C2.1 Purity and separating mixtures

Question 1

What is relative atomic mass?

Answer 1

the mean mass of an atom of an element compared to 1/12 of the mass of a carbon-12 atom

Question 2

What does a chemical formula tell you?

Answer 2

how many atoms of each element there are in a unit of a substance

Question 3

What is a relative formula mass?

Answer 3

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

Question 4

define empirical formula

Answer 4

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

Question 5

Calculate the relative formula mass of Mg(OH)2

Mg = 24.3, O = 16.0, H = 1.0

Answer 5

M(r) of Mg(OH)2 = 58.3

Question 6

How do you find the empirical formula of a chemical formula?

Answer 6

Divide the chemical formula by the highest common factor

Question 7

What does pure mean in scientific terminology?

Answer 7

a substance consisting of just one element or compound

Question 8

is water pure?

Answer 8

no, because it contains multiple minerals, distilled water is pure, however

Question 9

What is an example of a helpful mixture?

Answer 9

Alloys (a mixture of a metal with one of more other elements)

Question 10

What is the melting point of a substance?

Answer 10

The temperature at which it changes from the solid state to the liquid state

Question 11

How many melting points does a pure substance have?

Answer 11

Only one

Question 12

How many melting points does an impure substance have?

Answer 12

Multiple, however many elements there are in the compound

Question 13

How can you tell a substance is impure looking at its melting point?

Answer 13

• it’s melting point is less than of the pure substance

- it often melts over a range of temperatures, not just one temperature

Question 14

Does the temperature of a pure substance change while it melts?

Answer 14

No, it stays constant

Question 15

How must you heat a substance to find out its melting point and why?

Answer 15

• slowly
• stirring it
  So that the temperature of the whole sample can increase, not just the surface

Question 16

Why should you stir a substance while it melts?

Answer 16

To ensure that the entire sample is at the same temperature

Question 17

What does chromatography separate?

Answer 17

mixtures of soluble substances

Question 18

what happens when a substance dissolves?

Answer 18

• a solution forms

- the solute’s particles separate and become completely mixed with the solvent’s particles

Question 19

what does filtration do?

Answer 19

separates an insoluble substance in the solid state from substances in the liquid state

Question 20

what feature of the filter paper allows filtration to occur?

Answer 20

it has tiny microscopic holes which prevent insoluble substances from passing

Question 21

what is the substance called which is left behind during filtration?

Answer 21

residue

Question 22

what is the substance which passes through the filterpaper called

Answer 22

filtrate

Question 23

how does crystallisation work?

Answer 23

• a solution is heated slowly until it becomes a saturated solution (when no more solute can be dissolved at that temperature)
• crystals start forming at thus point so saturated solution is left to cool
• as solution cools, solubility of the solute decreases, so more crystals form
• the crystals can be separated from the remaining solution by filtration

Bite size:
To obtain large, regularly shaped crystals:

Put the solution in an evaporating basin
Warm the solution by placing the evaporating basin over a boiling water bath
Stop heating before all the solvent has evaporated
After the remaining solution has cooled down, pour the excess liquid away, or filter it. Dry the crystals using a warm oven or air-dry them.

Question 24

why must you heat a solution slowly (crystallisation)?

Answer 24

if you heat a solution too fast, a powder is formed (no crystals)

Question 25

how do you dry crystals?

Answer 25

- place them in a warm oven | - pat them with filter paper

Question 26

What is simple distillation used to separate?

Answer 26

to separate a solvent from a solution | eg. (oil and water)

Question 27

what does simple distillation rely on?

Answer 27

solvent having a much lower boiling point than solute

Question 28

describe simple distillation

Answer 28

1) solution is heated 2) solvent has low boiling point and so heats quicker than solute 3) solvent rises and condenses in vapour condenser 4) travels down the test tube

Question 29

what equipment is used during simple distillation?

Answer 29

condenser, test tube, bunsen burner, thermometer

Question 30

what is simple distillation useful for (in a lab)?

Answer 30

purifying water for experiments

Question 31

what does fractional distillation seperate?

Answer 31

Separates one or more liquids from a mixture of liquids

Question 32

how does fractional distillation separate substances?

Answer 32

depending on their boiling points

Question 33

how is fractional distaillation used industrially?

Answer 33

to separate crude oil into separate hydrocarbons

Question 34

describe the process of fractional distillation (in terms of an ethanol-water mixture)

Answer 34

1) vapours condenser on inside surface, heating it up 2) when the temperature reaches the boiling point of ethanol, ethanol vapour cannot condense any more, but water vapour can 3) water droplets fall back into the flask, and ethanol vapour passes into the condenser 4) in the condenser, the ethanol vapour is cooled and condensed back to its liquid state. It drips into the collecting container

Question 35

how does the fractioning column help improve the separation of the mixture?

Answer 35

- large surface area where vapours condense | - column becomes hottest at bottom and coolest at top

Question 36

what are the advantages of TLC (thin-layer chromatography) over paper chromatography?

Answer 36

- it is quicker - more accurate results (as TLC is more sensitive) - a smaller sample can be used - large range of stationary phases and solvents to choose from

Question 37

How does TLC’s stationary phase work?

Answer 37

a thin layer of silica or alumina powder is spread over a plate of glass or plastic

Question 38

how is gas chromatography different to paper or TLC?

Answer 38

- the stationary phase is silica or aluminium powder packed into a metal column - the mobile phase is an unreactive carrier gas

Question 39

what does gas chromatography do?

Answer 39

separates the components and measures their amounts on a chromatogram

Question 40

how is the the sample separated during gas chromatography?

Answer 40

depending on how strongly they bond to the stationary phase

Question 41

how do you calculate the rf value?

Answer 41

distance travelled by solute (or spot) / distance travelled by solvent

Question 42

how do you separate a soluble and insoluble substance?

Answer 42

filtration

Question 43

how do you separate two or more substances in the liquid state?

Answer 43

fractional distillation

Question 44

how do you separate coloured soluble substances?

Answer 44

chromatography

Question 45

if a component travels very high during chromatography, what property does it have?

Answer 45

highly soluble (dissolves easily in solvent)

Question 46

when calculating rf value, where do you measure the spot from, and where the solvent?

Answer 46

spot - from the PENCIL line to MIDPOINT on spot | solvent - from PENCIL line to where it stops

Question 47

How does gas chromatography work?

Answer 47

The sample is turned into the gas state when it is injected into the column, depending on how strongly they bond to the stationary phase. A detector sends a signal to a computer as each component leaves the column. The computer produces a chromatogram in which each component is a peak plotted against the travel time.