Chemical analysis Flashcards
What is a ‘pure’ substance, in chemistry?
- a pure substance is a single element or compound, not mixed with another substance
What 2 pieces of data can we use to determine if a substance is pure, why?
- pure elements and compounds MELT and BOIL at specific temperatures => MELTING POINT and BOILING POINT data can be used to distinguish pure substances from mixtures
From a heating graph, how can we know if a substance is pure?
- if the melting point and boiling point lines are straigh and horizontal
- and if these temperatures match the real temperatures from the internet
From a heating graph, how can we know that a substance isimpure?
- if the melting point and boiling point are not straight and completely horizontal
(the substance melts and boils over a range of temperatures) - more than one value for the melting/boiling point)
In everyday language, what is a ‘pure’ susbtance?
- a pure substance in this context can mean anything that has had nothing added to it, so it is unadulterated and in its natural state, eg. pure milk
What is a formulation?
- a complex mixture that has been designed as a useful product
How are formulations made?
- formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties
Give 7 examples of formulations.
- fuels
- cleaning agents
- paints
- medicines
- alloys
- fertilisers
- foods
Why is chromatography used?
- chromatography can be used to separate mixtures and give information to help identify substances
- paper chromatography allows us to separate substances based on their different SOLUBILITIES
What is the method for any general paper chromatography experiment?
1) Take a piece of special chromatography paper.
2) Draw a horizontal pencil line 2cm up.
3) Put a dot of the first colour on the pencil line and other dots along the line for any other colours (space them out).
4) Place the bottom of the paper in a solvent (a liquid that will dissolve substances) (not above the pencil line).
What is the stationary phase in chromatography?
What is the mobile phase in chromatography?
- the stationary phase is the paper because it does not move
- the mobile phase is the solvent because it does move
How can we identify a pure substance from an impure substance after carrying out paper chromatography?
- a pure chemical will produce a single spot in ALL solvents
- an impure substance may separate into different spots DEPENDING on the solvent
How does paper chromatography work?
- each chemical in the mixture will be attracted to the stationary phase to a different extent (chemicals that are weakly attracted will move further up the paper)
Why is the starting line always drawn in pencil, in paper chromatography?
- pencil will not dissolve in the solvent => will not affect the results (ink will be carried up the paper with the solvent)
What is the method for investigating how paper chromatography can be used to separate and tell the difference between coloured substances?
- we have a sample of food colouring which is a mixture of chemicals (U - unknown).
- we also have four known colourings that U could contain (A-D).
1) Use a ruler to draw a horizontal pencil line 2cm up on the chromatography paper.
2) Mark 5 pencil spotsat equal spaces across the pencil line.
3) Use a capillary tube to put a small spot of each of the knwon food colours and the unknown colour onto the pencil spots.
4) Pour water into a beaker to a depth of 1cm (solvent).
5) Attach the paper to a glass rod using tape and then lower the paper into the beaker so that the bottom of the paper just touches the water (make sure the pencil line is above the surface of the water). *also make sure that the sides of the paper do not touch the side walls of the beaker or it will interfere with the way that the water moves
6) Put a lid on the beaker to reduce evaporation of the solvent.
7) The water should be moving up the paper and the colours should be being carried up too.
8) Remove the paper from the beaker once the water has moved ~3/4 up the paper.
9) Draw a horizontal pencil line to mark where the water reached.
From the paper chromatography required practical, how can we know what colours were in the unknown colour?
- some of the spots in A, B, C, or D should line up with the spots in the unknown colour (this tells us that the unknown colour is a mixture of all the colours that line up)
What can we do with Rf values?
- we can use the Rf values to identify the chemicals in the colours used
What is the equation for an Rf value?
Rf = distance moved by substance / distance moved by solvent (water)
=> search this Rf value up in a database to identify the chemical
How can we increase the validity of the Rf value of a chemical?
- several different chemicals could have the same Rf value of one substance
=> repeat the experiment using a different solvent (instead of water)
How can we test for hydrogen?
- hold a burning splint at the open end of a test tube of hydrogen
=> hydrogen burns rapidly with a ‘squeaky’ pop sound
How can we test for oxygen?
- insert a glowing splint into a test tube of the oxygen gas
=> the splint relights
How can we test for carbon dioxide?
- use an aqeous solution of calcium hydoxide (limewater)
- shake or bubble carbon dioxide through limewater
=> the limewater turns milky (cloudy)
How can we test for chlorine?
- put damp litmus paper into chlrorine gas
=> the litmus paper is bleached and turns white
What are flame tests used for?
- flame tests can be used to idetify some metal ions (cations; positively charged)
What is the method for carrying out a flame test?
1) Place a small amount of the chemical onto a wire mounted in a handle.
2) Place the looped end of the wire into a BLUE bunsen burner flame.
3) The colour of the flame can be used to identify the metal ion present.
What colour flame do these cations produce:
- lithium?
- sodium?
- potassium?
- calcium?
- copper?
lithium => crimson
sodium => yellow
potassium => lilac
calcium => orange-red
copper => green
Name a metal ion that would produce a green flame. (Exam Q)
copper ion
What is especially true if there is only a low concentration of the metal compound when carrying out a flame test, or if a sample contains a mixture of metal ions?
- the colour of the flame test can be difficult to distiguish - - some flame colours can be MASKED)
Some fireworks contain a mixture of metal ions.
Why is it difficult to identify the metal ions from the colour of the flame? (Exam Q)
flame colours are masked
What are 3 advantages of using instrumental methods to detect and identify elements and compounds?
- rapid (metal ion can be identified more quickly in fes than by using flame tests)
- sensitive (works on a tiny sample)
- accurate (fes is more likely to identify a metal ion than flame tests)
How does flame emission spectroscopy work to identify a metal ion in a sample?
- a sample of the metal ion in solution is placed into a flame
- the light given out is then passed into a spectroscope machine
- the spectroscope converts the light into a line spectrum
- the positions of the lines in the spectrum are specific for a given metal ion
On a line spectrum, how can we tell if there is a higher concentration of the metal ion?
- lines are more intense
What metal hydroxide solution can be used to identify some metal ions (cations)?
sodium hydroxide solution
What is formed when sodium hydroxide solution is added to solutions of aluminium, calcium, and magnesium ions?
Out of the three cations, which metal hydroxide precipitate dissolves in excess NaOH?
white precipitates
- only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution => we know this solution had aluminium ions present
What is the balanced equation for the reaction between calcium nitrate solution and sodium hydroxide solution?
Ca(NO₃)₂ + 2NaOH → 2NaNO₃ + Ca(OH)₂
→ sodium nitrate solution + calcium hydroxide precipitate
=> calcium hydroxide then has to be flame tested to see whether magnesium or calcium ions were present (both of these produce a white precipitate and do not dissolve in excess NaOH)
What is the balanced equation for the reaction between magnesium nitrate solution and sodium hydroxide solution?
Mg(NO₃)₂ + 2NaOH → 2NaNO₃ + Mg(OH)₂
What is the balanced equation for the reaction between aluminium nitrate solution and sodium hydroxide solution?
Al(NO₃)₃ + 3NaOH → 3NaNO₃ + Al(OH)₃
How can the reaction between solutions of copper(II), iron(II), and iron(III) and sodium hydroxide solution be used to see if these ions are present?
- they form coloured compounds as they are transition metals
How do we know if copper(II) ions are present in the solution when added to sodium hydroxide solution?
- a blue precipitate of copper(II) hydroxide is formed
How do we know if iron(II) ions are present in the solution when added to sodium hydroxide solution?
- a green precipitate of iron(II) hydroxide is formed
How do we know if iron(III) ions are present in the solution when added to sodium hydroxide solution?
- a brown precipitate of iron(III) hydroxide is formed
What is the balanced equation to show the reaction between copper(II) nitrate solution and sodium hydroxide solution?
Cu(NO₃)₂ + 2NaOH → 2NaNO₃ + Cu(OH)₂
What is the balanced equation to show the reaction between iron(II) nitrate solution and sodium hydroxide solution?
Fe(NO₃)₂ + 2NaOH → 2NaNO₃ + Fe(OH)₂
What is the balanced equation to show the reaction between iron(III) nitrate solution and sodiumhydroxide solution?
Fe(NO₃)₃ + 3NaOH → 3NaNO₃ + Fe(OH)₃
How do we test for the carbonate ion (CO₃⁻)?
1) Add any dilute acid to the sample.
2) If the carbonate ion is present, the acid will react with the carbonate to make carbon dioxide gas; we will see effervescene (fizzing) but that does not prove that we have carbon dioxide gas.
3) Bubble the gas through limewater; if the limewater goes cloudy, we have carbon dioxide => this means we started with the carbonate ion.
How do we test for the halide ions (Cl⁻, Br⁻, I⁻)?
1) Add dilute nitric acid to the sample.
2) Add dilute silver nitrate solution.
3) If halide ions were present, a precipitate of the silver halide would form.
(each halide makes a different coloured precipitate)
What colour precipitate of silver halide do these halides form:
- chloride ions?
- bromide ions?
- iodide ions?
chloride => white precipitate
bromide => cream precipitate
iodide => yellow precipitate
Silver nitrate solution and dilute nitric acid are added to the solution.
A cream precipitate forms.
Which ion is shown to be present by the cream precipitate? (Exam Q)
bromide ion
How do we test for the sulfate ion (SO₄²⁻)?
1) Add dilute hydrochloric acid to the sample.
2) Add barium chloride solution.
3) If sulfate ions are present, a white precipitate will form (barium sulfate).
Describe a test to show the presence of sulfate ions in the solution.
Give the result of the test if there are sulfate ions in the solution. (Exam Q)
- add barium chloride (solution)
- add hydrochloric acid
- white precipitate produced
Sample Qs: what is the compound?
A - a lilac flame & a yellow precipitate with silver nitrate
B - an orange-red flame & effervescence with hydrochloric acid => gas produced turned limewater cloudy
C - a blue precipitate with sodium hydroxide solution & a white precipitate with barium chloride + dilute hydrochloric acid
A - potassium iodide
B - calcium carbonate
C - copper(II) sulfate
What name is given to a useful product such as methylated spirit? (Exam Q)
formulation
