C2.1 - Purity And Separating Mixtures

Question 1

What are the masses of subatomic particles described by?

Answer 1

• Their relative masses.

- Their masses compared to the mass of a proton.

Question 2

What is relative atomic mass?

Answer 2

Relative atomic mass, Ar, is the mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.

Question 3

What does a chemical formula show?

Answer 3

A chemical formula tells you how many atoms of each element there are in a unit of a substance.

Question 4

What is relative formula mass?

Answer 4

Relative formula mass, Mr, is the mean mass of a unit of a substance compared to 1/12 the mass of a Carbon-12 atom.

Question 5

What does an empirical formula show?

Answer 5

An empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound.

Question 6

What does a balanced chemical equation show?

Answer 6

A balanced chemical equation shows the formulae (how the atoms are rearranged) and the relative amounts of each substance involved.

Question 7

What does pure mean in everyday life and how does it differ from the scientific sense of purity?

Answer 7

• Pure in everyday life describes natural
  substances that have not been processed or
  changed.
• In science, a pure substance consists of just
  one element or compound.

Question 8

What is a mixture and are they pure or impure?

Answer 8

A mixture contains two or more different substances that are not chemically joined together, (two or more different compounds), so are therefore impure.

Question 9

Why is it difficult to obtain pure substances?

Answer 9

• Just one atom or molecule of something else
  makes the substance impure.
• Substances can easily become contaminated such
  as with the carbon dioxide from the air which itself
  is a mixture of nitrogen, oxygen, and other
  substances.

Question 10

What is an alloy?

Answer 10

An alloy is a mixture of a metal with at least one other element.

Question 11

What is the melting point of a pure substance?

Answer 11

The melting point of a substance is a single temperature.

Question 12

What is the melting point of an impure substance?

Answer 12

• Its melting point is less that that of the pure
  substance.
• It often melts over a range of temperatures.
• Greater the difference between the measured
  melting point and the accepted melting point, the
  lower its purity is likely to be.

Question 13

How can you determine melting point?

Answer 13

Heating the substance and:
- Measure the temperature at which it melts.
OR
- Measure its temperature at regular time intervals
and plot a temperature against time graph.

Question 14

What are the two important things to do when determining the melting point of a substance and why (what does this improve)?

Answer 14

1. Heating the substance slowly - Allows the
   temperature of the whole sample to
   increase.
2. Stirring the substance - Ensures the entire
   sample is at the same temperature.

• These two actions improve the accuracy of a
  measurement of the melting point of a sample.

Question 15

What is a solution?

Answer 15

A solution is a mixture formed when one substance dissolves in another.

Question 16

What is a solute and solvent?

Answer 16

• A solute is a substance that dissolves in a solvent.
• A solvent is a substance that can dissolve a
  solute to form a solution.

Question 17

What happens when a substance dissolves?

Answer 17

When a substance dissolves, its particles separate and become completely mixed with the particles of the solvent.

Question 18

What is a soluble substance and what is an insoluble substance?

Answer 18

• A soluble substance is one that can dissolve in
  the given solvent.
• An insoluble substance is one that can’t
  dissolve in the given solvent.

Question 19

What is the purpose of filtration and why does it work?

Answer 19

• Filtration separates an insoluble substance in the
  solid state from substances in the liquid state.
• It works because filter paper has tiny, microscopic
  holes where small molecules can pass through
  and larger insoluble substances can’t.

Question 20

What are the components in filtration and explain why scientists flute filter paper.

Answer 20

• Filter paper, funnel, residue, conical flask, filtrate.
• Scientists flute filtrate paper to produce a larger
  surface area for the filtrate to pass through.

Question 21

What is crystallisation?

Answer 21

Crystallisation is the process by which crystals are formed during evaporation of a solvent from a solution.

Question 22

What are the steps for crystallisation and explain each process?

Answer 22

• Use a water bath to heat up the solution,
  (filtrate - depending on context), until the
  solution becomes saturated (when no more
  solute can be dissolved at that temperature).
• Crystals will start forming at this point.
• Let the solution cool slowly - because the
  solution then decreases the solubility of the
  solute which results in more crystals
  forming.
• Separate the crystals from the remaining
  solution by filtration, and dry them in a warm
  oven or by patting them with filter paper.

Question 23

What happens if you heat a solution too strongly in crystallisation, and what should you do instead?

Answer 23

• If you heat the solution too strongly, you get a
  powder.
• If you allow the solvent to evaporate slowly, you
  get regularly shaped crystals.

Question 24

What does simple distillation separate and what does it rely on?

Answer 24

• Simple distillation separates a solvent from a
  solution.
• It relies on the solvent having a much lower
  boiling point than the solute.

Question 25

What happens during simple distillation?

Answer 25

- When the solution is heated, the solvent boils but the solute does not. - The solvent turns into vapour and travels from the solution to the condenser so it can be condensed back to its liquid state, leaving the previously dissolved solute behind.

Question 26

Why is simple distillation useful in the laboratory?

Answer 26

Simple distillation is useful in the laboratory if you want to purify a solvent.

Question 27

What is fractional distillation and what does it rely on?

Answer 27

- A separation technique which separates two or more liquids in a mixture. - It relies on each liquid having a different boiling point.

Question 28

What does the fractionating column improve?

Answer 28

The fractionating column improves the separation of the mixture due to its large surface area on which the vapours can continually condense.

Question 29

What happens in the fractionating column in the separation of bioethanol from water for use as a fuel?

Answer 29

- The vapours condense on the inside surface, heating the fractionating column up. - When the temperature inside reaches the boiling point of ethanol, ethanol vapour cannot condense any more, but water vapour can. - Water droplets fall into the flask, and ethanol vapour passes into the condenser...

Question 30

What are the two different chemical phases of chromatography?

Answer 30

- A stationary phase that does not move. | - A mobile phase that does move.

Question 31

What is the stationary and mobile phase in paper chromatography?

Answer 31

- The stationary phase is absorbent paper. - The mobile phase is a solvent in the liquid state, such as water or propanone

Question 32

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

Answer 32

The stationary phase is a thin layer of silica or alumina powder which is spread over a plate of glass or plastic. - The mobile phase is a solvent.

Question 33

What is a chromatogram and what does it show?

Answer 33

- A chromatogram is a pattern produced when separating a mixture using chromatography. - It shows the separate components of a sample.

Question 34

What does the pattern produced on a chromatogram depend on?

Answer 34

- The pattern produced depends on how each component is distributed between the two phases. - A component travels further up the plate if it forms stronger bonds with the mobile phase than with the stationary phase. - A component will not travel very far is if it forms stronger bonds with the stationary phase than with the mobile phase.

Question 35

What does gas chromatography do in addition to separating the components?

Answer 35

Gas chromatography separates the components of a mixture and also measures their relative amounts.

Question 36

What is the stationary phase and mobile phase in gas chromatography?

Answer 36

- The stationary phase is silica or alumina powder packed into a metal column. - The mobile phase is an unreactive carrier gas such as nitrogen, which does not react with the sample

Question 37

What are the steps in gas chromatography?

Answer 37

- The sample is turned into the gas state when it is injected into the column. - The carrier gas pushes the sample through the column. - The different components take different times to travel through the column, depending on how strongly they bond to the stationary phase. - A detector then sends a signal to a computer in which each component is a peak plotted against the retention time.

Question 38

- What is the Rf value? - How can you tell if two components are the same using the Rf value? - How to you calculate Rf value?

Answer 38

- The Rf value is the relative distance travelled by a substance during chromatography. - If two spots have the same Rf value and are the same colour, they are likely to be identical. - Rf = Distance travelled by substance / Distance travelled by solvent (distance from base line to solvent front).

Question 39

How do you interpret a gas chromatogram?

Answer 39

- Each peak represents a component /substance present in the mixture. - The area under each peak shows how the relative amount of each substance in the mixture. - The retention time is the time taken for a substance to travel to the chromatography column and is different for different substances.

Question 40

What are the advantages of TLC than paper chromatography?

Answer 40

- It is quicker. - It is more sensitive, so a smaller sample can be used. - There is a larger range of stationary phases and solvents to choose from. - You can also scrape an individual spot from a thin- layer chromatogram for further analysis, for example by gas chromatography.

Question 41

How do you separate a mixture which contains insoluble and soluble substances?

Answer 41

Dissolving followed by filtration.

Question 42

How do you separate a mixture which contains a solute dissolved in a solvent (a solution)?

Answer 42

Crystallisation to obtain the solute or simple distillation to obtain the solvent.

Question 43

How do you separate a mixture which contains two or more substances in the liquid state?

Answer 43

Fractional Distillation.

Question 44

How do you separate a mixture which contains coloured soluble substances?

Answer 44

Paper chromatography or thin layer chromatography.