C2.1 Purity and Separating Mixtures

Question 1

What is Relative atomic mass (Ar)?

Answer 1

The mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom( 12/6 C)

Question 2

What does a chemical formula tell you?

Answer 2

How many atoms of each element there are in a unit of substance.
E.G. Chemical formula of water is H2O
-this means each molecule of water contains 2 hydrogen atoms bonded to 1 Oxygen atom

Question 3

What is Relative Formula Mass? (Mr)

Answer 3

the mean mass of a unit of a substance compared to 1/12 the mass of a carbon-12 atom.

Question 4

How to calculate Mr?

Answer 4

add all the Ar values for all the atoms in a substance.

Question 5

Units

Answer 5

Ar and Mr do not have any units

Question 6

What is an empirical formula?

Answer 6

The simplest Whole-number ratio of the atoms of each element in a compound.
They are simplified by HCF

Question 7

What does a balanced equation show?

Answer 7

the formulae and number of units for all the substances in a reaction

Question 8

How do balanced equations also help?

Answer 8

They give you a way to check that you have correctly calculated Mr.

Question 9

What is a pure substance?

Answer 9

A substance which has not been processed of change.

A pure substance consists of just ONE element or compound.

Question 10

What is an impure substance?

Answer 10

A substance which contains more than one element or compound

Question 11

What is an alloy?

Answer 11

A mixture of a metal with one or more other elements.
Most metals you use are alloys.
EG - mixture of gold and copper is used in jewellery

Question 12

What is Melting point

Answer 12

The temperature at which it changes from solid to liquid

Question 13

Melting point of a pure substance:

Answer 13

A single temperature without any variation

Question 14

Melting Point of an impure substance:

Answer 14

(1) melting point is less than that of the pure substance
(2) It often melts over a range of temperatures NOT ONE temperature.
(3) Greater the difference between measured melting point and accepted MP - the lower its purity

Question 15

How to determine MP?

Answer 15

Heat the substance and then either measure the temperature at which it melts, or the temperature at regular intervals.

Question 16

What is it important to do when Measuring MP?

Answer 16

(1) Heat the substance slowly - allows temp of whole sample to increase
(2) Stir the substance as it melts - ensures the entire sample is at the same temperature

They both help improve accuracy.

Question 17

How is a solution formed?

Answer 17

When a solute (Substance which dissolves) dissolves in a solvent (substance it dissolves in).

Question 18

What happens when a substance dissolves?

Answer 18

Its particles separate and become completely mixed with particles of the solvent

Question 19

What is it meant when a substance is soluble?

Answer 19

Soluble - When a substance can dissolve in a particular solvent

Question 20

What is it meant when a substance is insoulble?

Answer 20

Insoluble - When a substance cannot dissolve in a particular solvent

Question 21

What does filtration do?

Answer 21

Separates insoluble substance in solid state from substances in liquid state

Question 22

How does filtration work?

Answer 22

Filter paper has tiny microscopic holes which allow the smaller liquid molecules to go through and stops the larger insoluble solid particles from going through

Question 23

What is the solid left in filtration called?

Answer 23

Residue

Question 24

What is the liquid which goes through the filter paper called?

Answer 24

Filtrate

Question 25

What is a saturated solution?

Answer 25

When no more solute can be dissolved into the solvent

Question 26

What happens when you heat a solution?

Answer 26

If you heat a solution, solvent evaporates leaving the solute behind / or a powder (if heated too strongly). If you allow solvent to evaporate slowly, you get a regularly shaped crystal

Question 27

How to do crystallisation?

Answer 27

Heat a solution gently until it becomes saturated. Crystals will start forming at this point, so let the solution cool slowly. As solution cools, solubility of solute decreases, so more crystals form.

Question 28

Two types of distillation:

Answer 28

Simple and Fractional

Question 29

Simple Distillation:

Answer 29

(1) separates solvent from a solution. (2) Relies on Solvent having much lower BP than solute. (3) When heated, solvent boils but solute does not. (4) solvent escapes solution in gas state and is then cooled and condensed back into a liquid by a Condenser

Question 30

Why is Simple distillation useful?

Answer 30

If you want to purify a solvent E.G. Providing purified water for chemists - tap water contains nacl and other dissolved salts.

Question 31

Fractional Distillation:

Answer 31

(1) Separates 2 or more liquids (EG ethanol and water) (2) relies on each substance having different BP (3) Uses fractioning column : - the vapours condense on the inside surface, heating it up - when inside temp reaches BP of ethanol, ethanol vapour cannot condense any more, but water can - Water droplets fall back into flask and ethanol vapour pass into the condensor (4) ethanol vapor is cooled and condensed into liquid. and drops into collecting container

Question 32

Fractions

Answer 32

Each substance separated by Frac Distil

Question 33

Types of Chromatography:

Answer 33

1) paper - separate coloured substances in Ink or dye 2) Thin-Layer - Separate sample into components for identification or analysis 3) Gas - Separates components of a mixture and also measures their amounts

Question 34

Two chemical phases:

Answer 34

Stationary phase | Secondary phase

Question 35

Stationary phases:

Answer 35

PC - absorbent paper TLC - Silica or alumina power spread over a plate of glass or plastic GC: Silica or alumina power packed into metal column

Question 36

Mobile Phases:

Answer 36

PC - Solvent in the liquid TLC - Solvent in liquid GC - Carrier gas which does not react W/ sample

Question 37

TLC in depth

Answer 37

1) Put solvent into a chromatography tank to depth of 1cm 2) Add small amount of sample to baseline, not damaging the power on the plate 3) Let solvent travel through the power, take plate out before it reaches the top 4) Analyse the pattern of coloured spots, chromatogram

Question 38

Rf values

Answer 38

used to compare different spots on a chromatogram - 2 identical spots have same Rf value and are the same colour.

Question 39

Rf formula

Answer 39

Distance travelled by substance / Distance travelled by solvent