Experimental Chemistry

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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
• 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

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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?
  
• between 90 C to 110 C
• between 80 C to 90 C
• below 80 C
• cannot be determined

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Separation Techniquesmethods of purification

• Filtration
• to separate insoluble
  solid from liquid

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Separation Techniquesmethods of purification

• Filtration
• Filters are used to purify
  water at water purification
  plants.

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Separation Techniquesmethods of purification

• Daily applications of filtration

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Separation Techniquesmethods of purification

• Evaporation (to dryness)
• to separate soluble solid from liquid

evaporating dish
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Separation Techniquesmethods of purification

• Evaporation (to dryness)
• Limitations
• Some substances decompose strongly when heated
  strongly.
• Soluble impurities may be left behind after all
  the water has been removed.

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Separation Techniquesmethods of purification

• Crystallization
• to separate soluble solid from liquid as crystals

BETTER METHOD!
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Separation Techniquesmethods of purification

• 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 Techniquesmethods of purification

• Decanting
• just pouring the liquid away!
• to separate a dense insoluble solid from liquid

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THINK!

• How can you separate a mixture containing salt
  and sand?

Add a suitable solvent, and filter
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Separating Salt and Sand

• Add some distilled water into the mixture of salt
  and sand. Stir and make sure all the salt has
  dissolved.
• Filter the mixture.

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Separating Salt and Sand

• Wash the residue with a little distilled water to
  remove all the salt solution from it.
• Evaporate the filtrate to dryness.

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Separation Techniquesmethods of purification

• Sublimation
• to separate a mixture of solids, one of which
  sublimes

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Separation Techniquesmethods of purification

• Some substances that sublime
• solid carbon dioxide
• anhydrous iron (III) chloride
• anhydrous aluminium chloride
• iodine
• ammonium chloride
• naphthalene

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THINK!

• How would you purify solid sodium nitrate
  contaminated with solid lead(II) sulfate?
• sodium nitrate is soluble in water, but lead
  sulfate is not

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Separation Techniquesmethods of purification

• 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 Techniquesmethods of purification

• Simple distillation
• 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 Techniquesmethods of purification

• Simple distillation
• Where is the bulb of the thermometer placed?

This ensures that the thermometer measures the
boiling point of the substance being distilled.
It is placed beside the side arm of the
distillation flask, and should not be dipped in
solution.
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Separation Techniquesmethods of purification

• 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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Around 97 of the water on Earth is seawater.
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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

Chemistry in Daily Life Pure Water for Singapore
page 25
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Separation Techniquesmethods of purification

• Fractional distillation
• to separate mixtures of miscible liquids with
  widely differing boiling points

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Separation Techniquesmethods of purification

• How does the temperature change as a solution of
  ethanol and water undergoes fractional
  distillation?

1. The temperature of the mixture increases as it
is heated.
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Separation Techniquesmethods of purification

• How does the temperature change as a solution of
  ethanol and water undergoes fractional
  distillation?

2. At 78 C, ethanol distils over. The
temperature remains constant until all the
ethanol has distilled out of the round-bottomed
flask.
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Separation Techniquesmethods of purification

• How does the temperature change as a solution of
  ethanol and water undergoes fractional
  distillation?

3. The temperature then increases until 100 C.
At 100 C, water distils over. The temperature
remains unchanged as water is being distilled.
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Separation Techniquesmethods of purification

• Fractional distillation
• the liquid with the lower boiling point will be
  distilled first
• Why is the fractionating column filled with glass
  beads (or plates/spirals)?

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Separation Techniquesmethods of purification

• Use of separating funnel
• to separate immiscible liquids

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Separation Techniquesmethods of purification

• Use of separating funnel
• pour the mixture into the separating funnel,
  ensuring the tap is closed

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Separation Techniquesmethods of purification

• Use of separating funnel
• support the separating funnel with a retort
  stand, placing a clean beaker below it

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Separation Techniquesmethods of purification

• Use of separating funnel
• 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 Techniquesmethods of purification

• Use of separating funnel
• 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

• red dye from a mixture of red and blue dyes in
  solution?
• oxygen from air?
• barium sulfate from a mixture of (insoluble)
  barium sulfate and water?

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QUIZ! How would you separate

• water from oil and water?
• petrol from crude oil?
• iodine crystals from iodine and copper(II) oxide?
• salt from sea water?