C8 (chemical analysis) Flashcards
what is a pure substance?
a substance that’s not mixed with any other substance (i.e. made of a single element/compound).
how do you check if a substance is pure using its melting/boiling point?
- pure substances melt and boil at fixed temperatures.
- impure substances melt and boil over a range of different temperatures, depending on how much of each substance there is in the mixture
- e.g. if a beaker of water melts at a temperature other than 0 degrees, or boils at a temperature other than 100 degrees, it’s impure.
what will having impurities in your substance generally do to its melting and boiling points?
will generally lower the melting point and increase the boiling point
what is the difference between a physical and chemical test?
- a physical test tests the physical properties of a substance (e.g. boiling point)
- a chemical test tests the chemical properties of a substance to find out what it is (e.g. by reacting it with another chemical)
what is a formulation?
a complex mixture that has been designed as a useful product using a specific formula. like a recipe
- in this, the quantity of each component is carefully measured, so the product has the desired properties and function, as each of the components contributes a different property
- examples include: fuels, cleaning products, paints, medicines, alloys, fertilisers, food and drink
what is the purpose of paper chromatography?
allows us to separate mixtures of substances based on their different solubilities, so we can identify them
- paper chromatography specifically separates the different dyes in an ink
what solvents are commonly used in chromatography?
- water
- ethanol
what is the method for chromatography?
- get chromatography/filter paper, draw pencil line near the bottom: baseline
- put dot of first colour ink on pencil line, dot of second colour next to it on the line
- place the bottom of the paper into a shallow amount of solvent in a beaker
- ensure the pencil line isn’t submerged
- place a lid on top, to stop the solvent from evaporating - solvent moves up the paper, and dissolves the ink and two coloured dots. they’re carried up the paper
what are the stationary and mobile phases in chromatography?
stationary phase: the substance/material the molecules can’t move in, usually a solid/thick liquid. in this case, it’s the paper
mobile phase: the substance the molecules can move in, always a liquid or gas. in paper chromatography, it’s the solvent we used
what are the different results in chromatography?
- pure compounds will produce a single spot in all solvents
- the compounds in a mixture will produce several spots, depending on the solvent
how does a substance act based on their solubility in chromatography?
a more soluble substance will travel further up the paper than a less soluble substance
- each of the different dyes will travel up the paper at different rates, so will end up separating out
- any insoluble chemicals won’t dissolve and move up the paper, they’ll just stay on the baseline
- the different chemicals will constantly change between the mobile and stationary phases: dissolving in the solvent, moving a bit, then binding to the paper, over and over again. therefore how much time they spend in each of these phases will determine the rate they move up the paper
- more soluble chemicals spend more time in the mobile phase, and therefore move faster
what is a chromatogram?
the patterned spots we have left after the paper chromatography process
why do we draw the chromatography starting line in pencil?
drawing the line in pen would mean the pen ink would move up the paper with the solvent, so the starting point wouldn’t be clear, and it would also make our results unclear
what is the method for chromatography to identify an unknown substance?
- place a dot of our chemical on our pencil line
- place the bottom of the paper in the solvent, meaning the chemical will start to move up the paper
- record with a line where the solvent moved to, compared to the chemical
- measure the distance from the pencil line to the centre of the chemical spot
- measure the distance moved by the solvent
- divide the distance moved by the substance, by the distance moved by the solvent, to get your Rf value
- look the Rf value up in a database, and it should help you identify the chemical
what is the possible disadvantage of using paper chromatography to identify an unknown chemical?
- several different substances may have the same Rf values, so we may need to repeat this experiment with a different solvent to narrow it down further
- if the substance has never been analysed before, then there will not be an Rf value for it on the database, so you’d need to carry out more analysis to identify it
- Rf values may change if the mobile or stationary phase has changed
what is the Rf value?
- the ratio of how far up the paper a certain spot moves compared to how far the solvent has travelled
- number between 0 and 1
- depends on the solvent and its temperature, and different substances will have different values
to calculate it, use:
Rf = distance moved by substance / distance moved by solvent
what happens if we do chromatography with a pure substance?
single spot on the paper, which we can calculate a specific Rf value for
how do you test for hydrogen gas?
- remove the bung of a test tube potentially containing hydrogen and insert a burning splint
- hydrogen gas will burn rapidly and produce a popping sound
- the heat energy provided by the flame causes the hydrogen to burn with the oxygen in the air to form water
what are the uses of hydrogen?
- making the fats in margarines solidify
- weather balloons
- an alternative to hydrocarbon fuels
how do you test for oxygen gas?
- place a glowing splint into a test tube potentially containing oxygen
- if oxygen is present, the splint will relight, as burning requires oxygen
what solution do you use to test for carbon dioxide?
limewater. it’s an aqueous solution of calcium hydroxide
what is the word equation for the production of carbon dioxide?
calcium carbonate (marble chips) + hydrochloric acid = calcium chloride + water + carbon dioxide
how do you test for carbon dioxide?
- have a test tube potentially containing carbon dioxide and a test tube containing limewater (calcium hydroxide)
- draw some of the gas into a plastic pipette
- bubble the gas through the limewater
- repeat this several times, and if carbon dioxide is present, the limewater will turn cloudy
- this is because the carbon dioxide and calcium hydroxide react to farm calcium carbonate and water
- calcium carbonate is a solid, so all of these solid particles will make it appear cloudy
how do you test for chlorine gas?
- insert damp litmus paper into the mouth of a test tube potentially containing chlorine
- the chlorine will bleach the litmus paper and turn it white (from blue)
why may the litmus paper briefly turn red before turning white when inserted into chlorine gas?
the chlorine as reacted with the water on the litmus paper, and briefly formed hydrochloric acid
what precaution must we take when testing for chlorine gas?
it’s poisonous, so wear a mask or do it in a fume cupboard
what is a flame test?
- used to identify metal ions
- take a platinum/nichrome wire loop and clean it by dipping it into dilute hydrochloric acid, rinsing it in distilled water and heating it over a Bunsen burner flame
- dip the wire loop into the compound you want to test
- hold the wire loop in the clear blue part of the Bunsen burner (hottest)
- see what colour the flame turns as the compound burns
why do you keep the end of the splint wet during flame tests?
- wet with the ion solution, otherwise you’d see the colour of the wood burning, not the ion
what colour flame does lithium create?
lithium produces a crimson flame
what colour flame does sodium create?
sodium creates a yellow flame
what colour flame does potassium create?
potassium creates a lilac flame
what colour flame does calcium create?
calcium creates an orange-red flame
what colour flame does copper create?
copper creates a green flame
what are the problems with using flame tests?
- colour of the flame can be difficult to distinguish, especially if there’s only a low concentration of the metal compound
- sometimes a sample contains a mix of metal ions which can mask the colour of the flame
what happens in flame emission spectroscopy/flame photometry?
- sample of the metal ion in solution is placed into a flame
- the light given out is then passed into a machine called a spectroscope
- the spectroscope converts the line into a line spectrum (showing the light’s individual emitted wavelengths). the position of the lines in the spectrum are specific to a given metal ion, as they have a unique pattern of wavelengths
- we can use this to identify the metal ion in the sample
what else can flame spectroscopy tell us?
the concentration of the metal ion in a solution, as the lines become more intense at a higher concentration
what is the main benefit of flame spectrometry?
- if a sample contains multiple different metal ions, the spectrum will show the lines for all of them on a combined spectra
- if this is unknown, we can compare this to the spectra of known metal ions and identify the ions in the unknown sample, and the concentrations of each ion
what is an instrumental method, and what are its advantages?
flame emission spectroscopy is an instrumental method, as it’s carried out by a machine
- rapid. analyses samples more quickly than manual flame tests
- sensitive. can work on even the tiniest sample of a compound
- accurate. FES is more likely to identify a metal ion accurately than flame tests
what are the benefits of manual tests (e.g. flame test)?
basic and cheap
describe the metal hydroxide tests:
- react metal ions with a solution of sodium hydroxide, and see what colour the solution turns
- metal ions form coloured precipitates when they react with hydroxide ions, and these coloured precipitates then determine the colour of the solution
how do you use sodium hydroxide solution to identify calcium, magnesium and aluminium?
- have solutions of aluminium, magnesium, and calcium ions in 3 test tubes
- add sodium hydroxide solution to these ions, they all produce a white precipitate
- add excess sodium hydroxide solution, and the aluminium precipitate will re-dissolve, and we can distinguish the aluminium solution
- to distinguish between the other two (show which one is calcium), you’d have to do a flame test
what are the reaction equations for reacting sodium hydroxide with calcium, magnesium, and aluminium?
calcium nitrate (also works for other solid calcium compounds) + sodium hydroxide = sodium nitrate + calcium hydroxide precipitate
same equation for magnesium and aluminium
what happens when copper II ions react with sodium hydroxide?
form a blue precipitate of copper II hydroxide
what happens when iron II ions react with sodium hydroxide?
form a green precipitate of iron II hydroxide
what happens when iron III ions react with sodium hydroxide?
form a brown precipitate of iron III hydroxide
how do you test for the carbonate ion?
- add a dilute acid (e.g. hydrochloric) to the sample
- if carbonate is present, the acid will react with it to make carbon dioxide gas. we will see effervescence (fizzing). (however, this doesn’t prove that we have carbon dioxide gas)
- therefore we must bubble the gas through limewater, and if it goes cloudy we’ll know we have carbon dioxide and that we started with a carbonate ion
why must we then test for carbon dioxide after testing for carbonates?
carbonate + acid = salt + carbon dioxide + water
how do you test for halide (chloride, bromide and iodide) ions?
- a.k.a. the silver nitrate test
- add dilute nitric acid to the sample to remove impurities
- add dilute silver nitrate solution
- halide ions produce a precipitate of the silver halide. each halide makes a different coloured precipitate
- the silver ions from the silver nitrate react with the halide ions to form a precipitate
what colour precipitate does a chloride ion produce?
white precipitate of silver chloride
what colour precipitate does a bromide ion produce?
cream precipitate of silver bromide
what colour precipitate does an iodide ion produce?
yellow precipitate of silver iodide
how do you test for sulfate ions?
- add dilute hydrochloric acid to the sample, to remove impurities such as carbonate or sulfate ions, as they would confuse our results
- add barium chloride solution (contains barium ions)
- if sulfate ions are present, we’ll see a white precipitate
barium + sulfate = barium sulfate (white precipitate)
what is the difference between a pure substance in chemistry and a pure substance in everyday life?
chemistry: single element/compound
everyday: substance that’s had nothing added to it
what is an anion?
negatively charged ion - includes carbonate, sulfate, and halide ions
if a chemical reaction is taking place in a test tube, what methods could be used to collect the gas?
- attach the top of the test tube to a gas syringe
- place an upside down measuring cylinder over the test tube to trap the gas
what is a cation?
a positively charged ion
- nearly all cations are metal ions, as metals form positive ions
TWO TESTS FOR CATIONS:
- flame test (see what colour flame metal ions produce when they burn)
- metal hydroxide test
