Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Crystallization is a physical process used to separate a pure solid in the form of crystals from a solution.
A Saturated Solution is a solution in which no more solute can be dissolved at a specific temperature. This is the starting point for crystallization.
The process involves heating a solution to the Crystallization Point (the stage where crystals begin to form on a glass rod held over the steam) and then cooling it slowly.
Slow cooling is essential because it allows the atoms/molecules to arrange themselves in a regular, repeating three-dimensional lattice, forming large and pure crystals.
Crystallization is superior to evaporation to dryness because it prevents the decomposition of heat-sensitive substances (like sugar) and removes soluble impurities which remain in the 'Mother Liquor'.
Many crystals contain a fixed number of water molecules chemically combined in their structure, known as Water of Crystallization. For example, Copper (II) Sulphate is represented as .
📐Formulae
💡Examples
Problem 1:
Explain why crystals of lose their blue color and shape when heated strongly.
Solution:
When is heated, it loses its Water of Crystallization.
Explanation:
The water molecules provide the geometric shape and the characteristic blue color to the crystal. Upon heating, the reaction is: . The resulting anhydrous is a white, amorphous powder.
Problem 2:
Describe the test to identify the 'Crystallization Point' during the purification of an impure sample of Alum ().
Solution:
The glass rod test is used.
Explanation:
A glass rod is dipped into the boiling solution and then taken out and blown upon. If tiny crusts or crystals of the solute appear on the rod, the solution has reached the crystallization point, meaning it is now a saturated solution at that high temperature.
Problem 3:
How can we obtain pure salt from a mixture of salt and sand using crystallization?
Solution:
Dissolve the mixture in water, filter the sand, then crystallize the salt from the filtrate.
Explanation:
- Add water to the mixture; dissolves while sand remains insoluble. 2. Filter the mixture to remove sand. 3. Evaporate the filtrate (salt solution) to the crystallization point. 4. Cool slowly to get pure crystals.