Topic 8: Chemical Analysis Flashcards
What is a pure substance?
A pure substance is a single element or compound, not mixed with any other substance
What is a formulation and how is it made? Give examples
- A formulation is a mixture of compounds in measured quantities that has been designed as a useful product.
- Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
- E.g. alloys are mixtures of metals; they are harder than pure metals, so have a particular purpose.
Describe paper chromatography
- draw a pencil line across the chromatography paper, 1 - 2 cm from the bottom
- use a pipette or capillary tube to add small spots of each ink to the line on the paper
- place the paper into a container with a suitable solvent in the bottom
- allow the solvent to move through the paper, but remove the
chromatogram - before it reaches the top, allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
How is Rf calculated?
Distance moved by the spot / distance moved by solvent
In a paper chromatography experiment, a compound A was found to have an Rf value of 0.85 - what does it tell you about the compound?
It has a higher affinity for the solvent than for the paper.
Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine
Hydrogen – pop with burning splint over gas
Oxygen – glowing splint relights
Carbon dioxide – turns limewater (Ca(OH)2) milky
Chlorine – bleaches damp litmus paper and makes it white
Describe the lithium flame test result
Lithium compounds– crimson red flame
Describe the sodium hydroxide test results and write the equations
Copper(II) forms a blue precipitate:
Cu2+ + 2OH− (aq) → Cu(OH)
Iron(II) forms a dirty green precipitate:
Fe2+ + 2OH− (aq) → Fe(OH)
iron(III) forms a brown precipitate:
Fe3+ + 3OH− (aq) → Fe(OH)3(s)
Al3+, Ca2+, Mg2+ form white precipitates but only the Al(OH)3 dissolves in excess NaOH to form a colourless solution.
Ca2 + 2OH− (aq) → Ca(OH)2(s)
Mg2+ + 2OH− (aq) → Mg(OH) (s)
Al3+ + 3OH− (aq) → Al(OH)3(s)
Describe the test for carbonate anions
- Add dilute acid, e.g. HCl
- Fizzing observed, as CO2 is released.
- E.g. Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2
Describe the test for sulfate anions
Add a solution containing Ba2+ cations, e.g. a solution of BaCl2
White precipitate of BaSO4 forms
E.g. K2SO4 + BaCl2 → 2 KCl + BaSO4
(!!!) can also be thought of a test for barium (II); add sulfates - white precipitate forms.
Describe the test for halide anions
Add a solution of AgNO3 (acidified with HNO3)
Chlorides - white precipitate, silver chloride; Ag+ + Cl− → AgCl
Bromides - cream precipitate, silver bromide; Ag+ + Br− → AgBr
Iodides - yellow precipitate, silver iodide; Ag+ + I− → AgI
What are instrumental methods?
They are accurate, sensitive and rapid methods which are useful when the amount of sample is very small
Describe the flame emission spectroscopy
- Flame emission spectroscope splits colored light from vaporized samples into an emission spectrum.
- Emission spectrum resembles a colored barcode with distinct lines.
- Each metal ion produces a unique emission spectrum.
- Metal in a sample is identified by comparing its spectrum with reference spectra.
- Reference spectra are emission spectra from known metal ions.
- Matching spectra indicate the presence of the same metal ion in the sample.
What are cations and anions?
Cations are the positive ions; anions are the negative ions.
Describe the sodium flame test results
Sodium compounds - yellow flame
