Chemistry Notes on Separation Techniques

Separation Techniques

Introduction

  • Mixtures can be separated into their components using physical separation techniques such as decantation, filtration, distillation, crystallization, etc.

Examples of Separable Mixtures

  • Salty water: can be separated into salt and water.
  • Water and oil: can be separated into water and oil.
  • Water and sand: can be separated into pure water and pure sand.
  • Blood: can be separated into its main constituents: plasma, white blood cells, red blood cells.

Separation Techniques Based on Mixture Type

Solid-Solid Heterogeneous Mixtures

  • Can be separated by:
    • Magnetism

Solid-Liquid Mixtures

  • Heterogeneous: can be separated by:
    • Decantation
    • Filtration
    • Centrifugation
  • Homogeneous: can be separated by:
    • Distillation
    • Crystallization
    • Chromatography

Liquid-Liquid Mixtures

  • Can be separated by:
    • Decantation
    • Fractional distillation

Heterogeneous Mixtures

Solid-Liquid Examples

  • Ice cubes in a drink
  • Sand and water mixture
  • Matte drink

Decantation

  • Removes insoluble (undissolved) solids from a liquid.
  • Based on density.
  • Impurities that settle at the bottom are called "sediments" or "precipitate".
  • Sedimentation: The process in which insoluble impurities settle at the bottom of the liquid to form sediment (e.g., obtaining clear water from muddy water).
  • Decantation: The process of gently pouring clear liquid into another beaker without disturbing the sediments.

Sedimentation

  • Sedimentation is a process of settling down of the heavier particles present in a liquid mixture.
  • Example: In a mixture of sand and water, sand settles down at the bottom.

Density and Decantation Speed/Precision

  • Decantation speed and precision depend on the density difference between the solid and liquid phases.
  • Water and sand:
    • Density of water = 1 \, \text{g/ml}
    • Density of sand = 1.6 \, \text{g/ml}
    • Sand settles down fast; decantation is fast and of high precision.
  • Orange juice and pulp:
    • Orange juice and pulps have very similar densities.
    • It takes too much time for pulps to settle down.
    • Decantation is slow and of low precision.

Filtration

  • Method for separating an insoluble solid from a liquid.
  • A filter paper is used to collect the solid particles.

Filtrate and Residue

  • The liquid that passes through the filter is called the filtrate.
  • The solid particles left on the filter paper are called residue.

Filtration Example: Sand and Water

  • Sand particles are retained on the filter paper (residue).
  • Water passes through the filter (filtrate).

Filter Paper Porosity

  • Filter paper acts as a porous surface.
  • Smaller pore size results in slower separation (filtrate takes longer to pass).

Vacuum Filtration

  • Used to accelerate a slow filtration.
  • Apparatus includes a Büchner funnel, filter paper, and vacuum flask connected to a vacuum source.

Liquid-Liquid Mixtures (Immiscible)

  • Immiscible = don't mix
  • Example: Water and oil mixture

Decantation of Immiscible Liquids

  • Utilizes a separating funnel.

Density and Layer Formation

  • This decantation also depends on density.
  • Oil is less dense than water, so it floats on top of the water layer.

Separation Process

  • The stopcock is opened to drain off the bottom layer (denser liquid) into a receiving beaker.
  • Separation is complete once the denser liquid is fully drained.

Solid-Solid Mixtures: Magnetism

  • Example: Iron and sulfur mixture.
  • Iron is attracted to the magnet, while sulfur is not.
  • Separation occurs based on magnetic properties.
  • The same principle applies to a mixture of sand and iron filings.

Homogeneous Mixtures

Solid-Liquid Examples

  • Salty water
  • Coffee
  • Sugar solution (water + sugar)

Distillation

  • Process to separate a homogeneous mixture by converting a liquid into vapor, which is then condensed back to liquid form.
  • Based on the boiling point.

Distillation Process

  • Vapor, obtained by heating a liquid, passes through a cool condenser, turning back into liquid.
  • The received condensed liquid is called “distillate”.

Condensation vs. Evaporation

  • Evaporation (heating): Liquid turns into gas.
  • Condensation (cooling): Gas turns back into liquid.
  • Example: The sun heats up water, which evaporates. Water vapor gets cold and condenses to form clouds.

Distillation Example: Salty Water

  • Water boils and evaporates upon heating.
  • Obtained vapor condenses in the condenser.
  • Condensate (liquid water) is collected in the receiving flask.
  • Salt remains in the distillation flask.
  • Pure water and pure salt are obtained; separation is complete.

Crystallization

  • Used to separate the dissolved solid from the liquid by evaporating the liquid.
  • Crystals are obtained.

Crystallization Process

  1. The impure solid is dissolved in a solvent.
  2. The solution is heated to evaporate most of the solvent.
  3. The hot solution is allowed to cool; the solid appears as pure crystals.
  4. The cold solution is poured off to obtain the crystals, which may be dried by pressing them between sheets of filter paper.

Crystallization Example: Salt from Sea Water

  • Salt extracted from sea water is obtained by crystallization.
  • When sea water evaporates, salt crystals form.

Sugar Crystals at Home

  • Prepare a saturated sugar solution by heating it.
  • Use some food coloring.
  • Keep it for a few days to evaporate, and sugar crystals will form.

Liquid-Liquid Mixtures (Fractional Distillation)

Examples

  • Water and alcohol mixture
  • Crude oil
  • Products made from a barrel of crude oil (%):
    • Gasoline: 47
    • Diesel Fuel & Heating Oil: 23
    • Other Products: 18
    • Jet Fuel: 10
    • Liquefied Petroleum: 4
    • Asphalt: 3

Fractional Distillation

  • Similar to simple distillation, but uses a fractionating column.
  • Based on the difference between boiling points of the liquids in the mixture.

Fractional Distillation Example: Ethanol and Water

  • Ethanol (alcohol) and water are separated using fractional distillation.
  • Ethanol, having a lower boiling point than water, condenses first and is collected in a receiving flask.
  • When the temperature reaches 100 \, ^{\circ}\text{C}, water boils, then condenses, and is collected separately.
  • Separation is then complete.