C2.1 Purity And Separating Mixtures

Question 1

Chemical formula

Answer 1

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

Question 2

What is relative atomic mass (Ar)

Answer 2

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

Question 3

What is relative formula/molecular mass (Mr)

Answer 3

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

Question 4

Empirical formula

Answer 4

Simplest whole number ratio of the atoms of each element in a compound

Example: C4H10 would be C2H5

Question 5

Why are alloys created for metals

Answer 5

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

Question 6

What is a melting point

Answer 6

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.

Question 7

How do you determine melting point

Answer 7

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

Question 8

What happens when a substance dissolves

Answer 8

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

Question 9

How filtration works

Answer 9

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

Question 10

How does crystallisation work

Answer 10

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.

Question 11

Simple distillation work

Answer 11

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

Question 12

What is simple distillation useful for in the lab

Answer 12

If you want to purify a solvent

Question 13

How does fractional distillation work
(Fraction as each substance separated is parts of the original mixture)

Answer 13

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

Question 14

How chromatography works

Answer 14

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

Question 15

Paper chromatography

Answer 15

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

Question 16

Thin layer chromatography (TLC)

Answer 16

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

Question 17

Rf values

Answer 17

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

Question 18

How gas chromatography (GC) works

Answer 18

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

Question 19

Choosing separation method

Answer 19

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

Question 20

Pure substances

Answer 20

Only 1 substance is present
1 atom or compound only

Question 21

Why formula for diamond is C but fullerene is C60

Answer 21

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

Question 22

Formulation mixture

Answer 22

Mixtures where substances are combined in exact amounts

Question 23

Retention time

Answer 23

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