Chemical analysis - purity, formulaions, chromatography + required practical chromatograhy Flashcards

1
Q

what can we to determine if a substance is pure

A

Melting point and boiling point data can be used to
distinguish pure substances from mixtures.

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2
Q

how to determine if a substance is pure

A

Pure elements and compounds melt and boil at specific (fixed) temperatures
impure substances melt and boil over a range of temperatures

you can test the purity of a sample by measuring its melting or boiling point and comparing it with the melting or boiling point of the pure substance (which you can find in a data book)

the closer your measured value is to the actual melting or boiling point, the purer your sample is

if we take a substance and heat it, we can measure the melting point and the boiling point, and these will tell us if the substance is pure

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3
Q

what is a pure substance

A

In chemistry, a pure substance is a single element or compound,
not mixed with any other substance

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4
Q

describe melting and boiling points of pure substances

A

Pure elements and compounds melt and boil at specific (fixed) temperatures

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5
Q

describe melting and boiling points of impure substances

A

impure substances melt and boil over a range of temperatures

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6
Q

experiment to test if a substance is pure
results of experiment: http://www.kentchemistry.com/images/links/matter/aim10.5.jpg

results of experiment using pure water

describe the results

A

We are starting with ice and gradually increasing the temperature
At a certain point, the temperature stops rising. This is the melting point
The melting point in this case is fixed at 0degrees - tells us this is a pure substance
Once the water has melted, the temperature increases again
And at a certain point, the temp. stops rising. this is the boiling point. - the boiling piont in this case is fixed at 100 degrees

BECAUSE BOTH THE MELTING AND THE BOILING POINTS ARE AT SPECIFIC FIXED TEMPERATURES, WE KNOW THAT THIS WATER IS PURE

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7
Q

experiment to test if a substance is IMPURE
results of experiment: https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTYL2ylbNcZ5h5WmzJEMHXeJmncFwjcL-Djelv96JOt3R9NI_S0

results of experiment using WATER AND AN IMPURITY
describe the results

A

In this case the water melts and boils over a range of temperatures
This tells us that the water is not pure

An impure sample of the compound would produce a gradual decrease in temperature as it freezes, as shown in the graph below

An impure sample of the compound would produce a gradual increase in temperature as it melts and boils, as shown in the graph below

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8
Q

what is a formulation

A

A formulation is a complex mixture that has been designed as a useful product

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9
Q

describe the features of a formulation - how is as formulation different to a mixture

A

In a formulation, the quantity of each component is carefully measured so that the product has the properties we need

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10
Q

In a formulation why is the quantity of each component carefully measured

A

In a formulation, the quantity of each component is carefully measured so that the product has the properties we need

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11
Q

Examples of formulations

A

Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and food

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12
Q

How can paper chromatography can be used to identify substances in a mixture

using paper chromatography to identify an unknown substance

A

benefit of chromatography - can be used to identify an unknown substance.

place a dot of the unknown chemical onto the pencil line and we place the bottom of the paper into a solvent

The solvent now makes its way up the paper and it dissolves the ink (which is in the dot
The ink is now carried up the paper as well

We can see that the chemical is pure as we only have one spot

Note the position the solvent moved to
Measure the distance moved by the unknown chemical. To dot that, measure from the pencil line to the centre of the spot. Record this distance
Measure the distance moved by the solvent. record this distance (measure from pencil line to position reached by the solvent)

Use these numbers to calculate the RF value for the substance
Rf - distance moved by substance / distance moved by solvent
No units for Rf values (f is subscript)

Look this RF value up in a database and that will tell us the identity of the substance

Different compounds have different Rf
values in different solvents,
which can be used to help identify the compounds.

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13
Q

problem with using paper chromatography to identify an unknown substance

A

Several different substances could have the same Rf value.
We might need to repeat this experiment using a different solvent (to narrow it down further)

If this substance has never been analysed before, then there will not be an Rf value on the database (so further analysis will need to be carried out to identify it)

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14
Q

How does paper chromatography work

A

Paper chromatography allows us to separate substances based on their different solubilities

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15
Q

suggest how chromatographic methods can be used for distinguishing pure substances from impure substances

e.g. A student has a pack of coloured pens. Wants to know which pens contain only one colour and which pens contain a mixture of colours

A

Take a piece of special paper called chromatography paper
Draw a pencil line near the bottom
Put a dot of our first colour onto the pencil line
Next to that we put a dot of our second colour
We can do this for several colours, as long as there is enough space on the paper

Place the bottom of the paper into a solvent (a liquid that will dissolve substances)

The solvent now makes its way up the paper and it dissolves the ink (which is in the two coloured dots).
And these are now carried up the paper as well

If a colour forms a single spot, we know that this colour is made from only one colour - it is a single pure colour

If the colour separates into two or more spots, we know that this colour was made from a mixture of two or more different colours

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16
Q

in paper chromatography, state the characteristics of a pure compound and how this is different from a compound in a mixture

A

A pure compound will produce a single spot in all solvents - since there is only one substance in the sample
The compounds in a mixture may separate into different spots depending on the solvent

The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

17
Q

explain why paper chromatography works

A

Paper chromatography works because different substances have got different solubilities

18
Q

how does the solubilitiy of a substance affect its movement up the paper

A

A more soluble substance travels further up the paper than a substance which is less soluble

19
Q

in paper chromatograph identify the stationary phase and mobile phase. Explain your answer

A

The paper is the stationary phase because it does not move
The solvent is the mobile phase because it does move

20
Q

explain how paper chromatography separates mixtures

A

Separation depends on the
distribution of substances between the phases.

Mobile phase - where the molecules can move
Stationary phase - where the molecules cant move

the amount of time molecules spend in each phase depends on two things
how soluble they are in the solvent
how attracted they are to the paper

molecules with a higher solubility in the solvent and which are less attracted to the paper, will spend more time in the mobile phase - and will be carried further up the paper

The distance travelled by each of the components depends upon their solubility in that solvent.3 Aug 20 - internet

21
Q

explain why we draw our starting line in pencil

A

If we drew the line in pen, the pen ink would move up the paper with the solvent

22
Q

describe how paper chromatography can be used to identify substances - required practical
to work out colours in food colouring e.g.

Sample of food colouring - which is a mixture of chemicals - we are calling it U for unknown
We also have four known food colourings that it could contain. We are going to label these A-D

A

Use a ruler to draw a horizontal pencil line on the chromatography paper
The line should be around 2cm from the bottom of the paper
Mark five pencil spots at equal spaces across the line. Leave at least 1cm clear at each side

Use a capillary tube to put a small spot of each of the known food colours and the unknown colour onto the pencil spots

A capillary tube is a very thin glass tube

It is important that we keep the spots relatively small. This prevents the colours from spreading into each other later

Pour water into a beaker to a depth of 1cm
In this case, the water is the solvent

Attach the paper to a glass rod using tape and lower the paper into the beaker
The bottom of the paper should dip into the water

Ensure that the pencil line with the spots of ink must be/is above the surface of the water. Otherwise the water will wash the ink off the line.

The sides of the paper must not touch the side walls of the beaker. If that happens, then it will interfere with the way the water moves.

We usually put a lid on the beaker to reduce evaporation of the solvent

At this stage, the water will move up the paper and the colours will be carried up
During this time, we need to be careful not to move the beaker

Remove the paper when the water has travelled around three-quarters up

At this stage, use a pencil to mark the point where the water reached
Finally Hang the paper up to dry

23
Q

REQUIRED Practical

HERE ARE THE RESULTS OF THE EXPERIMENT

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTfUMSA7QMSb7nQU2dDS9geeJDsfGyAboIWbBrnt92v3bjJlgIL

DESCRIBE THE RESULTS OF THE EXPERIMENT

describe how paper chromatography can be used to identify substances - required practical
to work out colours in food colouring e.g.

Sample of food colouring - which is a mixture of chemicals - we are calling it U for unknown
We also have four known food colourings that it could contain. We are going to label these A-D

A

The unknown colour has separated into three spots, telling us that this is a mixture of (at least - see CGP p.156) three colours

The spots in colours A, C and D line up with the spots in the unknown colour
This tells us that the unknown colour is a mixture of colours A, C, and D

The unknown colour does not contain colour B

24
Q

REQUIRED Practical

HERE ARE THE RESULTS OF THE EXPERIMENT

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTfUMSA7QMSb7nQU2dDS9geeJDsfGyAboIWbBrnt92v3bjJlgIL

WHAT IF WE WANTED TO IDENTIFY THE CHEMICALS IN THESE COLOURS

describe how paper chromatography can be used to identify substances - required practical
to work out colours in food colouring e.g.

Sample of food colouring - which is a mixture of chemicals - we are calling it U for unknown
We also have four known food colourings that it could contain. We are going to label these A-D

A

To identify the chemicals in these colours, we calculate the Rf values

Method:
Measure the distance from the pencil line to the centre of each spot
Measure the distance moved by the water from the pencil line (EASIER TO SEE IF YOU DRAW A PENCIL LINE OF WHERE THE SOLVENT REACHED)

To calculate the Rf value use this equation:
Rf = distance moved by substance / distance moved by solvent
Rf values do not have a unit
We can now look this Rf value up in a database and that will tell us the identity of the chemical

Several different chemicals could have this Rf value.
We might need to repeat this experiment using a different solvent to narrow it down further
If this chemical has never been analysed before then there will not be an Rf value on the database

distribution of substances between the phases.
The ratio of the distance moved by a compound (centre of spot from
origin) to the distance moved by the solvent can be expressed as its
Rf
value