Chemical Analysis Flashcards
what is a pure substance?
a single substance (element or compound) that is not mixed with anything else
what kind of MP and BP do pure substances have?
a fixed one
a mixture will melt or boil…?
over a range of temperatures
what is a formulation?
a mixture of different compounds in fixed proportions
what is the stationary phase in chromatography?
the paper
what is the mobile phase in chromatography?
the solvent
what does the separation in chromatography depend on?
the distribution of substances between the phases
what does Rf stand for?
retention factor
how do you calculate Rf?
distance travelled by dye / distance travelled by solvent
how does an Rf value change across solvents?
different compounds have different Rf values in different solvents
how many spots will a pure substance produce in chromatography?
one spot in all solvents
why is the baseline in chromatography drawn in pencil?
so that the pen ink doesn’t dissolve and move up the paper
why is the level of solvent in chromatography below the sample line?
if it wasn’t, it would spread out rather than going up
why do samples move at different speeds during chromatography?
the more soluble a substance is in the mobile phase, the further it will travel up the stationary phase
more soluble means what?
substance has a stronger attraction to the mobile phase and will move further up the stationary phase
what is the gas test for oxygen?
Glowing splint held in a test tube. if oxygen is present, splint relights
what is the gas test for hydrogen?
lighted splint held in a test tube. if hydrogen is present, a pop sound is heard
what is the gas test for carbon dioxide?
gas bubbled through limewater. if carbon dioxide is present, Limewater turns milky or cloudy white
what is the gas test for chlorine?
Damp litmus paper held in a test tube. if chlorine is present, Paper turns white
describe the process of flame test to identify metal ions in ionic substances
- dip nichrome wire in acid and then water to remove contaminants
- dry wire in roaring bunsen burner flame
- dip wire into ionic solution to be tested
- place wire into hottest part of flame and record any change in colour of the flame
what is the positive result for lithium in the flame test?
flame turns crimson red
what is the positive result for potassium in the flame test?
flame turns lilac
what is the positive result for calcium in the flame test?
flame turns orange-red
what is the positive result for sodium in the flame test?
flame turns yellow
what is the positive result for copper in the flame test?
flame turns green
give 2 limitations of the flame test to identify metal ions in ionic substances
- difficult to distinguish colours as they may be very similar
- in a mixture, only the dominant colour will be seen - a bold colour will mask a weaker one
describe the process of the precipitate test
- place 1-2cm^3 of ionic solution into test tube
- use pipette to drop 10 drops of NaOH into test tube
- observe and record result
- add a further 2-3cm^3 of NaOH to test tube and note any further changes
what is the positive result for aluminium in the precipitate test?
white precipitate forms, and after adding more NaOH it dissolves
what is the positive result for calcium in the precipitate test?
white precipitate forms, and after adding more NaOH it remains
what is the positive result for magnesium in the precipitate test?
white precipitate forms, and after adding more NaOH it remains
what is the positive result for Copper (II) in the precipitate test?
blue precipitate forms, and after adding more NaOH it remains
what is the positive result for iron (II) in the precipitate test?
green precipitate forms, and after adding more NaOH it remains
what is the positive result for iron (III) in the precipitate test?
brown precipitate forms, and after adding more NaOH it remains
describe the method of the carbonate test
- pour ~1cm depth of sample into test tubes
- place ~2cm depth of limewater into another test tube
- add ~1cm depth of dilute hydrochloric acid to test tube, and quickly place the bung into the neck of the test tube and the delivery tube into the limewater
what is the positive result for carbonate in carbonate test using dilute acid (HCl)?
bubbles turn limewater cloudy
what are the conclusions of carbonate test?
- carbonate ion (CO3^2-) is identified by producing CO2 gas when reacted with any acid
- CO2 gas will turn limewater cloudy when bubbled through it
describe the method of the sulfate test
- pour ~1cm depth of each of labelled sulfate solutions into 2 test tubes in the rack and a similar amount of tapwater into another test tube and distilled water in another
- add a few drops of dilute hydrochloric acid to each solution
- add ~1cm depth of barium chloride solution
what would carbonate ions in sulfate test produce?
bubbles of gas when acid was added
what is the positive result for sulfate ions in sulfate test?
white precipitate
what is the conclusion of the sulfate test?
sulfate ion (SO4^2-) produces a white precipitate when reacted with acidified (to remove false positives) barium chloride solution
describe the method of the halide test
- pour ~1cm depth of sample into test tube
- add a dew drops of nitric acid into test tube
- add ~1cm depth of silver nitrate solution
- record observations
why is nitric acid used in the halide test?
an acid is needed to remove any false positives, but HCl can’t be used as it contains chloride ions which will give a positive result
what is the positive result of chloride ion in halide test?
white ppt
positive result of bromide ion in halide test?
cream ppt
positive result of iodide ion in halide test?
yellow ppt
what is the conclusion of halide test?
halide ions produce a precipitate when reacted with acidified silver nitrate solution, and the colour of precipitate deepens going down group 7
what would carbonate ions in halide test produce?
bubbles of gas when acid was added
how is chemical vs instrumental analysis done?
- chemical analysis is performed by scientists who analyse chemical reactions and make conclusions from their observations
- instrumental analysis is performed by machines and instruments
give some examples of chemical analysis
- flame tests
- precipitate tests
- identifying gases
- titrations
give some examples of instrumental analysis
- flame emission spectroscopy
- mass spectroscopy
- infra-red spectroscopy
what are some advantages of chemical analysis?
- often simple to perform
- no expensive equipment required
what are some disadvantages of chemical analysis?
- Time-consuming (slow)
- requires a large sample size
what are some advantages of instrumental analysis?
- Highly accurate
- uses a small sample size
- can analyze complex mixtures quickly (fast) and provides a lot of information
what are some disadvantages of instrumental analysis?
- expensive equipment
- requires frequent calibration
how does flame emission spectroscopy work?
- every element has its own distinctive spectrum with specific wavelengths of light on it
- instead of using flame colour to help identify an element a spectrum of light of the flame is detected instead
what is flame emission spectroscopy used for?
- elements to be identified in distant stars and planets
- kidney dialysis
how is flame emission spectroscopy used in kidney dialysis?
- determining the concentration of sodium ions
- calibration curve will be produced using known concentrations of sodium ions and light intensity will be measured for those concentrations
- calibration curve used to determine ion concentration from the sample’s light intensity
