Separation Methods and Crystallisation

Principles of Separation Methods and Mixture Properties

• Definition of Mixtures:

  • Mixtures are composed of two or more pure substances (elements or compounds) that are physically combined rather than chemically bound.
  • Each pure substance within a mixture retains its own unique characteristics and properties designed for specific functions.

• Necessity of Separating Components:

  • Components of a mixture must be separated so that each individual pure substance can be studied and utilized according to its specific properties.

Crystallization Method

• Suitable Mixture Types for Crystallization:

  • This method is used to separate and purify mixtures containing two solids where one compound is present as a small quantity of unwanted impurity, and both solids are soluble in the same solvent.
  • It is commonly employed in the purification of Copper Sulphate ($CuSO_4$), where impurities like Sodium Chloride ($NaCl$) are present in lesser amounts.

• Role of Sulphuric Acid in Crystallization:

  • A drop of sulphuric acid ($H_2SO_4$) is added to the Copper Sulphate solution during crystallization for the following reasons:
  • It aids in making pure crystals by preventing unwanted dissolution reactions.
  • It supports any residual copper or copper oxide ($CuO$) present in reacting to form additional Copper Sulphate ($CuSO_4$).

• Applications of Crystallization:

  • Purification of common salt ($NaCl$) obtained from sea water.
  • Obtaining pure samples of Copper Sulphate ($CuSO_4$) from impure samples.
  • Isolation of pure crystals of alum from impure samples.

• Comparison Between Crystallization and Simple Evaporation:

  • Disadvantages of Evaporation:
  • Evaporation does not fully purify the solid; after the solvent evaporates, non-volatile impurities remain mixed with the solute.
  • Evaporation cannot be used for components that undergo decomposition or charring when heated to dryness.
  • Advantages of Crystallization:
  • Crystallization isolates pure solids in crystalline form.
  • It prevents the thermal decomposition and charring of heat-sensitive solids, making it a preferred technique over simple evaporation.

• Step-by-Step Activity: Crystallization of Copper Sulphate:

  • Aim: To obtain pure crystals of Copper Sulphate ($CuSO_4$) from an impure sample.
  • Procedure:
  1. Dissolution: Place approximately $5\,g$ of impure Copper Sulphate in a China dish and dissolve it completely using the minimum amount of water.
  2. Filtration: Filter the solution to remove all insoluble impurities.
  3. Evaporation: Heat the filtered solution in the China dish to evaporate water until a saturated solution is obtained.
  4. Cooling: Cover the China dish with a watch glass, stop heating, and leave it undisturbed to cool slowly at room temperature for $1\,day$
  • Observation: Beautiful, blue-colored pure crystals of Copper Sulphate form at the bottom of the China dish.
  • Conclusion: Crystallization is a process that separates a pure solid in the form of crystals from its solution and is superior to simple evaporation because it prevents solid decomposition.

Distillation Method

• Suitable Mixture Types for Distillation:

  • Used for separating homogeneous mixtures of two or more miscible liquids that have a difference in boiling points of at least $25\,^\circ C$.
  • It can also be used to separate a liquid solvent from a solution containing dissolved non-volatile solids.

• Examples of Separation via Distillation:

  • Separation of a miscible mixture of acetone (boiling point $56\,^\circ C$) and water (boiling point $100\,^\circ C$).
  • Separation and recovery of water from a common salt solution.

• Step-by-Step Activity: Separation of Acetone and Water:

  • Setup:
  • Pour the mixture of acetone and water into a distillation flask.
  • Fit the distillation flask with a thermometer and connect it to a water condenser.
  • Procedure:
  1. Assembly: Ensure the distillation setup is properly secured with the thermometer placed at the flask neck and the outlet connected to the condenser.
  2. Heating: Heat the liquid mixture using a Bunsen burner while continually monitoring the temperature reading on the thermometer.
  3. Vaporization: As the temperature reaches $56\,^\circ C$, acetone reaches its boiling point, vaporizes, and turns into gas.
  4. Condensation: Acetone vapor passes through the water condenser where it cools down and converts back into liquid form.
  5. Collection: Liquid acetone drips out of the condenser outlet and is collected in a separate beaker.
  6. Result: Water, having a higher boiling point ($100\,^\circ C$), remains behind in the distillation flask.

Paper Chromatography

• Suitable Mixture Types and Principle:

  • Paper chromatography is primarily used to separate mixtures of colored components that are soluble in the same solvent but possess different solubilities.
  • Mechanism: Based on the difference in solubility within the solvent, the component that has higher solubility moves more rapidly along with the solvent as it travels across the paper.

• Applications of Paper Chromatography:

  • Separating individual colors from a dye.
  • Separating natural pigments from natural colors.
  • Separating and identifying drugs present in blood.