C2.1 Purity And Seperating Mixtures

Question 1

What is relative atomic mass?

Answer 1

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

Question 2

Define relative formula mass

Answer 2

The mean mass of a unit of a sub trance compared to the mass of a carbon 12 atom

Question 3

What is an empirical formula?

Answer 3

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

Question 4

How do you work out the empirical formula?

Answer 4

Divide by the HCF

Question 5

How do you work out the empirical formula from percentages?

Answer 5

1) Divide each elements percentage by its mass number
2) Divide ALL the answers by the smallest answer
3) Write it out

Question 6

Why do scientists use empirical formulas?

Answer 6

It makes little sense to write out formulas with huge numbers

Question 7

What does pure mean?

Answer 7

A substance which consists of only one element or compound

Question 8

What is a mixture

Answer 8

An impure substance made of more than one element or compound

Question 9

Why are some mixtures useful?

Answer 9

Mixture can be useful for alloys which are a mixture of metals with more than one element. This adds strength and other beneficial qualities

Question 10

How do you use melting points to distinguish between pure and impure?

Answer 10

Impure substances melt over a range of temperatures and the melting point is less than that of a pure substance, eg seawater has a lower melting point than pure water

The greater the difference, the more impure it is

Question 11

Why should you heat a substance slowly and stir it?

Answer 11

It allows the temperature of the whole substance to increase evenly and mixing ensures that the whole thing is the same temperature

Question 12

What is a solution?

Answer 12

a mixture formed when one substance dissolves in another

Question 13

What is a solvent?

Answer 13

The substance a solute dissolves in

Question 14

What is a solute?

Answer 14

The substance that dissolves in the solvent

Question 15

What does soluble mean?

Answer 15

If a substance can dissolve in a solvent

Question 16

What does insoluble mean?

Answer 16

If a substance cannot dissolve in a solvent

Question 17

What is filtration used for?

Answer 17

Separating insoluble substances in solid state from a liquid.

Question 18

How does filtration work?

Answer 18

1) fold some filter paper into a cone and put it in a funnel
2) put the funnel in a conical flask
3) pass the mixture through the filter paper
4) you are left with residue in the filter paper and the liquid in the flask because the filter paper has holes which are too small for solid to pass through

Question 19

What is residue?

Answer 19

What is left behind after filtration

Question 20

What is filtrate?

Answer 20

The substance that passes through the filter paper

Question 21

What is solubility?

Answer 21

How well a substance can dissolve

Question 22

What is crystallisation used for?

Answer 22

To separate a solute from a substance

Question 23

Describe and explain how crystallisation works?

Answer 23

You gently heat the solution under a Bunsen burner until more than half has evaporated(saturated solution).

Then leave the remaining solution to cool so that the remaining solvent evaporates until you are left with crystals

Question 24

What is simple distillation used for?

Answer 24

To separate a solvent from a solution which have different boiling points. It can be used to make distilled water by separating salts from water which have a higher boiling point.

Question 25

What is fractional distillation used for?

Answer 25

For separating 2 or more substances in liquid state from a mixture with different boiling points

Question 26

What is the stationary and mobile phase for thin layer chromatography?

Answer 26

Stationary - Silica or alumina powder on the plate Mobile - Solvent

Question 27

What is the mobile and stationary phase in paper chromatography?

Answer 27

Stationary - absorbent paper Mobile - solvent

Question 28

What is the mobile and stationary phase in gas chromatography?

Answer 28

Stationary - alumina or silica powder Mobile - an unreactive carried gas such as nitrogen

Question 29

Rf formula

Answer 29

Distance travelled by substance / distance travelled by solvent

Question 30

Explain how you know which are the *more soluble* substances and *less soluble* substance from *TLC* or *PC*

Answer 30

More soluble substances are more attracted to the mobile phase , therefore move further up the plate Less soluble substances are more attracted to the stationary phase so they get dropped of on the plate sooner

Question 31

What are the advantages of *Thin layer* chromatography compared to *paper* chromatography

Answer 31

It is quicker It is more sensitive, so a smaller sample can be used A wide variety of stationary and mobile phases can be used You can extract the spots for further analysis

Question 32

What does gas chromatography do additionally from separating the components of the mixture

Answer 32

Measures the amounts of the components of a mixture

Question 33

What does the term *(Retention time)* mean in gas chromatography

Answer 33

Retention time is the time taken for the *sample* to pass through the metal column

Question 34

Explain how *simple distillation* works

Answer 34

1. Pour the solution into a flask and gently heat it 2. The liquid solvent boils but the solute doesn't, as the solvent has a lower boiling point 3. The solvent evaporates and its vapours rise up and pass through the condenser 4. The condenser is surrounded with cold water so the vapour molecules cool down and turn back into a liquid 5. The liquid droplets are collected up in the beaker/test tube

Question 35

Why should you draw the baseline with a *pencil*

Answer 35

Pencil marks are insoluble and won't move with the solvent. Pencil doesn't contain ink, so it wont smudge

Question 36

What is solvent front?

Answer 36

Where the solvent reached at the end of the analysis

Question 37

What is a chromatogram?

Answer 37

Results of a chromatography analysis

Question 38

Describe two main *uses* of chromatography in day to day life

Answer 38

1.- Checks if a sample is pure 2- Tells us the relative solubility of each substance in a sample

Question 39

Explain fully how to carry out *TLC* (thin layer chromatography)

Answer 39

1* - Put solvent into a chromatography tank to the depth of 1 cm . If solvent is flammable make sure there are no naked flames * 2* - Draw a baseline on the plate using a pencil * 3* - add a small amount of sample to the baseline of the TLC plate, taking care not to damage the powder on the plate * 4* - Place the plate inside the tank * 5* - let the solvent travel through the powder and take the plate before it reaches the top * 6* - analyse the pattern of coloured spots

Question 40

What is a substance?

Answer 40

A collection of particles held together by im forces

Question 41

How does gas chromatography work?

Answer 41

1* - Sample is turned into the gas state when it is injected into the column * 2* - The carrier gas pushes the sample through the column and each substance of the sample separate * 3* - The different substances take different times to travel through the column, depending on how strongly they bond to either phase * 4* - A detector sends a signal to a computer as each substance leaves the column and the computer produces a chromatogram based on the results