C8: Chemical analysis Flashcards
How would you tell whether a substance is pure?
- Testing the melting/boiling point. If not what expected, it is a mixture.
- For an ink/dye, chromatography can be used
What are formulations?
Mixtures, designed to be useful products, made by mixing the components in exact quantities.
E.g. fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
What is chromatography used for?
Can be used to separate mixtures (inks/dyes) and can give information to help identify substances.
What are the mobile and stationary phases in chromatography?
- Mobile phase = solvent (bc it moves up the paper).
- Stationary phase = pencil line (it doesn’t move).
Practical
Explain how paper chromatography can be used to separate mixtures.
- Draw a line near the bottom of a sheet of filter paper in pencil (since it’s insoluble).
- Add a spot of ink to this line.
- Place the sheet in a beaker of solvent (depends on what’s being tested), making sure the ink doesn’t touch the solvent.
- Put a lid on the container to stop the solvent evaporating.
- The mobile phase (solvent) seeps up the paper, carring the ink with it.
- Each dye within the ink moves up the paper at a different rate; the dyes separate out into spots.
- If any of the dyes are insoluble, they stay on the stationary phase (pencil line).
- When the solvent has nearly reached the top, the paper is taken out and allowed to dry; the result is a chromatogram.
How would you calculate Rf value in chromatography?
distance moved by substance / distance moved by solvent
How would you test for hydrogen?
- Hold a burning splint over a test tube of the gas
- Positive result: burns rapidly and makes a “pop” sound
How would you test for oxygen?
- Insert a glowing splint into a test tube of the gas
- Positive result: the splint relights
How would you test for carbon dioxide?
- Use an aqueous solution of calcuim hydroxide (limewater)
- Bubble the gas through the limewater
- Positive result: limewater goes cloudy/milky
How would you test for chlorine?
- Place damp litmus paper into the gas
- Positive result: litmus paper turns white (is bleached)
Practical
How would you carry out a flame test?
- Clean a nichrome wire loop by dipping into hydrochloric acid, holding over a blue bunsen burner flame and redipping in the acid.
- Dip the loop into the metal compound being tested.
- Hold the loop in the flame and record the colour to identify the ion.
• Only works with one type of ion in the sample, because some flame colours may be masked in mixtures
What colour flame is produced by heating lithium ions?
Crimson
What colour flame is produced by heating sodium ions?
Yellow
What colour flame is produced by heating potassium ions?
Lilac
What colour flame is produced by heating calcium ions?
Orange-red (scarlet)
What colour flame is produced by heating copper ions?
Green
What may happen if you flame tested a mixture of cations?
Some flame colours may be masked by others.
What colour precipitate do dissolved calcium ions form when sodium hydroxide solution is added to the solution?
White
What colour precipitate do dissolved magnesium ions form when sodium hydroxide solution is added to the solution?
White
What colour precipitate do dissolved aluminium ions form when sodium hydroxide solution is added to the solution?
White, but redissolves in excess sodium hydroxide solution.
What colour precipitate do dissolved copper (II) ions form when sodium hydroxide solution is added to the solution?
Blue
What colour precipitate do dissolved iron (II) ions form when sodium hydroxide solution is added to the solution?
Green
What colour precipitate do dissolved iron (III) ions form when sodium hydroxide solution is added to the solution?
Brown
Write the balanced ionic equation for the reaction between dissolved iron (III) ions and sodium hydroxide solution. Include state symbols.
Fe3+(aq) + 3OH-(aq) →Fe(OH)3 (s)
How would you test for carbonate ions?
- They react with dilute acids to form carbon dioxide gas
- CO2 can be identified by bubbling it through limewater
- If CO2 is present, the limewater goes cloudy
How would you test for chloride ions?
- Add dilute nitric acid to a solution of the ions tested
- Then add silver nitrate solution
- Positive result: a white silver chloride precipitate forms
How would you test for bromide ions?
- Add dilute nitric acid to a solution of the ions tested
- Then add silver nitrate solution
- Positive result: a cream silver bromide precipitate forms
How would you test for iodide ions?
- Add dilute nitric acid to a solution of the ions tested
- Then add silver nitrate solution
- Positive result: a yellow silver iodide precipitate forms
How would you test for sulfate ions?
- Add dilute hydrochloric acid to a solution of the ions being tested
- Then add barium chloride solution
- Positive result: a white precipitate forms
Elements and compounds can be detected and identified using instrumental methods, such as flame emission spectroscopy, instead of chemical tests.
Name 3 advantages of using instrumental methods.
- accurate
- sensitive - only an tiny sample needed
- rapid + can be automated
Explain how flame emission spectroscopy works.
- Used to analyse metal ions in solutions
- Sample is put into a flame
- The light given out is passed through a spectroscope
- The output is a line spectrum
- This can be analysed to identify the metal ions in the solution, and measure their concentrations
Why is flame emission spectroscopy more useful than flame tests?
It can be used to identify different ions in mixtures, whereas flame tests can only identify single ions.
In flame emission spectroscopy, each type of ion produces an individual line spectrum. What do the combination of wavelengths emitted by an ion depend on?
- its charge
- its electron arrangement
What indicates the concentration of an ion in flame emission spectroscopy?
The intensity of the colours.
