| technique |
basis for separation |
apply this technique to: |
| adsorption / desorption |
phase transfer to a solid surface |
liquid or gaseous mixtures that contain at least
one component that adsorbs |
| chromatography |
phase transfer from a mobile mixture to a stationary
phase |
liquid or gaseous solutions that contain several
components with differing affinities for the stationary phase |
| condensation |
phase separation by condensing gases in the mixture
to liquids |
gaseous mixtures containing at least one gas
with a much higher boiling point than the others |
| dialysis |
phase transfer through a porous membrane that
allows some molecules to pass through, but not others |
solutions containing small molecules mixed with
very large molecules |
| effusion |
gases with faster molecules flow through tiny
pinholes faster than gases with slow molecules |
gaseous mixtures containing gases with different
molecular weights |
| dissolution (washing,
solvent extraction) |
soluble components can be washed away, leaving
behind insoluble components (phase transfer to a washing solvent) |
mixtures of solids with different solubilities |
| electrorefining |
separate a metal from impurities by dissolving
it and then plating it onto an electrode |
solid mixtures with a metal as one component
|
| filtration |
collect solid particles on a filter |
heterogeneous mixture containing a solid phase |
| floatation |
dense components sink, and lighter ones float |
heterogeneous mixture with phases with different
densities |
| ion exchange |
ions in the mixture bind to surfaces with oppositely
charged sites (phase transfer to an ion exchange resin) |
solutions containing ions |
| precipitation |
convert solutes to an easily separated solid
form |
solutions containing a solute that can be precipitated |
| scrubbing |
bubble mixture through a solution that selectively
absorbs a component (phase transfer from gas to solution) |
gaseous mixtures containing a solute that can
be selectively absorbed by a scrubbing solution |
| stripping |
a gas bubbled through the mixture carries off
the most volatile components
(phase transfer from solution to gas)
|
a liquid mixture containing at least one volatile
component |
| volatilization
(drying, distillation,
sublimation)
|
components with widely differing volatility can
be driven out of the mixture by heating (phase change from solid or liquid
to gas) |
a mixture containing components with differing
volatility |
Adsorption and desorption
- some solids bind gases and organic materials to their surfaces, removing
them from mixtures
- adsorbed gases or liquids can recovered from the adsorbent material
by washing with a solvent
- examples
- activated charcoal adsorbs many gases and liquids
- used as a "universal antidote" for poisoning
- used in water purifiers (removes particulates, lead, copper, mercury,
chlorine, hypochlorite, organics)
- used to adsorb drugs from the blood of overdose victims
- silica gel absorbs moisture from air
Condensation
- cooling a vapor causes components with the highest boiling points to
condense as liquids first
- examples
- separating steam and air
- separating oxygen and nitrogen in air
Dialysis
- a semipermeable membrane allows some components in a mixture through,
but not others
- how does the membrane distinguish components?
- some membranes act as a "molecular sieve" that discriminates
between large and small molecules
- some membranes dissolve one component better than others
- development of new membranes is an active area of research in industry and government
- components flow spontaneously from the high concentration to low concentration
side
- pressure applied to the low concentration side can stop or even reverse
this flow (reverse osmosis)
- examples
- purification of blood in dialysis machines
- purification of seawater by reverse osmosis
- separation of pollutants from drinking water
Effusion
- use porous membranes to separate light gases from heavy ones
- average speed of gas molecules depends on the masses of their molecules
- heavy molecules in a mixture move slower on average than light ones
- gases made of light molecules diffuse through pores in membranes faster
than heavy molecules
- differences from dialysis
- membrane is permeable, not semipermeable: all gas molecules in the
mixture can pass through it
- size of molecules isn't usually important: pores in membrane are much
larger than gas molecules
- ...molecular velocity (and so, molecular mass) is the basis for separation,
not size
- examples
- separating helium from oxygen
- separating uranium isotopes as volatile UF6
Dissolution (washing)
- separate solids by washing away those that are soluble
- examples
- separating sand and salt by water washing
- separating feldspars from quartz in rocks by washing with hot concentrated
phosphoric acid
- separating organic stains from clothing by washing with organic solvents
(dry cleaning)
Electrorefining
- used to separate metals from impurities
- strategy
- dissolve the impure metal
- plate it on an electrode, using a strong electric current
- pure metal deposits on the electrode, and the impurities stay in solution
Filtration
- pass a mixture that contains solid particles through a porous filter
- if pores are smaller than particles, solid particles stay on filter
and liquid/gaseous components pass through
- often used after separation by precipitation
Ion exchange
- used to separate ions from mixtures
- pass the mixture over a surface that is covered with charged sites
- some ions stick to the charged sites
- examples
Precipitation
- precipitation is the conversion of a solute to solid form by chemical
or physical change
- solids are then separated by filtration or floatation
- examples
- separating mud and bacteria from water
- a gooey aluminum hydroxide precipitate is formed in the water to carry
particulates and bacteria to the bottom of a vat
- clean water is drawn off the top
- separating sulfate ions from water by adding barium ions
- barium ion + sulfate = insoluble barium sulfate
- water softening with washing soda
- carbonate + calcium ion = insoluble calcium carbonate
Scrubbing
- scrubbing is bubbling a gas stream through a solution that traps some
components
- examples
- CO2 can be separated from air by bubbling it through a solution
of barium hydroxide
- H2S can be removed from air by bubbling it through a zinc
acetate solution
Solvent extraction
- a component moves into a solvent shaken with the mixture
- works best with solvents that dissolve only one component
Solvent extraction can be used to extract vanillin from vanilla
beans. Shaking the beans with an organic solvent like chloroform transfers
organic compounds (including the vanillin) to the chloroform. Shaking the
chloroform with a sodium hydroxide solution transfers the vanillin into
the sodium hydroxide solution.
Pure Substances
Elements and compounds are both examples of pure substances.
Pure substances cannot be separated into simpler substances by physical or mechanical means such as sifting, filtering, crystallization, distillation, etc.
eg, distilling pure water (H2O) does not separate water into hydrogen and oxygen, it only produces water vapour.
Pure substances display a sharp melting and boiling point.
On a graph of temperature vs time, this is shown as flat line where the temperature does not change over time until all the pure substance has melted or boiled.
Pure substances will have a constant appearance, colour and density throughout the sample.
Pure substances have constant chemical composition, eg,
pure water (H2O) is always composed of 2 hydrogen atoms and 1 oxygen atom chemically bonded.
pure gold (Au) is only made up of gold atoms
Types of mixtures
A mixture refers to the physical combination of two or more substances on which the identities are retained and are mixed in the form of alloys, Simple mixtures, solutions, suspensions, and colloids.
Simple mixtures can involve various combinations of solids, liquids and gases.
Solid in solid
Solid in liquid
Solid in gas
Liquid in liquid
Liquid in gas
Gas in liquid
Gas in gas
| Type of Mixture |
Example |
|---|
| gas in gas |
The atmosphere is a mixture of gases, mostly nitrogen and oxygen. |
|
| liquid in liquid |
Wine is a mixture of mostly ethanol and water. |
|
| solid in solid |
Alloys, such as brass, are made up of a mixture of metals. |
|
| gas in liquid |
Soft drinks, such as cola, are mixtures of mainly carbon dioxide gas and water. |
|
| solid in liquid |
Sea Water is a mixture of salts dissolved in water. |
|
| solid in gas |
Smoke is mixture of tiny solid particles in atmospheric gases. |
Solutions are homogeneous mixtures.
Solution- a homogeneous mixture in which one substance (the solute) is dissolved in another substance (the solvent). Example: salt water (Water, the solvent, plus salt, the solute, produces the solution of salty water.)
Suspension- a heterogeneous mixture in which the particles are large enough to be seen by a microscope or the unaided eye (eventually, they settle out of the mixture). Example: stirring a teaspoon of dirt in a glass of water.
Colloid- a mixture where the size of particles in the mixture are between those of a solution and a suspension. NOTE: The particles appear evenly distributed. Examples: fog, cheese, butter, jellies, whipped cream.
Separating mixtures
How can a mixture be separated?
The components of a mixture usually can be separated by physical means such as distillation, evaporation, precipitation, filtration, solvent extraction, or chromatography.
How Does Fractional Distillation Work?
Distillation is the process of separating two or more liquids based on differences in their boiling points. When the liquids' boiling points are very similar, however, separation by normal distillation becomes ineffective or impossible. Fractional distillation is a modified distillation process that allows the separation of liquids with similar boiling points.
Boiling Points
A liquid's boiling point is the temperature at which it transforms to vapor. Liquids retain their characteristic boiling point even when mixed with other liquids. This represents the underlying principle of distillation---that liquids can be separated by converting the liquid with the lowest boiling point to vapor, then converting that vapor back to the liquid state after it has transferred to a separate container.
Distillation
In the process of distillation, the liquid mixture is placed in a boiling flask, which is connected to a cooling column called a condenser, the opposite end of which is connected to a receiving flask. The condenser sits horizontal with a slight downward slope so that the vapor that reaches the condenser and is converted back to liquid can be collected in the receiving flask.
Fractional Distillation
A fractional distillation setup includes an additional column that sits vertically on top of the boiling flask and to which the condenser is connected. Its purpose is to increase the distance that the vapor must travel to reach the condenser. The columns are typically packed with glass beads or pieces of ceramic to increases the surface area the vapor must come into contact with as it transports to the condenser.
During normal distillation, a substantial amount of the higher-boiling liquid also will vaporize and transport to the collection flask, essentially becoming an impurity in the distilled product. This is especially problematic when the liquids being separated have similar boiling points. The more surface area the higher-boiling liquid contacts along the way, the more likely it is to condense back to a liquid and return to the boiling flask. Fractional distillation uses this increased surface area to improve the efficiency of the distillation.
Uses
The two primary applications of fractional distillation are the refining of crude oil and the manufacturing of spirits (alcoholic beverages).
Crude oil contains numerous different chemicals, many of which have similar boiling points. Refineries separate these chemicals by boiling point into various products. The lower-boiling fractions become petroleum gas or gasoline, the intermediate-boiling fractions become fuel oil, diesel fuel, or kerosene, and the highest-boiling fractions become paraffin wax or asphalt.
The fermentation of sugars into alcohol stops when the alcohol content approaches 13 percent because the yeast cannot survive at higher alcohol concentrations. The boiling points of alcohol (78.5 degrees Celsius) and water (100 degrees Celsius) are similar enough that distilleries must use fractional distillation to concentrate the alcohol to about 50 percent (which is then called "spirits").
The distillation process at an oil refinery consumes 2 barrels of oil in energy for every 100 barrels of refined oil.
Exercise
It keeps the properties of the materials that make it up.
compound mixture
mixture
It is difficult to separate into the materials that make it up.
compound mixture
Correct:compound
There was a chemical reaction when it was formed.
compound mixture
Correct:compound
Salt (sodium and chlorine)
compound mixture
Correct:compound
The air (oxygen, nitrogen and carbon dioxide)
compound mixture
Correct:mixture
Water (hydrogen and oxygen)
compound mixture
Correct:compound
Rock salt (sand and salt)
compound mixture
Correct:mixture
How do we extract salt from seawater?
How does distillation work?
How can mixtures be separated?
Why are the physical properties of matter important in separating mixtures?
What occurs during filtration of a mixture?
What occurs during evaporation of a mixture?
How is caffeine separated from coffee?
How can artist's pigments be separated by paper chromatography?
How can NaCl be separated from other dissolved salts?
How are mixtures analyzed for drugs of abuse, like THC?
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