SCIENCE CHEMISTRY (Sec3): Chpt 1_Methods of purification & analysis Flashcards
pure substance (definition) [2pts]
- made up of only one element or one compound
- does not contain any other substances or impurities
if substance is pure…[2pts]
- solid to liquid at fixed temp. under fixed conditions: melting point
- liquid to gas at fixed temp. under fixed conditions: boiling point
presence of impurities in a substance [2pts]
- substance melts below its m.p., over a range of temp.s
- substance boils higher than its b.p., over a range of temp.s
relationship bwt. amt of impurities & temp substance melts at
greater amt of impurities. lower temp it melts below its m.p.
mixture (definition) [1pt]
formed when two or more substances that are added together do not combine chemically
solid-solid mixture: magnetic attraction [1pt + examples of magnetic substances]
- magnets attract magnetic substances, separating non-magnetic substances in a mixture
- magnetic substances (e.g.): iron, nickel, cobalts, steel (alloy)
can be attracted with magnet
ferromagnetic
cannot be attracted with magnet
non-ferromagnetic
solid-solid mixture: sieving [2pt]
- based on particle size
- only size of particles smaller than pore size of sieve can pass through the pores
fine substances [def. + example]
substances with small particle size
e.g. flour
coarse or lumps [def. + example]
substances with large particle size
e.g. sand
solid-solid mixture: using suitable solvents [1pt]
- when mixture contains only one soluble solid, suitable solvent used to dissolve soluble solid
solute [def + example]
substance that dissolves in a solvent
e.g. salt dissolves in water, salt is the solvent
solvent [def + example]
substance that dissolves a solute
e.g. water dissolves salt, water is the solvent
solubility [def]
refers to the ability of a solute to dissolve in a solvent
solid-solid mixture: sublimation [3pts + example]
- used to separate solid that can change into gas directly without melting
- heating is carried out
- common substances that sublimes: iodine crystals, dry ice (carbon dioxide), naphthalene (in mothball)
- solid sublimes and its vapour changes back into solid deposit when cooled on the inverted filter funnel –> to yield pure crystals of the solid
solid deposit (sublimation) [def]
sublimate
solid-liquid mixture: filtration [3pts]
- physical method
- filter paper used: very tiny pores
- only dissolved substances with particle size smaller than pore size of filter paper can pass through pores (insoluble solids too large to pass through them)
residue [def]
substance collected on filter paper
filtrate [def]
solution collected in the flask which passes through the filter paper
solid-liquid mixture: evaporation to dryness [4pts + examples]
- physical method
- used to recover pure soluble solid from a solution
- solid is heat-stable: evaporation used by heating solution to dryness
- solid remains as crystallined residue after solvent has evaporated
- e.g. sodium chloride solution, sodium chloride: heat-stable salt, obtain sodium chloride crystals
solid-liquid mixture: crystallisation [4pts]
- compound is not heat-stable, strong heating to dryness would decompose it
- soluble solid is not heat-stable, solution heated until it becomes saturated (or 2/3 of its solvent has evaporated
- saturated solution then allowed to cool
- size of crystals formed depends on how cooling is carried out
saturated solution (crystallisation)
type of solution in which no more solute can dissolve in it
cooling takes place quickly (crystallisation)
size of crystals is smaller or powdery
cooling takes place slowly (crystallisation)
size of crystals is bigger
crystallisation steps [4 steps]
- gently heat (solution) in an evaporating dish to evaporate most of the liquid, until the solution is saturated. Stop heating solution before all the solvent has evaporated
- cool solution gradually until (__) crystals appear within solution
- carefully pour mixture through funnel lined with filter paper to collect the solid crystals
- (__) crystals are washed with some cold distilled water to remove impurities. they can be dried with a few sheets of filter paper
solid-liquid mixture: simple distillation [3pts]
- liquid heated to reach its boiling point – changes to vapour
- two physical state changes: boiling and condensation
- substances in the mixture must have large diff.s in b.p. for pure liquid to be obtained
simple distillation steps [3 steps]
- (__) heated. boiling chips added to ensure smooth boiling. at 100°C, water boils. vapour rises and enters condenser through exit sidearm of distillation flask
- water vapour cools in condenser and condenses back to liquid water. pure water collected in conical flask as distillate
- as more water vaporises, (__) solution becomes more concentrated. eventually, solid residue of salt remains in the distillation flask
bulb of thermometer placed near opening of condenser (simple distillation)
can measure b.p. of substance accurately
boiling chips added (simple distillation)
for smooth boiling, prevent bumping during boiling
conical flask used as receiving flask instead of beaker (simple distillation) [2pts]
- narrow neck
- can reduce loss of distillate from splashing out of flask
receiving flask (conical flask) placed in water bath (simple distillation) [2pts]
- if distillate is volatile: can evaporate easily due to heating effect from heat source
- water bath shields heat from heat source
water in condenser flows opposite direction to flow of vapour (simple distillation)
ensures permanent cold surface to condense vapour effectively before vapour leaves condenser
liquid-liquid mixture: fractional distillation
- separate two or more miscible liquids with slightly different b.p.s
- liquids distilled according to their b.p.s
- liquid with lowest b.p. being distilled first
miscible distillation (fractional distillation)
Liquids which dissolve in each other completely in all proportions are called miscible liquids
tall fractionating column (fractional distillation)
- ensure separation of diff. liquids more efficiently
- packed with glass beads or glass rings: increase surface area for both evaporation & condensation of substances with diff. b.p.s to take place
- closer the b.p.s of diff. liquids, the taller the fractionating column should be
liquid-liquid mixture: separating funnel [2pts]
- used to separate immiscible liquids
- liquid with higher density forms lower layer, liquid with lower density forms upper layer
separating funnel steps [3 steps]
- mixture forms two layers in the funnel
- tap is opened and lower layer is drained off into a beaker first
- tap is closed once the lower layer has been drained off completely
immiscible liquids (separating funnel) [def]
refers to two or more liquids that do not mix completely together and form heterogenous mixture (substances not mixed uniformly throughout mixture)
liquid-liquid mixture: chromatography [2pts]
- separate mixtures based on extent of solubility of different substances in a given solvent
- more soluble to substance in solvent, further distance it travels on paper
solvent = mobile phase (chromatography)
moves across chromatography paper
chromatography paper = stationary phase (chromatography)
placed stationary in the solvent
spot (chromatography)
shows component in sample
starting line (chromatography)
marks position in which sample if placed
solvent (chromatography)
to dissolve sample
chromatography paper (chromatography)
allows separation of components in sample
solvent front (chromatography)
marks end position of solvent travelling on paper
lid (chromatography)
prevent evaporation of solvent
chromatography steps [4 steps]
- use pencil to draw starting line
- drop of (sample) is placed at starting line
- edge of paper is submerged into a solvent, keeping (sample) spot just above solvent level
- as solvent is absorbed by the paper, it travels towards the opposite edge, separating components in the sample
solvent front should not travel to edge of paper (chromatography) [2pts]
- prevents components of sample being unaccountable on chromatogram
- allows actual distance travelled by solvent to be measured
solvent front should be as high as possible (chromatography) [1pt]
allows all components to be fully separated
starting line is drawn with graphite pencil (or other insoluble substances) instead of ink (chromatography) [1pt]
- graphite pencil: insoluble in solvent, will not be carried away by solvent to interfere results
sample is applied as small as possible (chromatography) [2pts]
- tailing and smudging of diff. components will not occur
- can identify respective spots easily
starting line should be above solvent (chromatography) [1pt]
- prevents sample from dissolving in solvent so that components in sample can be moved along by solvent on chromatography paper
beaker containing solvent is covered by a lid (chromatography) [3pts]
- prevents evaporation of solvent
- helps maintain saturation of solvent vapour: solvent will not evaporate from paper too quickly
- separation of components by solvent can be more efficient
chromatogram (chromatography)
result of separated components on chromatography paper
number of spots (chromatography)
indicated number of substances in the sample
any spots horizontally matched (chromatography)
likely to be same substance
single spot produced by a sample (chromatography)
pure substance
two or more spots produced by a sample (chromatography)
indicates sample is a mixture
+ve of using chromatography [3pts]
- gives quick & accurate analysis of sample
- requires only a small amount of sample
- able to separate complex mixtures (e.g. food dyes + flavourings)
Rf value (chromatography) [4pts]
- retention factor
- distance of substance travelled compared to distance travelled by solvent
- formula: dist. travelled by spot/ dist. travelled by solvent
- larger the Rf value, more soluble solute is in solvent
comparing known samples (chromatography)
spots match horizontally to spots of ‘known’ samples in same chromatography
use of locating agents (chromatography)
for separation of colourless compounds
locating agent: ninhydrin (chromatography) [2pts]
- deep blue in colour
- react with (__) to produce coloured substances
locating agent: ultraviolet (UV) light (chromatography) [1pt]
used on colourless components that fluoresce under UV light
