C2.1 Purity And Separating Mixtures

How many atoms of each element there are in a unit of a substance or compound

Masses of sub atomic particles compared to mass of a proton Masses of atoms compared to a standard atom

Mass of a compound and can be calculated using relative atomic masses (Ar) of all elements contained in the compound

Simplest whole number ratio of the atoms of each element in a compound Example: C4H10 would be C2H5

To give them strength Pure metals are too soft The atoms are of different sizes, so this gives the metal strength, as the layers of metals cannot then slide over each other

Temperature at which substance changes from solid state to liquid state Melting point of a pure substance is single temperature If a substance is impure, its melting point is less than that of the pure substance NOT MORE It also often melts over a range of temperatures, instead of 1 temperature The greater the difference between measured melting point for a substance and its accepted melting point, the lower its purity is likely to be.

Heating substance then either measure temperature at which it melts, or measure its temperature at regular time intervals and then plot a graph to identify state of change Important to heat the substance SLOWLY - allows temperature of whole substance to increase Stir substance as it melts - ensures that entire sample is at same temperature These actions improve accuracy of a measurement of melting point of a sample

A solution is formed when 1 substance (solute) dissolves in another (solvent) Solute’s particles separate and become completely mixed with the particles of the solvent If it dissolves then it is soluble, if not then it is insoluble

Filtration separates an insoluble solid substance from liquid Residue - what stays behind (solid) Filtrate - What passes through (liquid and some solids)

If you heat a solution, the solvent evaporates leaving the solute behind. If heated too strongly, powder formed. If solvent allowed to evaporate slowly, regularly shaped crystals formed: Solution is to be gently heated until it becomes a saturated solution - when no more solute can be dissolved at that temperature- crystals form at this point, and need to be cooled slowly where solubility of solute decreases so more crystals form - can be separated by filtration and dried in a warm oven or patting them with filter paper.

Separates a solvent from a solution - relies on the solvent having a much lower boiling point than the solute - when solution heated, solvent boils and escapes from the solution as gas which then is condensed back to liquid state using a condenser and solute doesn’t boil away and remains The water in the condenser goes down to up to avoid air bubbles, and that the whole condenser is cool

If you want to purify a solvent

Separates 2 or more liquid mixtures Relies on each substance having a different boiling point Uses fractionating column that has large surface areas on which the vapours can continually condense. Inside fractionating column: Vapours condense on the inside surface, heating it up Glass beads can be added to increase surface area and increase temperature gradient When temperature inside reaches boiling point of substance A (78 degrees), it cannot condense anymore but Substance B (100 degrees) can. Substance B falls back into the flask, substance A passes into the condenser to be cooled and condensed back to liquid state

Relies on 2 different chemical phases: Stationary phase and mobile phase that moves phase is a substance in solid, liquid or gas state

Stationary phase: Absorbent paper Mobile phase: Solvent in the liquid state, such as water or propanone

Stationary phase: Thin layer of silica or alumina powder spread over a plate of glass or plastic Mobile phase: Solvent in the liquid state, such as water or propanone 1) Put solvent into chromatography tank to a depth of about 1cm - if flammable make sure that there are no naked flames and room is well ventilated 2) Add a small amount of sample to the baseline 3) Let solvent travel through powder and take plate out before it reaches top 4) Analyse pattern of coloured spots, called chromatogram Pattern produced depends on how each component is distributed between the 2 phases travels further up the plate if it forms stronger bonds with the mobile phase than with the stationary phase component will not travel far if forms stronger bonds with stationary phase. Advantages over paper chromatography: Quicker, more sensitive - smaller sample can be used, can scrape individual spot for further analysis - gas chromatography, larger range of stationary phases and solvents to choose from

Used to compare different spots on a chromatogram if 2 spots have the same Rf value, they are the same colour and likely to be identical. Rf = distance travelled by substance / distance travelled by solvent Rf values vary from 0 to 1 NO UNITS

Stationary phase: Silica or alumina powder packed into a metal column Mobile phase: inert carrier gas - such as nitrogen - doesn’t react with the sample GC separates components of a mixture and also measures their amounts Sample turned to gas state when injected into the column Carrier gas pushes sample through the column depending on how strongly they bond to the stationary phase, therefore taking different times Detector sends signal to computer as each component leaves the column Computer produces a chromatogram Each component is a peak plotted against the travel time

Insoluble and soluble substances - dissolving followed by filtration Solute dissolved in a solvent - crystallisation to obtain the solute, simple distillation to obtain the solvent 2 or more liquids - fractional distillation Coloured soluble substances - paper chromatography or thin layer chromatography

Only 1 substance is present 1 atom or compound only

Fullerene is likely to be a buckyball and has certain arrangement of particles Diamond has a giant covalent structure - no fixed number of atoms - its empirical formula, C, is used

Mixtures where substances are combined in exact amounts

Time taken for chemical being analysed to travel through equipment / reach detector