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GCSE Chemistry > C2.1 Purity and separating mixtures > Flashcards

C2.1 Purity and separating mixtures Flashcards

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1
Q

Define dissolves

A

This happens to a substance when it’s particles separate and become completely mixed with the particles of the solvent

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2
Q

If an atom’s atomic mass is greater than 12.0…

A

its mass is more than a carbon-12 atom

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3
Q

Define an empirical formula

A

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

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4
Q

What does a balanced equation show?

A

A balanced equation shows the formulae and number of units for all the substances in a reaction.

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5
Q

Calculate the relative formula mass of:

a) Mg(OH)2
b) Al(OH)3
c) (NH4)2CO3

A

a) 58
b) 78
c) 96

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6
Q

Define a pure substance

A

A pure substance completely consists of a single element or compound.

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7
Q

Define an alloy

A

An alloy is a mixture of a metal with one or more other elements.

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8
Q

Define melting point

A

The melting point of a substance is the temperature at which it changes from the solid state to a liquid state.

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9
Q

Describe the melting point of a pure substance.

A

The melting point of a pure substance is a single temperature.

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10
Q

If a substance is impure…

A
  • its melting point is less than of the pure substance

- it often melts over a range of temperatures, not just one temperature.

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11
Q

Describe the relationship between the melting point of a substance and its purity.

A

The greater the difference between the measured melting point for a substance and its accepted melting point, the lower its purity is likely to be.

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12
Q

Describe the boiling point of an impure substance.

A

The boiling point of an impure substance will be higher than of a pure substance.

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13
Q

How can you determine the melting point of a substance?

A

You can determine the melting point of a substance by heating it. You can either measure the temperature at which it melts or measure its temperature at regular intervals and plot a graph.

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14
Q

When determining the melting point of a substance why is it important to heat the substance slowly?

A

Heating the substance slowly allows the temperature of the whole sample to increase.

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15
Q

When determining the melting point of a substance, why is it important to stir the substance as it melts?

A

Mixing ensures that the entire sample is at the same temperature.

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16
Q

What are the two ways to improve the accuracy of a measurement of the melting point of a sample.

A

By heating the substance slowly

By stirring the substance as it melts

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17
Q

Define solution

A

One substance dissolved in another

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18
Q

Define solvent

A

The substance which dissolves the solute

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19
Q

Define solute

A

A substance which is dissolved in the solvent

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19
Q

What does simple distillation rely on?

A

It relies on the solvent having a much lower boiling point than the solute.

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21
Q

Define soluble

A

When a substance can dissolve in a particular substance

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22
Q

If the atom’s atomic mass is below 12.0…

A

It has less mass than a carbon-12 atom

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23
Q

Define filtration

A

Separating an insoluble substance in the solid state from substance in a liquid state by passing it through a porous layer

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24
Q

Define relative atomic mass

A

It 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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25
Q

Define residue

A

The solid which remains in the filter paper after filtration

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26
Q

Define evaporate

A

When a solid changes from liquid to a gas

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27
Q

What is the mobile phase in thin layer chromatography?

A

A solvent in the liquid state

28
Q

In TLC, what does it mean if the component doesn’t travel very far? And what does it say about is solubility?

A

A component will not travel very far if it forms stronger bonds with the stationary phase than with the mobile phase.
The molecules have a relatively low solubility in the substance.

29
Q

Define crystallisation

A

Dissolving a substance until the solution is saturated and then allowing to cool slowly.

30
Q

What is fractional distillation used for?

A

Fractional distillation is used to separate two or more substances from a mixture in the liquid state.

31
Q

Define solubility

A

How much a substance will dissolve in a given volume.

32
Q

Describe how you crystallise a product.

A

1) Pour the solution into an evaporating dish and gently heat the solution. Some of the solvent (which usually will be water) will evaporate and the solution will get more concentrated.
2) Once some of the solvent has evaporated or when you see crystals start to form, remove the dish from the heat and leave the solution to cool.
3) Filter the crystals out of the solution and leave them in a warm place to dry. You could also use a drying oven or a desiccatior.

33
Q

If a mixture contains two or more substances in a liquid state, what separation technique would you use?

A

fractional distillation

34
Q

Describe how you would carry out simple distillation to get pure water from seawater.

A

1) Pour you sample of sea water into the distillation flask.
2) Set up your apparatus with a thermometer in the distillation flask and a condenser connected to the distillation flask and connect to that is beaker where the pure water will be collected. Connect the bottom end of the condenser to a cold tap using rubber tubing. Run cold water through the condenser to keep it cool.
3) gradually heat the distillation flask. The part of the solution that has the lowest boiling point will evaporate so the water in this case.
4) the water vapour passes into the condenser which turn it back into a liquid. It them flows into the beaker where it is collected.
5) Eventually you will end up with salt in the flask and pure water in the beaker.

35
Q

What happens in the condenser?

A

Solvent vapour is turned back into a liquid

36
Q

If a mixture contains a soluble solute dissolved in a solvent, what separation method would you use to obtain:

a) the solute
b) the solvent

A

a) crystallisation

b) simple distillation

37
Q

What does simple distillation do?

A

Simple distillation is used to separate a solvent from a solution.

38
Q

How is fractional distillation all used industrially?

A

Fractional distillation is used industrially to separate bio ethanol from water for use as a fuel.

39
Q

What is the problem with simple distillation?

A

The problem is that you can only use it to separate things with very different boiling points

40
Q

Define relative formula mass

A

It is the mean mass of a unit of a substance compared to 1/12 the mass of a carbon-12 atom.

41
Q

Define a saturated solution

A

A solution which cannot dissolve anymore solute

42
Q

Describe how you would carry out fractional distillation of crude oil.

A

1) Put your mixture in a flask. Attach a fractionating column and condenser above the flask.
2) Gradually heat the flask. The different liquids will all have different boiling points so they will evaporate at different temperatures.
3) The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, it will reach the top of the column.
4) Liquids with higher boiling points might also start to evaporate but the column is cooler towards the top, so they will only get part of the way up before condensing and running back down towards the flask.
5) When the first liquid has been collected, raise the temperature until the next one reaches the top.

43
Q

Advantages of TLC over paper chromatography

A

It is quicker
It is more sensitive, so a smaller sample can be used
There is a large range of stationary phases and solvents to choose from.

44
Q

What does the fractionating column improve in fractional distillation?

A

It improves the separation of the mixture. It has a large surface area on which the vapours can continually condense. During fractional distillation, the column becomes hottest at the bottom and coolest at the top.

45
Q

Define insoluble

A

When a substance can’t dissolve in a substance

46
Q

What is chromatography used for?

A

Chromatography is used to separate and identify the substances in a mixture.

47
Q

Define stationary phase.

A

A stationary phase is a phase where the molecules can’t move. This can be a solid or really thick liquid

48
Q

Define mobile phase

A

A phase where the molecules can move. This is always a liquid or gas

49
Q

What is the stationary phase in chromatography?

A

Absorbent paper

50
Q

Define filtrate

A

The liquid which is passed through the filter paper during filtration

51
Q

What is the mobile phase in chromatography?

A

A solvent in the liquid state, such as water or propane.

52
Q

How would you set up and carry out thin-layer chromatography?

A

1) Draw a pencil line near the bottom of the tray.
2) Put a spot of the mixture on the pencil line, taking care not to damage the powder on the plate.
3) Put the solvent in a chromatography tank to a depth of about 1cm. It should be below the pencil line.
4) the solvent will start to move up the plate. When the chemicals in the mixture dissolve in the solvent, they will move up the plate too.
5) Let the solvent travel through the powder, and take the plate out before it reaches the top.

53
Q

What is a desiccatior?

A

A desiccatior contains chemicals that remove water from the surrounding.
This helps some to dry quicker.

54
Q

In TLC, what does it mean if a component travels further up? And what does it say about its solubility?

A

A component travels further up the plate if it forms stronger bonds with the mobile phase when with the stationary phase.
The molecules have a high solubility in the solvent.

55
Q

What is the mobile phase in gas chromatograph?

A

An unreactive carrier gas such as nitrogen

56
Q

What is the stationary phase in thin layer chromatography?

A

Thin layer of silica or alumina powder spread over a plate of glass plastic.

57
Q

What is the stationary phase in gas chromatography?

A

Silica or alumina powder packed into a metal column

58
Q

Explain the process of gas chromatography.

A

The sample is injected into the column.
The mixture moves along the the tube with the mobile phase until it comes out the other end. The different components will take different times to travel through the column, depending on how strongly they bond to the stationary phase.
As each component leaves the column a detector sends a signal to a computer which produces a chromatogram.

59
Q

What does the retention time of a substance mean?

A

The time it take a chemical to travel through the tube.

It is used to identify the chemicals.

60
Q

What do the peaks represent on a gas chromatography chromatogram?

A

Represents each chemical

61
Q

What do the areas under the peaks on a chromatogram represent?

A

The relative amounts of each chemical in the substance.

62
Q

If a mixture contains coloured soluble substance, what separation method would you use?

A

paper chromatography or thin-layer chromatography

63
Q

What is the equation to work out the Rf values for each substance?

A

Rf= distance travelled by substance/distance travelled by

solvent

64
Q

If a mixture contains insoluble and soluble substance, what separation method would you use?

A

Dissolving followed by filtration

65
Q

What is a locating agent?

A

A locating agent might be used to make colourless spots on a chromatogram visible.

GCSE Chemistry (9 decks)

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