to heat up and transfer energy as light. The light passes through a machine called a spectroscope, which can detect different wavelengths of light to produce a line spectrum. Different ions emit different wavelengths of light, so each ion gives a different pattern of wavelengths, and has
a different line spectrum.