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Physical and Chemical Changes

Crystallisation

Crystallisation is the process of formation of crystals. Crystal, the word makes us think of a solid shiny object which may be rare or precious. We performed two experiments to demonstrate crystallisation. Before going into experiments let us understand what are crystals, and the process of crystallisation.

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Crystals

A crystal is a chunk of a material in the Solid Form. It is the typically the purest form of the material. At the microscopic level, crystals have a regular pattern. This means that the atoms and molecules in the crystals follow a particular repeating structure.

Examples in Nature

Crystallisation

Objects ranging from the rarest precious diamonds all the way to grains of salt are crystals. The beautiful phenomena of snowflakes occur as a result of Crystallisation. You would also find this process occurring in the remoteness of a cave. It leads to the formation of Stalactites and Stalagmites.

Crystallisation Process

Crystallisation process results in a physical change in objects. It leads to the formation of crystal structures. Clear distinguish between processes of crystallisation is not possible. But we can identify two categories of crystallisation processes namely Cooling Crystallisation and Evaporative Crystallisation.

Browse more Topics under Physical Chemical Changes

Cooling Crystallisation

Many substances, when dissolved in solvent, form crystals on cooling in favorable conditions in this process. However, this method is not very common and has many limitations. Usually, the substance is heated and then allowed to cool down in this method.

Evaporative Crystallisation

It is more common than cooling crystallisation. Evaporating sea water leaves salt behind. Pure Crystals of Substance are deposited after evaporation of solutions. But this method requires a high concentration of the substance in the solution. Therefore, its solute/solvent mass ratio should be high.

Simple Experiments to Demonstrate Crystallisation

Experiment 1

  • First of all, take some water in a beaker. Add a few drops of dilute Sulphuric Acid (H2SO4).
  • Heat the mixture on a burner. Add Copper Sulphate Powder when it starts boiling.
  • Keep stirring the solution. Keep adding Copper Sulphate Powder to the solution until it is soluble.
  • Now filter the solution and allow it to cool down.
  • You will see crystals of copper sulphate suspended in the solution.

Learn about Rusting of iron here.

Experiment 2

  • Put 160 grams of sodium acetate in a flask and add 30 mL of water.
  • Heat the flask on a burner or a hot plate until it starts boiling.
  • Stir the solution unless the sodium acetate crystals dissolve.
  • Now remove the flask from the heat and let it cool down for a while.
  • Finally, place one or two sodium acetate crystals on a surface and pour the solution on top of it.
  • Try to keep the stream of solution steady.
  • The solution will solidify and form a vertical structure.

Solved Examples for You

Question: Which of the following represents the correct statement(s)?

  1. Evaporation of water results in the formation of salts.
  2. The salt obtained during crystallisation is pure and its crystals are large.
  3. The shape of crystals can be seen clearly after crystallization.
  4. Large crystals of pure substances cannot be formed from their solution.

Solution: The option A represents the correct statement. Salt can be obtained by the evaporation of sea-water. The salt obtained during crystallisation is not pure and its crystals are small. After crystallisation, the shape of the crystals cannot be seen clearly. Large crystals of pure substances can be formed from their solutions.

Question: Process of removing insoluble impurities from solution during crystallization:

  1. drying
  2. filtration
  3. heating
  4. cooling

Solution: Heating is the process of increasing the temperature of a body from its original temperature. Cooling is the process of decreasing the temperature of a body from its original temperature. Filtration is the process of separating small particles of solid matter from a liquid, by causing the liquid to pass through the pores of some substance, called a filter. Further, Filtration removes insoluble impurities from solution during crystallization. Drying is the process of drying up the liquid present on any material by heating the material. Option B is correct.

Question: Evaporation of seawater results in :

  1. Crystallisation
  2. Sedimentation
  3. Galvanization
  4. Amalgamation

Solution: Crystallisation

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