Mixtures and Separation Techniques

A mixture is made from different substances that are not chemically joined.
For example powdered iron and powdered sulphur mixed together makes a mixture of iron and sulphur. They can be separated from each other without a chemical reaction, in the way that different coloured sweets can be picked

out from a mixed packet and put into separate piles.

Mixture and compounds

Mixtures have different properties from compounds. The table summarises these differences.

An example – iron, sulphur and iron sulphide

Remember that iron and sulphur react together when they are heated to make a compound called iron sulphide. What are the differences between a mixture of iron and sulphur, and iron sulphide? Here are some of them:

• The mixture can contain more or less iron, but iron sulphide always contains equal amounts of iron and sulphur.
• The iron and sulphur atoms are not joined together in the mixture, but they are joined together in iron sulphide.
• The iron and sulphur still behave like iron and sulphur in the mixture, but iron sulphide has different properties from both iron and sulphur.
• You can separate the iron from the mixture using a magnet but this does not work for iron sulphide.

SEPARATION OF MIXTURES USING DIFFERENT TECHNIQUES

Our Objective

To separate the components of a mixture using the following techniques:

• Separating funnel
• Chromatography
• Centrifugation
• Simple distillation
• Fractional distillation

The Theory

How is a homogeneous mixture different from a heterogeneous mixture?

Most materials in our surroundings are mixtures of two or more components. Mixtures are either homogeneous or heterogeneous. Homogeneous mixtures are uniform in composition, but heterogeneous mixtures are not uniform in composition.

Air is a homogeneous mixture and oil in water is a heterogeneous mixture. Homogeneous and heterogeneous mixtures can be separated into their components by several physical methods. The choice of separation techniques is based on the type of mixture and difference in the chemical properties of the constituents of a mixture.

What are types of separation techniques?

Various types of separation processes are: 

• Crystallization
• Filtration
• Decantation
• Sublimation
• Evaporation
• Simple distillation
• Fractional distillation
• Chromatography
• Centrifugation
• Separating funnel
• Magnetic separation
• Precipitation

Let’s discuss some of the separation techniques

Separating mixtures

• components in a mixture retain their identities
• exploit properties that distinguish the components to separate mixtures

Some manufacturers add iron filings to cereal to increase its iron content! The bits of iron will stick to a magnet, but the cereal won’t. So you can easily separate the mixture by stirring a bar magnet through a slurry of water and finely crushed cereal.

the more similar the properties are, the more difficult it is to separate them

Many elements occur in forms with slightly different masses. For example, uranium occurs as uranium-235, which can be used to construct atomic bombs, and uranium-238, which can’t. The two are very difficult to separate because they are nearly identical otherwise. The technical difficulties in separating this mixture is one of the factors that has limited the proliferation of nuclear weapons.

• basic strategies
  • phase conversion: convert components of the mixture into other forms that are easy to isolate
  • phase transfer: add a new phase that collects some components from the mixture, but not others

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
  • water deionization

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.

Stripping

• a stream of gas bubbled through the mixture will carry off the most volatile components

Blowing air through a straw in a glass of soda will cause it to go flat, because the air carries off the volatile carbon dioxide.

• volatile components can be reclaimed from the gas by scrubbing

Volatilization

• heating a mixture can cause low-boiling components to volatilize (vaporize)
• several variations
• distillation is collecting and condensing volatilized components

Alcohol can be separated from fermented corn mash by heating the mash to vaporize the alcohol. The vapor is collected and passed through coils of copper tubing, where it cools and condenses as a liquid once again. Moonshiners sometimes used old car radiators for the condensation step; the soldered joints added a toxic quantity of lead to the shine!

• drying is complete volatilization of some components in the mixture

Separation of water from clothes on a clothesline is one obvious example. The separation of salt from seawater using evaporating pools is another.

• sublimation is volatilization of a solid (without melting!)

Dry ice (solid carbon dioxide) is probably the most familiar example of a solid that sublimes. But water ice can also be converted directly into water vapor without melting, at low pressure. Snow on mountain peaks disappears without moistening the soil.

Separation by sublimation is sometimes called ‘freeze drying’. Instant coffee is manufactured by freeze drying. (Boiling the coffee destroys the delicate molecules that give coffee its flavor, and so does exposure to air after a certain time, so distillation or simple drying isn’t used). Fresh coffee is frozen to form a mixture of ice and coffee crystals. The pressure over the mixture is lowered so that the ice sublimates, leaving the coffee crystals behind.

Bottom of Form

EVALUATION

1. Define and give one example each of i. A homogenous mixture and ii. A heterogenous mixture.
2. Define the following terms.i. filtrate ii. distillate  iii sublimate
3. How would you separate a mixture of salt and sand?Explain.
4. How would you separate the component of black ink.
5. List and explain all the separation techniques .Describe their industrial application.