chemical analysis Flashcards
pure substances and formulations
-a pure substance can be a single element
-a pure substance can also be a single compound
-a pure substance is not mixed with any other substance
what is true about a pure susbatnce’s melting and boiling point (Pick 2)
A) a pure substance melts over a range of temperatures
B) a pure substance boils at a specific fixed temperature
C) a pure substance melts at a specific fixed temperature
D) a pure substance boils over a range of temperatures
B) a pure substance has a specific fixed boiling point
C) a pure substance melts at a specific fixed fixed temperature
impure substances melt and boil over a range of temperatures
if a sample of water boils over a range of temperatures is it pure or impure?
is the sample boils over a range of temperatures than it is an impure substance because impure substances boil over a range of temperatures
what are formulations
formulations are a complex mixture that has been designed as a useful product
in a formulation quantity of each component is carefully measured so that the product has the properties we need.
examples of formulations
- fuels
- cleaning products
- alloys
- fertilisers
- medicine
- paint
All separation techniques including paper chromatogrpahy are physical processes
why
because they do not involve chemical reactions, no new substances are made
what is paper chromatography
paper chromatography allows us to separate substances based on their different solubilities
how do we carry out paper chromatography
we take a piece of chromatography paper and draw a pencil line at the bottom. we put a dot of our first colour on the pencil line
now we put a dot of our second colour
we must do this for several colours as long as there is enough space on the paper.
now place bottom of paper into a solvent
solvent makes its way up the paper and dissolves the ink. these are carried up the paper as well
how do we carry out paper chromatography
we take a piece of chromatography paper and draw a pencil line at the bottom. we put a dot of our first colour on the pencil line
now we put a dot of our second colour
we must do this for several colours as long as there is enough space on the paper.
now place bottom of paper into a solvent
solvent makes its way up the paper and dissolves the ink. these are carried up the paper as well
what is the definition of a solvent
liquid that will dissolve substances
what is the stationary phase and what is the mobile phase
stationary phase- paper ( does not move)
mobile phase- solvent ( moves up the paper)
pure substances will produce a single spot because…
pure substances will produce a single spot because they are not mixed with any other substance.
in context: if one of the colours has separated into two different spots what does it mean
it is a mixture of two different substances
why does paper chromatography work
because each chemical in the mixture is attracted to the stationary phase to a different extent.
chemicals that are strongly attracted to the stationary phase will:
A) move further up the paper
B) not move very far
A) chemicals that are strongly attracted to the stationary phase will not move very far
chemicals that are weakly attracted to the stationary phase will move:
A) further up the paper
B) not very far
A) chemicals that are weakly attracted to the stationary phase will move further up the paper
remember a pure substance will produce a single spot in ALL solvents
- a pure substance will produce a single spot in ALL solvents
chemicals in a mixture may separate into different spots depending on the solvent
chemicals in a mixture may separate into different spots depending on the solvent
why do we draw the starting line in pencil
pen ink would move up the paper with the solvent
testing for gases
describe how to test for hydrogen gas?
- remove the bung of the test tube and bring a burning splint towards mouth of test tube. hydrogen will burn rapidly and produce a squeaky pop sound
describe how to test for oxygen gas?
- bring a glowing splint towards the mouth of the test tube
- if the splint relights then oxygen gas is present
describe how to test for carbon dioxide gas
- draw up some gas into a plastic pipette.
- bubble gas through test tube
- if limewater turns cloudy then carbon dioxide is present
describe how to test for chlorine gas
-bring damp litmus paper into mouth of test tube
chlorine would bleach the damp litmus paper white
what is limewater? ( could ask in exam)
limewater is an aqueous solution of calcium hydroxide
Flame tests- how does it work explain steps briefly
1) First we place a small amount of our chemical onto a wire mounted in a handle
2) place end of this into a roaring (blue) bunsen burner flame
3) the colour of the flame can help us identify the metal ion
list all the colours of the flames of these metal ions: lithium sodium potassium calcium copper
- lithium= produces a crimson flame
- sodium=produces a yellow flame
- potassium= produces a lilac flame
- calcium= produces an orange red flame
- copper=produces a green flame
identify the problems of using flame tests to identify metal ions:
- sometimes, the colour can be difficult to distinguish this is especially true if there is a low concentration of the metal compound
- sample may contain a mixture of different metal ions which could mask the colour of the flame
flame emission spectroscopy is an instrumental method.
what is an instrumental method
-a method carried out using a machine
explain how flame emission spectroscopy works:
a sample of metal ion in solution is placed into a flame
the light given of by the flame is passed into a machine called a spectroscope.
the spectroscope converts light into a line spectrum
the position of the lines in the spectrum are specific for a given metal ion.
the light given out from the flame is passed into a machine of called a spectroscope.
what does a spectroscope do?
- when light given out from the flame is passed into a spectroscope, it converts the light into a line spectrum
the position of the lines in the spectrum are specific for a given metal ion so we can use the line spectrum to identify the metal ion present.
as well helping us identify the metal ions that are present. what else can flame emission spectroscopy show?
-the concentration of the metal ion because the line becomes more intense at a higher concentration
what are the three advantages of using flame emission spectroscopy?
- rapid= flame emission spectroscopy can be used to analyse samples more rapidly than using flame tests
- sensitive-flame emission spectroscopy will work on even a tiny sample of the metal compound
- accurate- more likely to identify the metal ions correctly than using a flame test
METAL HYDROXIDES- sodium hydroxide solution can be used to identify some metal ions
which three metals form white precipitates when sodium hyddroxide solution is added to them:
-aluminium
-magnesium
-calcium
( memorise balanced equations for all of each metal nitrates reaction with sodium hydroxide eg aluminium nitrate+ sodium hydroxide= sodium nitrate+ aluminium hydroxide) the aluminium hydroxide is solid it is a precipitate
how can we distinguish the white precipitates formed when reacting aluminium, magnesium and calcium with sodium hydroxide to identify the metal ions.
to identify if a precipitate has alumnium ions inside it, we can add excess sodium hydroxide and the precipitate should redissolve
to work out which precipitate is calcium we must do a flame test.
solutions of copper (ii) ions iron (iii) ions iron (ii) ions form coloured precipitates when sodium hydroxide is added.
name the precipitates for each ion
copper (ii) ions= copper (ii) hydroxide blue precipitate
iron (ii) ions hydroxide= green precipitate
iron (iii) ions= iron (iii) hydroxide brown precipitate.
(memorise balanced equations)
testing FOR NON METAL IONS: testing for carbonates:
- add dilute acid to sample.
- the dilute acid will react with the carbonate to make carbon dioxide gas we will see an effervscence (the fizzing does not prove the gas is co2).
- now we bubble this gas through limewater.
- if the limewater goes cloudy, then this will prove that we have co2.
Testing for NON METAL IONS: hallide ions
1) add dilute nitric acid to the sample
2) add dilute silver nitrate solution
3) hallide ions produce precipitates of the silver halide
4) each hallid produces a different coloured precipitate
name the coloured precipitates that each hallide ion produces when reacted with silver nitrate solution and nitric acid
chloride ions
bromide ions
iodide ions
- chloride ions produce a white precipitate of silver chloride
- bromide ions produce a cream precipitate of silver bromide
- iodide ions produce a yellow precipitate of silver iodide
testing for NON METAL IONS: sulfate ions
1) add dilute hydrochloric acid to a sample
2) add barium chloride solution
3) if sulfate ions present we will see a white precipitate
specification points: 4.8.1.1 pure substances
in chemistry, a pure substance is a single element or compound not mixed with any other substance
-pure elements and compounds melt and boil at specific fixed temperatures
example of a pure substance in everyday language= a substance which has nothing added to it such as pure milk
specification points 4.8.1.2 formulations
-A formulation is a complex mixture that has been designed as a useful product. formulations are made by mixing components in carefully measured quantities to ensure the product has the required properties
examples of formulations- fuels, paints, cleaning products, alloys, fertilisers, medicines and foods
specification points 4.8.1.3 Chromatography
chromatography can be used to separate mixtures and give information to help identify substances . chromatography involves the stationary phase ( paper) and the mobile phase (solvent) . separation depends on the distribution of substances between the phases. chemicals that are strongly attracted to the stationary phase will not move very far. chemicals that are weakly attracted to the stationary phase will move further up the paper.
rf value- distance moved by solute/ distance moved by solvent
different compounds have different rf values in different solvents which can be used to help identify the compounds.
the compounds in a mixture may separate into different spots depending on the solvent but pure compounds will produce a single spot in all solvents.
specification points 4.8.2.1 identifying gases
- 8.2.2
- 8.2.3
- 8.2.4
-test for hydrogen= the test for hydrogen uses a burning splint held at open end of test tube of the gas. hydrogen burns rapidly with a squeaky pop sound
-test for oxygen=the test for oxygen uses a glowing splint inserted into a test tube of gas. splint relights in oxygen
test for carbon dioxide= the test for carbon dioxide uses an aqueous solution of calcium hydroxide (limewater) when carbon dioxide is shaken with or bubbled through limewater, limewater turns cloudy.
test for chlorine=to test for chlorine use litmus paper. when damp litmus paper is put into chlorine gas, the litmus paper is bleached and turns white.
specification points 4.8.3 identifying ions using flame tests and flame emission spectroscopy
Flame tests can be used to identify some metal ions (cations).
Lithium, sodium, potassium, calcium and copper compounds
produce distinctive colours in flame tests:
• lithium compounds result in a crimson flame
• sodium compounds result in a yellow flame
• potassium compounds result in a lilac flame
• calcium compounds result in an orange-red flame
• copper compounds result in a green flame.
If a sample containing a mixture of ions is used some flame colours
can be masked.
specification points: 4.8.3.6 instrumental methods
Elements and compounds can be detected and identified using
instrumental methods. Instrumental methods are accurate, sensitive
and rapid.
Students should be able to state advantages of instrumental
methods compared with the chemical tests in this specification.
specification points- flame emission spectroscopy 4.8.3.7
Flame emission spectroscopy is an example of an instrumental
method used to analyse metal ions in solutions.
The sample is put into a flame and the light given out is passed
through a spectroscope. The output is a line spectrum that can be
analysed to identify the metal ions in the solution and measure their
concentrations.
specification points metal hydroxides 4.8.3.2
-sodium hydroxide solution can be used to identify some metal ions
-solutions of aluminium, calcium and magnesium ions form white precipitates when sodium hydroxide solution is added. but only aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.
-solutions of copper (ii) ions, iron (ii) ions and iron (iii) ions form coloured precipitates when sodium hydroxide solution is added.
copper (ii) forms blue precipitate
iron (ii) forms green precipitate
iron (iii) forms brown precipitate
specification points carbonates hallides and sulfates
- 8.3.3
- 8.3.4-
- 8.3.5-
4.8.3.3-carbonates react with dilute acids to form carbon dioxide gas. carbon dioxide gas can be identified by limewater
4.8.3.4-hallide ions in solution produces precipitates with silver nitrate solution in the prescence of dilute nitric acid
silver chloride is white precipitate
silver bromide- cream precipitate
silver iodide- yellow precipitate
4.8.3.5-sulfate ions in solution produce white precipitate with barium chloride solution in the presence of dilute hydrochloric acid
