What happen when two Substances we obtain by crystallisation
Answers
Answer:
Evaporation of water results in the formation of salts. The salt obtained during crystallisation is pure and its crystals are large. The shape of crystals can be seen clearly after crystallization. Large crystals of pure substances cannot be formed from their solution.
Answer:
To crystallize an impure, solid compound, add just enough hot solvent to it to completely dissolve it. The flask then contains a hot solution, in which solute molecules - both the desired compound and impurities - move freely among the hot solvent molecules. As the solution cools, the solvent can no longer hold all of the solute molecules, and they begin to leave the solution and form solid crystals. During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute. After the solution has come to room temperature, it is carefully set in an ice bath to complete the crystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent.
This first series of diagrams shows what happens if you let a crystallization proceed slowly: first by setting the flask at room temperature undisturbed until crystals form, and then carefully on ice. The red bar to the right of each image is a thermometer, to indicate the temperature. The yellow triangles are an impurity in the hot solution of orange hexagons. If the solution is allowed to cool slowly, the impurities may attach briefly to the growing crystal lattice, but they soon leave as a compound with a more suitable geometry comes in to take their place. Suitable hexagons stay more readily in the growing lattice, and eventually pure crystals of orange hexagons are formed.
This second series of diagrams shows what happens if you cool the solution too quickly. The yellow triangle impurities are trapped inside the crystals being formed by the orange hexagons, thus, the crystals isolated are impure. Note that slow crystallization gives larger crystals than fast crystallization. Small crystals have a large surface area to volume ratio and impurities are located on the surface of the crystals as well as trapped inside the matrix.
