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๐ฌ Think about it
Mishri (candy sugar) grows as beautiful crystals. Snowflakes and frost are crystals too. If you cool a hot, full solution slowly, pure crystals appear. How does this help us purify a solid? Let us see.
What is crystallization?
- It is forming crystals from a saturated solution.
- A crystal is a solid with particles in a regular geometric pattern.
- Cooling a hot saturated solution makes the extra solute separate as crystals.
- It is based on solubility being different at different temperatures.
Example:
Cooling a saturated solution of compound B from 60 °C to 40 °C drops out solid because less can stay dissolved.
Why is crystallization useful?
- It separates two solids when one is in small quantity and both dissolve in the same solvent.
- It is used for the purification of solids.
- It separates a desired pure substance from unwanted impurities.
In this Activity, we will grow blue copper sulfate crystals by cooling a hot saturated solution slowly.
โ Safety first:
Copper sulfate is toxic. Do the experiment under adult supervision and do not touch it with bare hands. The teacher should add the sulfuric acid; handle it very carefully.
Procedure
1. Collect copper sulfate (blue vitriol); if unavailable, use common salt.
2. Put 1 g of copper sulfate in a 100 mL beaker, add 25 mL water and a drop of dilute sulfuric acid. Heat gently in a water bath while stirring. (Sulfuric acid helps make pure crystals by preventing unwanted reactions.)
3. Add more copper sulfate until the solution is saturated.
4. Filter the hot solution to remove insoluble impurities; collect it and cover with a watch glass.
5. Let it cool slowly, undisturbed. Larger, shiny, well-shaped blue crystals form.
6. Filter the crystals, rinse with cold water and dry them on a watch glass.
2. Put 1 g of copper sulfate in a 100 mL beaker, add 25 mL water and a drop of dilute sulfuric acid. Heat gently in a water bath while stirring. (Sulfuric acid helps make pure crystals by preventing unwanted reactions.)
3. Add more copper sulfate until the solution is saturated.
4. Filter the hot solution to remove insoluble impurities; collect it and cover with a watch glass.
5. Let it cool slowly, undisturbed. Larger, shiny, well-shaped blue crystals form.
6. Filter the crystals, rinse with cold water and dry them on a watch glass.
Crystallization Step by Step
Make a hot saturated solution
↓
Filter hot
Remove insoluble impurities.
↓
Cool slowly
Particles come together as crystals.
↓
Filter and dry pure crystals
โ Summary
- Make a hot saturated solution.
- Filter while hot.
- Cool slowly, undisturbed.
- Get pure blue crystals.
In this Activity, we will describe how salt crystals are obtained from seawater.
Procedure
Observe Fig. 5.9, which shows how salt crystals are obtained from seawater, and describe the process in your own words.
Salt from Seawater
Seawater
→
Evaporates
Sun and wind remove water, leaving a saturated solution.
→
Salt crystals
โ Summary
- Collect seawater in pans.
- Sun evaporates the water.
- Solution becomes saturated.
- Salt crystals are left.
Important Points
- Crystallization forms pure crystals from a saturated solution.
- It separates and purifies solids using solubility differences.
- Slow cooling gives larger, better-formed crystals.
๐งช Think as a Scientist
-
If a hot saturated copper sulfate solution is cooled rapidly in ice-cold water, smaller and less well-formed crystals form than if cooled slowly. How would you design an experiment to test this?
Prepare a hot saturated copper sulfate solution and divide it into two equal parts. Cool one part quickly in ice water and the other slowly at room temperature, then compare the size and shape of the crystals formed.
๐งฌ Pause and Ponder
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If equal masses of hot saturated solutions of compounds A and B are cooled from 80 °C to 60 °C, which is likely to deposit more solid?
Compound B, because its solubility falls much more steeply with cooling, so more of it drops out as solid. -
Will there be any change in the size of common salt crystals if the rate of evaporation is increased or decreased? Explain.
Yes — slow evaporation gives larger, well-formed crystals because particles have time to arrange neatly; fast evaporation gives smaller crystals.
๐ India's Scientific Contributions
- Crystallization of salt was an ancient process in coastal India.
- Panga salt was made by boiling concentrated sea brine.
- Karkatch salt was made by evaporating seawater.
- These methods produced salt crystals of different sizes.
๐ Ready to Go Beyond
- Large crystal deposits form naturally in mines, caves and the Earth's crust.
- The Mawsmai Cave in Sohra (Cherrapunji) is known for such formations.
- Quartz is one of the beautiful crystals found in nature.
โ Test Yourself
-
What is a crystal?
View Answer
A solid whose particles are arranged in a regular geometric pattern. -
What does crystallization separate?
View Answer
A pure solid (crystals) from a saturated solution, leaving impurities behind. -
Why cool the solution slowly?
View Answer
Slow cooling gives larger, well-formed crystals.
Important Definitions
- Crystallization — forming pure crystals from a saturated solution.
- Crystal — a solid whose particles are arranged in a regular geometric pattern.
