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Chemistry > 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 called?

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 is something that contains only one type of particles, it has a fixed melting and boiling point
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8
Q

What is a pure substance ?

A

A pure substance is something that contains only one type of element or compound, it has a fixed melting and boiling point

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9
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 elements or compounds), so are therefore impure.

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10
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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11
Q

What is an alloy?

A

An alloy is a mixture of different metals ( more than one element )

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12
Q

What is the melting point of a pure substance?

A

The melting point of a substance is a single temperature, a fixed melting point

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13
Q

What is the melting point of an impure substance?

A
  • Its melting point is less than that of the pure
    substance.
  • It often melts over a range of temperatures, does not have a fixed melting point/ a variable melting point
  • 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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14
Q

Do you know how to draw cooling/heating curves for pure and impure substances ?

A

yes I hope.

This is for a pure substance since the lines when there is a change of state, showing it has a fixed melting boiling point

For a mixture, the lines representing a change of state would be slanted since a mixture has a variable melting/ boiling point - KNOW HOW TO DRAW THAT TOO

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15
Q

Why are lines on a cooling, heating curve horizontal during a change of state ?

A

At a change if state the temperature does not change as the energy is involved in making/breaking bonds

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16
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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17
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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18
Q

What is a solution?

A

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

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19
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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20
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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21
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.
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22
Q

Why does filtration work ?

A

It works because filter paper has tiny, microscopic holes where small molecules can pass through and larger insoluble substances can’t.

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23
Q

What is filtration used to separate ?

A

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

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24
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.
25
Q

explain how sand and water are separated using filtration ?

A
  • The mixture of sand and water is poured into the filter funnel, which is lined with filter paper.
  • The filter paper has tiny microscopic holes
  • The water molecules are small enough to pass through the paper to collect in the beaker.
  • The large insoluble sand particles cannot pass through the filter paper and collect in the filter funnel.
26
Q

What is crystallisation?

A

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

27
Q

What are the steps for crystallisation and explain each process?

A
  • Use a water bath/ evaporating basin to heat up the solution (filtrate - depending on context) until it becomes a saturated solution (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.
28
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.
29
Q

What does simple distillation rely on?

A

It relies on the solvent having a much lower boiling point than the solute, the substances being separated having a large difference in boiling points

30
Q

What does simple distillation separate ?

A
  • There is quite a difference in boiling points
  • The two substances being separated are miscible
31
Q

What happens during simple distillation?

A
  • When the bunsen is lit, the substance with the lowest boiling point evaporates first
  • The substance rises up towards the thermometer. The vapour travels through the condenser, were the cold water “jacket” cools and condenses the ethanol into a liquid
  • When all the substance with the lowest boiling is collected the temperature rises, and eventually reaches the temperature if the boiling point of the other substance
  • It is now the other substance’s vapour that rises, travels through the condenser, cools and condenses to liquid water
32
Q

Label the equipment in simple distillation ?

A
33
Q

Why is simple distillation useful in the laboratory?

A

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

34
Q

What does fractional distillation rely on?

A

It relies on each liquid having a different boiling
point.

35
Q

What is fractional distillation used to separate ?

A
  • Mixtures with substances that have similar boiling points
  • When there are many substances, a mixture with many different substances
36
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.

37
Q

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

A
  • The substance with the lowest boiling point evaporates first and travels up the column
  • It condenses, as the beads inside the column are cold and falls back down
  • This continues until the beads are hot enough for the vapour not to condense
  • A temperature then increases as the second substance evaporates, rises up the column and the beads at the top of the column get hotter
  • Each substance that evaporates and condenses and is collected at a different temperature is called a fraction

eg. Crude oil ( a black mixture containing lots of liquids with different boiling points )

38
Q

What are the two different chemical phases of chromatography?

A
  • A stationary phase that does not move.

- A mobile phase that does move.

39
Q

Why is chromatography used ?

A

It is used to separate substances contained in a mixture when they are in small quantities and they are coloured.

40
Q

What is the stationary and mobile phase in paper chromatography?

A
  • The stationary phase is paper.
  • The mobile phase is a solvent in the liquid state,
    such as water or propanone
41
Q

What is a mobile and stationary phase ?

A

Stationary : This is the substance that the mobile phase moves on

Mobile : This is the substance that moves up/along the stationary phase, usually a solvent

42
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.
43
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.
44
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 longer retention time
  • A component will not travel very far is if it forms
    stronger bonds with the stationary phase than
    with the mobile phase, a shorter retention time
45
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.

46
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
47
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 inert carrier gas ( hydrogen or nitrogen ) pushes the sample through the column ( the column is a very long coil of thin layer of silica coated on it - the stationary phase )
  • The different components take different times to
    travel through the column, depending on how
    strongly they bond to the stationary phase and eventually travel through it and reach the detector
  • A detector then sends a signal to a computer in
    which each component is a peak plotted against
    the retention time.
48
Q
  • What is the Rf value?
A

The Rf value is the relative distance travelled by
a substance during chromatography.

49
Q

What is Rf value used for ?

A

It is used to compare different spots on a chromatogram.

eg. if two spots have identical Rf values and are the same colour - most probably the same substance

50
Q

How can you tell if two components are the same using the Rf value?

A

If two spots have the same Rf value and are the same colour, they are likely to be identical.

51
Q

How to you calculate Rf value?

A

Rf = Distance travelled by solute ( spot ) / Distance
travelled by solvent front

52
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 highest peak therefore means the highest percentage of that component
  • The further the peak the longer the retention time
53
Q

What is retention time ?

A

Retention time is the time taken for a
substance to travel to the chromatography column
and is different for different substances.

54
Q

What are the advantages of TLC ( thin layer chromatography ) 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 - sample can be retreived
55
Q

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

A

Dissolving followed by filtration.

56
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.

57
Q

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

A

Fractional Distillation.

58
Q

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

A

Paper chromatography or thin layer chromatography.

Chemistry (17 decks)

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