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LGS GCSE OCR Chemistry A - Aayan 😎 > C2.1 - Purity And Separating Mixtures > Flashcards

C2.1 - Purity And Separating Mixtures Flashcards

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1
Q

What are the masses of subatomic particles described by?

A
  • Their relative masses.

- Their masses compared to the mass of a proton.

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2
Q

What is relative atomic mass?

A

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.

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3
Q

What does a chemical formula show?

A

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

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4
Q

What is relative formula mass?

A

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.

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5
Q

What does an empirical formula show?

A

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

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6
Q

What does a balanced chemical equation show?

A

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

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7
Q

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

A
  • 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.
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8
Q

What is a mixture and are they pure or impure?

A

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

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9
Q

Why is it difficult to obtain pure substances?

A
  • 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.
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10
Q

What is an alloy?

A

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

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11
Q

What is the melting point of a pure substance?

A

The melting point of a substance is a single temperature.

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12
Q

What is the melting point of an impure substance?

A
  • 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.
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13
Q

How can you determine melting point?

A

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.

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14
Q

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

A
  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.
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15
Q

What is a solution?

A

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

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16
Q

What is a solute and solvent?

A
  • A solute is a substance that dissolves in a solvent.
  • A solvent is a substance that can dissolve a
    solute to form a solution.
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17
Q

What happens when a substance dissolves?

A

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

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18
Q

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

A
  • 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.
19
Q

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

A
  • 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.
20
Q

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

A
  • Filter paper, funnel, residue, conical flask, filtrate.
  • Scientists flute filtrate paper to produce a larger
    surface area for the filtrate to pass through.
21
Q

What is crystallisation?

A

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

22
Q

What are the steps for crystallisation and explain each process?

A
  • 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.
23
Q

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

A
  • If you heat the solution too strongly, you get a
    powder.
  • If you allow the solvent to evaporate slowly, you
    get regularly shaped crystals.
24
Q

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

A
  • Simple distillation separates a solvent from a
    solution.
  • It relies on the solvent having a much lower
    boiling point than the solute.
25
Q

What happens during simple distillation?

A
  • 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.
26
Q

Why is simple distillation useful in the laboratory?

A

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

27
Q

What is fractional distillation and what does it rely on?

A
  • A separation technique which separates two or
    more liquids in a mixture.
  • It relies on each liquid having a different boiling
    point.
28
Q

What does the fractionating column improve?

A

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

29
Q

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

A
  • 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…
30
Q

What are the two different chemical phases of chromatography?

A
  • A stationary phase that does not move.

- A mobile phase that does move.

31
Q

What is the stationary and mobile phase in paper chromatography?

A
  • The stationary phase is absorbent paper.
  • The mobile phase is a solvent in the liquid state,
    such as water or propanone
32
Q

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

A

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.

33
Q

What is a chromatogram and what does it show?

A
  • A chromatogram is a pattern produced when
    separating a mixture using chromatography.
  • It shows the separate components of a sample.
34
Q

What does the pattern produced on a chromatogram depend on?

A
  • 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.
35
Q

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

A

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

36
Q

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

A
  • 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
37
Q

What are the steps in gas chromatography?

A
  • 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.
38
Q
  • 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?
A
  • 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).
39
Q

How do you interpret a gas chromatogram?

A
  • 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.
40
Q

What are the advantages of TLC than paper chromatography?

A
  • 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.
41
Q

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

A

Dissolving followed by filtration.

42
Q

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

A

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

43
Q

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

A

Fractional Distillation.

44
Q

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

A

Paper chromatography or thin layer chromatography.

LGS GCSE OCR Chemistry A - Aayan 😎 (17 decks)

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