T8: chemical analysis Flashcards
What is a pure substance?
How do chemically pure substances melt/boil?
they melt or boil at a specific temperature.
How can you test the purity of a sample?
- By measuring its melting and boiling point and comparing it with the melting or boiling point of the pure substance.
- The closer your measured value is to the actual value, the purer your sample is
What will impurities in your sample do?
- lower the melting point
- increase the melting range
- increase the boiling point
- may result in the sample boiling at a range of temperatures.
What do formulations contain?
Each component in the formulation is present in a measured quantity and contributes to the properties of the formulation so that it meets its required function.
What is a formulation?
A formulation is a mixture that has been designed as a useful product.
What are many products of formulations?
Many products are complex mixtures in which each chemical has a particular purpose.
What are paints composed of?
Pigment: gives the paint colour e.g. titanium oxide is used as a pigment in white paints.
Solvents: used to dissolve the other components and alter the viscosity.
Binder (resin): forms a film that holds the pigment in place after it’s been painted on.
Additives: added to further change the physical and chemical properties of the paint.
What does the amount of chemicals in a paint depend on and why?
Depending on the PURPOSE of the paint, the chemicals used and their amounts will be changed so the paint produced is right for the job.
Give a reasons why formulations are useful in the pharmaceutical industry?
By altering the formulation of a pill, chemists can make sure it delivers the drug to the right part of the body at the right concentration, that it’s consumable and has a long enough shelf life.
What are the uses of formulations (in every day life)?
fuels, cleaning agents, paints, cosmetics, medicines, metal alloys, fertilisers, drinks and foods
What does the information on the packing of a product tell you?
The ratio or percentage of each component tells you the product is a formulation.
Allows you to choose a formulation with the right composition for your particular use.
What is chromatography?
An analytical method used to separate the substances in a mixture and can give information to help identify substances.
What are the different stages in chromatography?
mobile and stationary phase
What is the mobile phase?
Where the molecules can move. This is always a liquid or gas.
What is the stationary phase?
Where the molecules cannot move. This can be a solid or a really thick liquid.
During the chromatography experiment, what do the substances in the sample do?
Substances in the sample constantly move between the mobile and stationary phases- an equilibrium is formed between the two phases.
What does the mobile phase move through?
The mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves with it.
What does the speed a chemical moves depend on?
How quickly a chemical moves depends on how it’s ‘distributed’ between the 2 phases- whether it spends more time in the mobile or stationary phase.
What happens to the chemicals that spend more time in the mobile phase than the stationary phase?
move further
How do the components in a mixture normally separate and why?
Will normally separate through the stationary phase, so long as the components spend different amounts of time in the mobile phase.
How can the number of spots change?
The number of spots may change in different solvents as the distribution of the chemical will change depending on the solvent.
What is the chromatography of a pure substance like?
A pure substance will only ever form one spot in any solvent as there is only one substance in the sample.
In paper chromatography, what is the stationary and mobile phase?
stationary: the chromatography paper (often filter paper)
mobile: the solvent (e.g. water or ethanol)
In paper chromatography, what does the amount of time the molecules spend in each phase depend on?
- how soluble they are in the solvent
- how attracted they are to the paper
What happens to molecules with a higher solubility and which are less attracted to the paper?
will spend more time in the mobile phase- they’ll be carried further up the paper.
What is the result of chromatography called?
a chromatogram
What is the Rf value?
The Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.
How can you calculated Rf values?
Rf= distance traveled by the substance ÷ distance traveled by the solvent
How does chromatography test if a certain substance is present in a mixture?
Run a pure sample of that substance (a reference) alongside the unknown mixture.
If the Rf values of the reference and one of the spots in the mixture match, the unknown substance may be present (but you have not proved they’re the same)
What is the Rf value dependent on?
the solvent: if you change the solvent the Rf value for a substance will change.
How can you test for both the mixture and reference in a number of different solvents?
- If the Rf value of the reference compound matches the Rf value of one of the spots in the mixture in all of the solvents, then it’s likely the reference compound is present in the mixture.
- If the spots in the mixture and the spot in the reference only have the same Rf value in some of the solvents, then the reference compound isn’t present in the mixture.
In the chromatography of flowers, what is the solvent used and why?
ethanol as the flower colours are soluble in ethanol but insoluble in water so the colours wouldn’t move/dissolve.
What are 4 mistakes that can happen in chromatography?
1) water level above the start line: food colours would dissolve into the ink.
2) start line drawn in ink: the ink will ‘run’ on the paper
3) no lid: the solvent can evaporate (more quickly)
4) side walls of the paper touching the side walls of the beaker: interfere with the way that water/solvent moves
Explain how the different dyes in X are separated by paper chromatography. (4)
- solvent moves through paper
- different dyes have different solubilities in solvent
- and different attractions for the paper
- and so are carried different distances
What is these and result for chlorine gas?
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
What is these and result for oxygen?
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
What is these and result for carbon dioxide?
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water).
When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky / cloudy.
What is these and result for hydrogen?
The test for hydrogen uses a burning/ lit splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a squeaky pop sound. (the noise comes from the hydrogen burning quickly with the oxygen in the air to form H20)
When testing for chlorine gas, why may the litmus paper turn red for a moment first?
the solution of chlorine is acidic.
What do tests for anions give?
often give precipitates
What are carbonates?
substances that contain CO3^2- ions.
How can you test if a substance contains carbonate ions?
-put a sample in a test tube
-use a pipette to add a few drops of dilute acid
-connect the test tube to a test tube of limewater
- if carbonate ions are present, carbon dioxide with be released, turning the limewater cloudy when it bubbles through it.
How can sulfate ions (SO4^2-) be tested for?
- use a dropping pipette to add a couple drops of dilute HCL acid followed by a couple drops barium chloride solution (BACL2) to the unknown solution in the test tube.
What would be a positive result of sulfates?
If sulfate ions are presents, a white precipitate of barium sulfate will form.
(Ba^2+)aq +(SO4^2-)aq → BaSO4(s)
How do you test for halides?
- add a couple drops of dilute nitric acid(HNO3) followed by a couple drops of silver nitrate solution, AgNO3, to the unknown solution.
What are the different results when testing for halide ions? (chloride, bromide and iodide)
- a chloride gives a white precipitate of silver chloride
- a bromide gives a cream precipitate of a silver bromide
- a iodide gives a yellow precipitate of a silver iodide
What do flame tests do?
Flame tests can be used to detect and identify some metal ions (cations).
Lithium, sodium, potassium, calcium and copper compounds produce distinctive colours in flame tests
What colour does lithium ions burn with?
crimson flame
What colour does sodium ions burn with?
yellow flame
What colour does potassium ions burn with?
lilac flame
What colour does calcium ions burn with?
orange-red flame
What colour does copper ions (CU2+) burn with?
green flame
How do you carry out the flame test?
- clean a platinum wire loop by dipping it in some dilute HCL.
- then holding it in a blue flame from a bunsen burner until it burns without any colour.
- Then dip the loop into the sample you want to test and put it back in the flame.
- record the colour of the flame
What type of samples does flame tests only work for?
Samples that contain a single metal ion.
Why won’t a flame test for a sample with a mixture of metal ions work?
If a sample containing a mixture of ions is used some flame colours can be masked. (the flame colours of ions may be hidden by the colours of others).
What properties of metal hydroxides allow them to be tested for?
- many metal hydroxides are insoluble and precipitate out of the solution when formed.
- some of these hydroxides have a characteristic colour.
What is used to test for a metal hydroxide?
Sodium hydroxide solution can be used to identify some metal ions (cations).
What is the process for testing for a metal hydroxide?
- Add a few drops of sodium hydroxide solution to your solution of your unknown compound.
- An insoluble hydroxide will form.
- If you get a coloured insoluble hydroxide you can often tell which metal was in the compound.
Using the metal test, how can you test for calcium ions? (and what is the ionic equation)
white precipitate
Ca^2+ (aq) + 2OH^- (aq) → Ca(OH)2 (s)
Using the metal test, how can you test for copper ions? (and what is the ionic equation)
Blue precipitate
Cu^2+ (aq) + 2OH^- (aq) → Cu(OH)2 (s)
Using the metal test, how can you test for iron (II) ions? (and what is the ionic equation)
Green precipitate
Fe^2+ (aq) + 2OH^- (aq) → Fe(OH)2 (s)
Using the metal test, how can you test for iron (III) ions? (and what is the ionic equation)
Brown precipitate
Fe^3+ (aq) + 3OH^- (aq) → Fe(OH)3 (s)
Using the metal test, how can you test for aluminium ions? (and what is the ionic equation)
White precipitate at first. But then redissolves in excess NaOH to form a colourless solution.
Al^3+ (aq) + 3OH^- (aq) → Al(OH)3 (s)
Using the metal test, how can you test for magnesium ions? (and what is the ionic equation)
White.
Mg^2+ (aq) + 2OH^- (aq) → Mg(OH)2 (s)
What happens during the flame emission spectroscopy?
- a sample is placed in the flame
- as the ions heat up their electrons become excited.
- when the ions drops back to their original energy level, they transfer energy as light
- the light passes through the spectroscope
What does a spectroscope do?
detect different wavelengths of light to produce a line spectrum
What does the combination of wavelengths emitted by an ion depend on?
its charge and its electron arrangement.
How are does each ion have a different line spectrum?
Since no 2 ions have the same charge AND the same electron arrangement, different ions emit different wavelengths of light.
So each ion produces a different pattern of wavelengths and has a different line spectrum.
What does the intensity of the spectrum indicate?
concentration of the ions in that solution.
What can line spectrums be used for?
to identify ions in a solution and calculate their concentrations.
Why is flame emission spectroscopy more useful than the flame tests?
Flame emission spectroscopy can be used to identify different ions in mixture whereas flame tests only work for substances that only contain a single metal ion.
Why do chemists often use instrumental analysis (tests using machines e.g. flame emission spectroscopy) instead of conducting tests?
advantages of using machines:
- very sensitive (can detect even the tiniest amounts of substances)
- very rapid/fast and tests can be automated.
- very accurate
