Separating Mixtures | Chemical Earth

Need for Separating Mixtures

Need for Separation

  • Often substances we require from the natural world need to be separated from impurities, which is separating mixtures.
  • This can be done through a number of techniques.
  • The techniques presented are based on physical properties and can’t be used to separate compounds into elements.

Separating Solids of Different Size

Separating Solids of Different Size

  • Two common methods sieving and centrifuging for separating mixtures.
  • Sieving uses a fine mesh to allow objects smaller than the pore size to slip through while blocking larger objects.
  • This process is used to ensure a fine distribution of flour for baking.

Centrifuging

  • Centrifuging is a common separation technique that exploits the density differences of different substances.
  • When solids are suspended in a liquid gradually the densest substance will settle on the bottom.
  • This is known as sedimentation.
  • Centrifuging accelerates this process by spinning the suspension at high speed.
  • This process is used in the separation of whole blood into its components as well as to remove fine particles from gas streams.

Separating Solids from Liquids

Separating Solids from Liquids

  • Filtration is a common method of removing solids from fluid streams for separating mixtures.
  • Relies on the insolubility of the solid.
  • Filtration uses a fine mesh to allow objects fluid particles to slip through while blocking larger objects.
  • Commonly used in water purification plants to remove large particles.
  • This is done with a sand or carbon filter.
  • Froth floatation is another method to remove solids from liquids.
  • Used in mining to remove minerals from waste rock.
  • Crushed rock mixture is placed in a tank of water, detergent and some oily chemicals.
  • Air is pumped through this liquid forming a froth.
  • Minerals adhere to froth while waste rock does not.
  • Froth is then scraped off the top and minerals extracted.

Separating Dissolved Solids from Liquids

  • Evaporation is the most commonly used method to remove water from dissolved solids.
  • Energy is used to heat the water and evaporate it.
  • The dissolved solid crystallises when there is no more water.
  • Process is contingent on the solid having a much higher boiling point than the water.
  • Used in the production of table salt.
  • Sea water is evaporated leaving the salt behind.
  • Distillation is the process used if the liquids are soluble in one another (e.g. ethanol and water).
  • Relies on the two liquids having different boiling points.
  • Mixture is heated to the boiling point of one of the liquids (the liquid with the lowest boiling point).
  • This liquid evaporates leaving the other liquid in the flask.
  • The vapours are then condensed elsewhere forming two separate liquids.
  • This process is used in the refinement of petroleum and ethanol.
  • If the liquids are not soluble then a separating funnel can be used.
  • Relies on the insolubility and density difference of the liquids.
  • One liquid will sit on top of the other (due to density differences).
  • The valve is opened to let the liquid out of the bottom and closed once it has been removed.

Separating Mixtures of Gases

Separating Mixtures of Gases

  • Similar methods to separating liquids that are soluble in one another.
  • Distillation is used.
  • Relies on boiling point differences.
  • Gases are cooled till they condense.
  • Gas with the highest boiling point is removed first.
  • Essentially the reverse of the liquid distillation.
  • This process is used to separate the components of air.
  • Gas centrifuges are also used to separate mixtures.
  • When the gaseous mixture is spun at high speed in a gas centrifuge, the heavier molecules collect at the side.
  • This leaves a mixture near the centre with a higher proportion of the lighter molecules.
  • This process relies on the gases having different molecular weight.
  • Used in the production of radioactive material.

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