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

• if not prefect mark not perfect….be honest here *

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 atom

• if not prefect mark not perfect….be honest here *

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 the melting point of a substance?

Answer 9

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

Question 10

How many melting points does a pure substance have?

Answer 10

Only one

Question 11

How many melting points does an impure substance have?

Answer 11

Multiple, however many elements there are in the compound

Question 12

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

Answer 12

• it’s melting point is less than of the pure substance
• it often melts over a range of temperatures, not just one temperature

Question 13

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

Answer 13

No, it stays constant

Question 14

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

Answer 14

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

Question 15

Why should you stir a substance while it melts? (1)

Answer 15

To ensure that the entire sample is at the same temperature

Question 16

What does chromatography separate?

Answer 16

mixtures of soluble substances

Question 17

what happens when a substance dissolves, and what is formed? (2)

Answer 17

-a solution forms
- the solute’s particles separate become completely mixed with the solvent’s particles (solvent particles clump around solute)

Question 18

what does filtration do?

Answer 18

separates an insoluble substance from a solvent/solution

Question 19

what feature of the filter paper allows filtration to occur?

Answer 19

it has tiny microscopic holes which prevent insoluble substances from passing

Question 20

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

Answer 20

residue

Question 21

what is the substance which passes through the filterpaper called

Answer 21

filtrate

Question 22

how does crystallisation work?

Answer 22

• a solution is heated slowly until it becomes a saturated solution
• saturated solution is left to cool
• crystals form

Question 23

why must you heat a solution slowly (crystallisation)?

Answer 23

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

Question 24

how do you dry crystals?

Answer 24

• place them in a warm oven
• pat them with filter paper

Question 25

What is simple distillation used to separate?

Answer 25

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

Question 26

what does simple distillation rely on?

Answer 26

solvent having a much lower boiling point than solute

Question 27

describe simple distillation

Answer 27

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 28

what equipment is used during simple distillation?

Answer 28

condenser, test tube, bunsen burner, thermometer

Question 29

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

Answer 29

purifying water for experiments

Question 30

what does fractional distillation seperate?

Answer 30

Separates one or more liquids from a mixture of liquids

Question 31

how does fractional distillation separate substances?

Answer 31

depending on their boiling points

Question 32

how is fractional distaillation used industrially?

Answer 32

to separate crude oil into separate hydrocarbons

Question 33

describe the process of fractional distillation (in terms of an ethanol-water mixture) (this answer is not the best but hopefully you remember what coggins said)

Answer 33

1) vapours condenser on inside surface (heating it up) 2) vapour rises to fractioning column 3) water condenses and falls back down from flask (less than 100 degrees) 4) bioethanol fuel passes into the condenser (lower boiling point) 5) bioethanol fuel condenses

Question 34

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

Answer 34

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

Question 35

what are the advantages of TLC (thin-layer chromatography) over paper chromatography? (3/4)

Answer 35

- it is quicker - iffy one - 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 36

How does TLC’s stationary phase work?

Answer 36

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

Question 37

how is gas chromatography different to paper or TLC?

Answer 37

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

Question 38

what does gas chromatography do?

Answer 38

separates the components and measures their amounts on a chromatogram

Question 39

how is the the sample separated during gas chromatography?

Answer 39

depending on how strongly they bond to the stationary phase

Question 40

how do you calculate the rf value?

Answer 40

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

Question 41

how do you separate a soluble and insoluble substance?

Answer 41

filtration

Question 42

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

Answer 42

fractional distillation

Question 43

how do you separate coloured soluble substances?

Answer 43

chromatography

Question 44

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

Answer 44

highly soluble (dissolves easily in solvent)

Question 45

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

Answer 45

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