Unit 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
a) A start line is drawn near the bottom of the paper. The mixture is spotted on the line.
b) A beaker is filled with small amount of solvent (it cannot touch or go above the start line
when paper is placed in a beaker)
c) Paper is hung on a rod and placed in a beaker.
d) Solvent travels up the paper, thus separating the components.
e) Before solvent level reaches the end, the paper is taken out and the finish line is
marked. The paper is dried.
f) The procedure works when different compounds have different affinities for the
solvent/paper. Stronger attraction for the paper - travels slowly with the solvent etc.
g) Paper is called the stationary phase - it doesn’t move. Solvent is the mobile phase.
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 flame test results
Lithium compounds– crimson red flame
Sodium compounds - yellow flame
Potassium compounds - lilac flame
Calcium compounds – orange red flame
Copper compounds -green flame.
Describe the sodium
hydroxide test results and
write the equations
Copper(II) forms a blue precipitate:
Cu2+
(aq) + 2OH−
(aq) → Cu(OH)2(s)
Iron(II) forms a dirty green precipitate:
Fe2+
(aq) + 2OH−
(aq) → Fe(OH)2(s)
iron(III) forms a brown precipitate:
Fe3+
(aq) + 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+
(aq) + 2OH−
(aq) → Ca(OH)2(s)
Mg2+
(aq) + 2OH−
(aq) → Mg(OH)2(s)
Al3+
(aq) + 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
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
a) instrumental method used for identifying metal ions in solution or measuring
their concentration
b) Spectroscope measures the exact wavelength of the light emitted by a metal
ion
c) That allows for definite identification - sometimes colours are difficult to
distinguish.
d) Concentrations are found by measuring the intensity of light emitted. The
more intense light, the greater the concentration of the metal ion in a solution.
e) From the intensity vs concentration graph, you can read off a relevant
concentration value at a given intensity.
What are cations and anions?
Cations are the positive ions; anions are the negative ions.
