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 do you test for carbonate ions?
1) Drop a few drops of dilute acid into a test tube containing the unknown substance
2) Connect this test tube to one containing limewater. IF CARBONATE IONS ARE PRESENT, THEIR REACTION WITH THE ACID WILL RELEASE CO2, TURNING THE LIMEWATER CLOUDY
PRACTICAL: How do you test for sulphate ions with HCl and BaCl?
1) Use a dropping pipette to add a few drops of dilute hydrochloric acid, then barium chloride solution to the test tube containing the unknown substance
2) IF SULPHATE IONS ARE PRESENT, A WHITE PPT. OF BARIUM SULPHATE WILL FORM
PRACTICAL: How do you test for halides (Cl, Br and I) with HNO3 and AgNO3?
1) Add a few drops of dilute nitric acid, followed by silver nitrate solution to the unknown solution
2) - IF Cl IS PRESENT, A WHITE PPT. OF SILVER
CHLORIDE IS GIVEN OFF
- IF Br IS PRESENT, A CREAM PPT. OF SILVER
BROMIDE IS GIVEN OFF
- IF I IS PRESENT, A YELLOW PPT. OF SILVER IODIDE IS
GIVEN OFF
PRACTICAL: What colour flame does lithium produce?
Crimson / brick red
PRACTICAL: What colour flame does sodium produce?
Yellow
PRACTICAL: What colour flame does potassium produce?
Lilac
PRACTICAL: What colour flame does calcium produce?
Orangy-red
PRACTICAL: What colour flame does copper produce?
Green
PRACTICAL: How do you test for metal hydroxide?
Add a few drops of sodium hydroxide solution to the unknown compound
PRACTICAL: What colour PPT. do calcium ions produce?
White
PRACTICAL: What colour PPT. do copper (II) ions produce?
Blue
PRACTICAL: What colour PPT. do iron (II) ions produce?
Green
PRACTICAL: What colour PPT. do iron (III) ions produce?
Brown
PRACTICAL: What colour PPT. do aluminium ions produce?
White AT FIRST, then it redissolves in excess NaOH to form a colourless solution
PRACTICAL: What colour PPT. do magnesium ions produce?
White
What is flame emission spectroscopy used for?
Identifies different ions in solutions ( + calculate their concentration) and mixtures
How does flame emission spectroscopy work?
1) A sample is placed in a flame. As it heats, the electrons move to higher energy levels. When they drop back to their original energy levels, they release energy as light
2) The light passes through a spectroscope, which detects different wavelengths of light to produce a line spectrum
3) The combination of wavelengths emitted by an ion depends on its charge and electron arrangement
4) Different ions will emit different wavelengths of light. So, each ion produces a different pattern of wavelengths
5) The intensity of the spectrum indicates the concentration of that ion in the solution, meaning that line spectra can be used to identify different ions in solution and calculate different concentrations
What are some advantages of using machinery for flame emission spectroscopy?
- Very sensitive - they detect the tiniest amount of substances
- Very fast
- Very accurate
