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Experimental Chemistry
Purification of Substances

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The Need for Pure Substances:
Definitions
A pure substance… is a single substance not mixed with anything else
A mixture..
contains two or more substances

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The Importance of Pure Substances
 Example #1:
Impurities in medicine may cause undesirable side effects.

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The Importance of Pure Substances
 Example #2:
Impurities in metals weaken its structure more than it should be .

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The Importance of Pure Substances
 Example #3:
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Chemicals are added to food and beverages to make them last longer, taste better or look more attractive/appealing.
Safe-for-consumption chemicals

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Recognizing Pure Substances
 Checking the melting and boiling points of the substance
 Performing chromatography
Melting and boiling points
Pure substance Impure substance
(mixture)
Fixed & unique Over a range of temperatures

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Effect of impurities on M.P. and B.P.
Presence of impurities:
 Lowers the melting point
(greater % of impurity = lower m.p.)
 Raises the boiling point
(greater % of impurity = higher b.p.)

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Concept check!
An impure substance of X melts at around
90 ° C. What is most likely to be the melting point of X?
A.
between 90 ° C to 110 ° C
B.
between 80 ° C to 90 ° C
C.
below 80 ° C
D.
cannot be determined

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Separation Techniques methods of purification

Filtration
 to separate insoluble solid from liquid

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Separation Techniques methods of purification
 Filtration
Filters are used to purify water at water purification plants.

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Separation Techniques methods of purification

Daily applications of filtration

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Separation Techniques methods of purification
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Evaporation (to dryness)
 to separate soluble solid from liquid evaporating dish

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Separation Techniques methods of purification
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Evaporation (to dryness)
Limitations:
1.
Some substances decompose strongly when heated strongly.
2.
Soluble impurities may be left behind after all the water has been removed.

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Separation Techniques methods of purification

Crystallization
 to separate soluble solid from liquid as crystals

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Separation Techniques methods of purification
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Crystallization
 Heating is stopped when a hot saturated solution is formed.

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How do we test for a saturated solution?
Procedure:
 A clean glass rod can be used to test whether a solution is saturated.
 It is dipped into the solution and removed. There will be a small amount of solution on the rod.
 If small crystals form on the rod as the solution cools, the solution is saturated.

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Separation Techniques methods of purification

Decanting
 just pouring the liquid away!
 to separate a dense insoluble solid from liquid

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THINK!

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Separating Salt and Sand
1.
Add some distilled water into the mixture of salt and sand. Stir and make sure all the salt has dissolved.
2.
Filter the mixture
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Sand
Salt solution

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Separating Salt and Sand
3.
4.
Wash the residue with a little distilled water to remove all the salt solution from it.
Evaporate the filtrate to dryness.
Salt solution
Sand

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Separation Techniques methods of purification
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Sublimation
 to separate a mixture of solids, one of which sublimes

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Separation Techniques methods of purification
Some substances that sublime:
1.
solid carbon dioxide
2.
3.
4.
5.
6.
anhydrous iron (III) chloride anhydrous aluminium chloride iodine ammonium chloride naphthalene

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THINK!
sodium nitrate is soluble in water, but lead sulfate` is not

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Separation Techniques methods of purification
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Simple distillation
 to separate a pure liquid from a solution
 the process of boiling the liquid and condensing the vapour

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Separation Techniques methods of purification
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Simple distillation
1.
2.
3.
4.
solution is heated until it boils pure liquid turns into vapour and leaves the flask vapour is then cooled by a condenser, which changes it
back into a liquid the liquid is collected in the conical flask, and is called the distillate

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Separation Techniques methods of purification
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Simple distillation
Where is the bulb of the thermometer placed?
It is placed beside the side arm of the distillation flask, and should not be dipped in solution.

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Separation Techniques methods of purification
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Simple distillation
Where does the water enter and leave the condenser?
Cold running water enters from the bottom of the condenser and leaves from the top.

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Obtaining Pure Water from Seawater:
Desalination
 by distillation of seawater
[one way]
 disadvantage: expensive method because oil is the main fuel used to heat the water in the distillation process, and a lot of oil is required, which makes the process very costly

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Separation Techniques methods of purification
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Fractional distillation
 to separate mixtures of miscible liquids with widely differing boiling points

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Separation Techniques methods of purification
How does the temperature change as a solution of ethanol and water undergoes fractional distillation?
Temperature / o C
1. The temperature of the mixture increases as it is heated.
Time / s

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Separation Techniques methods of purification
How does the temperature change as a solution of ethanol and water undergoes fractional distillation?
Temperature / o C
2. At 78 ° C, ethanol distils over.
The temperature remains constant until all the ethanol has distilled out of the round-bottomed flask.
Time / s

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Separation Techniques methods of purification
How does the temperature change as a solution of ethanol and water undergoes fractional distillation?
Temperature / o C
3. The temperature then increases until 100 ° C.
At 100 ° C, water distils over.
The temperature remains unchanged as water is being distilled.
Time / s

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Separation Techniques methods of purification
 Fractional distillation

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Separation Techniques methods of purification
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Use of separating funnel
 to separate immiscible liquids

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Separation Techniques methods of purification
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Use of separating funnel
1.
pour the mixture into the separating funnel, ensuring the tap is closed

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Separation Techniques methods of purification
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Use of separating funnel
2.
support the separating funnel with a retort stand, placing a clean beaker below it

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Separation Techniques methods of purification
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Use of separating funnel
3.
4.
allow some time for the liquids to separate completely open the tap to allow the denser liquid at the bottom to drain into the beaker
* close the tap before the liquid in the top layer runs out! the denser liquid

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Separation Techniques methods of purification
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Use of separating funnel
5.
place another beaker below the separating funnel and allow a little of the liquid in the top layer into it
* dispose this liquid collected
The mixture is separated!

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QUIZ!
How would you separate…
(a) red dye from a mixture of red and blue dyes in solution?
(b) oxygen from air?
(c) barium sulfate from a mixture of
(insoluble) barium sulfate and water?

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QUIZ!
How would you separate… d) water from oil and water?
e) petrol from crude oil?
f) g) iodine crystals from iodine and copper(II) oxide?
salt from sea water?