The extra solute cannot stay dissolved and comes out as a pure solid — as crystals
The whole method rests on one fact —
solubility changes with temperature
.
Dissolve extra solute at a high temperature, then cool it, and the extra solute must separate out.
Let us see this on the solubility curve of Activity 5.2
Using Fig. 5.6
Solubility of compound B at
60 °C
=
287 g
per 100 g of water.
Solubility of compound B at
40 °C
=
241 g
per 100 g of water.
Take a saturated solution made by dissolving 287 g of B in 100 g of water at 60 °C, and cool it gradually to 40 °C.
Only 241 g can now remain in solution.
Solid separated out
=
287 g − 241 g =
46 g
That 46 g does not vanish — it comes out as
crystals
.
What is a crystal, and what is crystallization?
A
crystal
is a solid made up of particles arranged in a
regular geometric pattern
, with a fixed design.
Crystals shine because their faces are flat and smooth.
In laboratories, the process of forming crystals from a saturated solution is called
crystallization
.
Definition
Crystallization
is the process of forming crystals from a saturated solution. A
crystal
is a solid whose particles are arranged in a regular geometric pattern.
Rock salt
crystals, which you have surely seen.
Sugar crystals
that grow while making candy sugar, called
mishri
in Hindi.
Snowflakes
are crystals too, formed when water vapour freezes in the air.
Frost
on windows is also ice crystals. This is how crystals form naturally.
What is crystallization used for?
Two uses of crystallization
Crystallization
Crystals from a saturated solution
Use 1 — Getting a pure substance out of a homogeneous mixture
Example:
Pure salt from sea water
Use 2 — Separating two solids, when one is in small quantity and both dissolve in the same solvent
Example:
Purifying impure copper sulfate
Use 1 — Pure substance from a homogeneous mixture
Sea water contains salt along with other dissolved substances — it is a homogeneous mixture.
On crystallization, pure salt crystals separate out and the impurities stay behind in the leftover solution, called the
mother liquor
.
Use 2 — Separating two solids
Impure copper sulfate has a small amount of impurity mixed in, and both dissolve in water.
Heat the solution and cool it, and only pure blue copper sulfate crystals form.
The impurity was present in such a small amount that it never reaches the concentration needed to crystallize, so it stays dissolved in the solution.
Both uses share one principle — the wanted substance comes out pure as it crystallizes, and the small-quantity impurities are left behind in the mother liquor.
🌊
Activity 5.4 — Let us describe a process
In this Activity, we will look at how salt crystals are obtained from seawater, and describe the process in our own words.
From seawater to salt crystals
Seawater
filled into wide, shallow pans on the shore
↓
Saturated solution
sunlight and air evaporate the water until it can hold no more salt
↓
Salt crystals form and deposit at the bottom, ready to be collected and cleaned
Observation
The figure shows three stages — Seawater, Saturated solution, and Salt crystals.
In our own words
Sea water is filled into wide, shallow pans built along the shore. "Shallow" means not deep. Sunlight and air keep evaporating the water. As the water reduces, the solution becomes thicker and at one point becomes saturated. After that, any further evaporation means the salt can no longer stay dissolved, so salt crystals form and deposit at the bottom. These are then collected and cleaned.
◆ Summary
Shallow pans hold seawater at the shore
Sun and air evaporate the water till the solution saturates
Salt then crystallizes out and is collected
🔵
Activity 5.3 — Let us prepare
In this Activity, we will grow our own copper sulfate crystals and see why slow cooling gives better crystals.
Materials needed
Copper sulfate (blue vitriol), a 100 mL beaker, water, a drop of dilute sulfuric acid, a glass rod, a water bath, a tripod stand, a burner, filter paper with a conical funnel, and a watch glass.
Safety first
Copper sulfate is
toxic
. Perform the experiment under adult supervision and do not touch it with bare hands. It is advised that the
teacher
adds the drop of sulfuric acid — handle sulfuric acid very carefully.
Procedure
1. Collect a sample of copper sulfate (blue vitriol). If it is unavailable, common salt can be used instead.
2. Take 1 g of copper sulfate in a 100 mL beaker, add 25 mL of water and a drop of dilute sulfuric acid. "Dilute" means thinned down with water. Heat it gently in a water bath while stirring constantly. A water bath means keeping the beaker in a container of hot water so the flame does not touch it directly.
3. Gradually add more copper sulfate until the solution becomes saturated.
4. Filter the hot solution to remove insoluble impurities. Collect the filtrate in a clean beaker and cover it with a watch glass.
5. Allow the solution to cool slowly without disturbing it.
6. Filter the crystals, rinse them with cold water and let them dry on a watch glass.
Observation
Large, shiny, well-shaped, blue-coloured crystals of copper sulfate are formed.
Explanation
Sulfuric acid helps in making pure crystals by preventing unwanted reactions. Slow cooling gives the particles enough time to come together and set properly, which is why the crystals turn out large, shiny and well shaped.
Impurities
means unwanted substances mixed in, and the
filtrate
is the clear liquid that passes through the filter.
You may also try
Place 1–2 mL of the saturated copper sulfate solution on a small glass plate or a lamination sheet and leave it for some time. Did you get crystals? If yes, is this a good way to experiment?
Answer
You will get crystals, because the water evaporates and the solid is left behind. But it is
not
a good method. In the open, water evaporates quickly, the particles get no time to set, and the crystals turn out small and irregular in shape. On top of that, dust can settle on an open plate.
Crystallization is a common technique for separating pure substances from homogeneous mixtures. New compounds are often accompanied by unwanted impurities, and crystallization separates the desired pure substance from them.
→ Slow cooling means big, clean crystals; fast cooling means small, poorly formed ones.
◆ Summary
Make a hot saturated solution and filter it hot
Cool it slowly and undisturbed
Large, shiny, pure blue crystals form
🔬 Think as a Scientist
A
hypothesis
is a guess that can be tested.
The hypothesis here is — if a hot saturated solution of copper sulfate is cooled rapidly in ice-cold water, smaller and less well-formed crystals will form than if it is cooled slowly at room temperature.
Question:
how would you design and perform an experiment to test this hypothesis? The book's hint is to prepare a hot saturated solution and divide it into two equal parts.
Suggested method
Keep one part in ice-cold water and the other on the table at room temperature.
Keep everything else the same — the amount of solution, the concentration, and the size of the beaker.
This is called a
fair test
: only one thing is changed, and here that one thing is the speed of cooling.
After a few hours, compare the crystals. The rapidly cooled part generally gives small grains, while the slowly cooled part gives large, clearly shaped crystals.
🇮🇳 India's Scientific Contributions
Crystallization of salt was an ancient process used by local communities of the
coastal areas
of India. "Coastal" means along the seashore.
Panga salt
was obtained by boiling concentrated sea brines.
Karkatch salt
was produced by the evaporation of sea water.
Salt crystals of different sizes were produced by these methods.
🚀 Ready to Go Beyond
Have you ever been to a place where large crystal
deposits
can be seen in nature? A deposit means a heap collected in one place.
Such crystals are found in mines, caves and even within the Earth's crust.
One example is the
Mawsmai Cave
in Sohra (Cherrapunji).
Quartz
is also one of the beautiful crystals found in nature.
Pause and Ponder
🤔 Pause and Ponder — Questions 4 and 5
4. Refer to the solubility curves given in Activity 5.2. If equal masses of hot, saturated solutions of compounds 'A' and 'B' are cooled from 80 °C to 60 °C, which solution is likely to deposit more solid?
View answer
Hide answer
Answer
Compound B
will deposit more solid.
Explanation
This answer comes straight off the lines of Fig. 5.6 — not from a guess.
Compound
At 80 °C
At 60 °C
Solid deposited
B (blue line)
about 356 g
about 287 g
about 69 g
A (red line)
about 65 g
about 56 g
about 9 g
From the same amount of water, B deposits about
seven times
as much solid as A.
One fine point — the question says equal
mass of solution
, not equal water. B's solution is so concentrated that 100 g of it holds only about 22 g of water, while A's solution holds about 60 g. Even after allowing for this, 100 g of B's solution deposits about 15 g of solid against about 5 g for A. The answer stays
B
either way.
B's solubility falls much more sharply on cooling, so more of it must come out as solid.
5. Will there be any change in the size of common salt crystals if the rate of evaporation is increased or decreased? Explain.
View answer
Hide answer
Answer
Yes.
Fast evaporation gives small crystals; slow evaporation gives large ones.
Explanation
A crystal grows large only when the particles get time to join one by one and settle into their proper places.
In fast evaporation, a great many particles come out of solution together, so many small crystals form instead of a few large ones.
In slow evaporation, the particles have time to arrange themselves properly, so the crystals grow large and well shaped.
This is exactly the same point we saw in Activity 5.3, where slow cooling gave big, shiny copper sulfate crystals.
Slower is bigger — time is what lets a crystal grow.
📝 Summary - What we have learnt
Crystallization
is forming crystals from a saturated solution.
It works because solubility
falls when the solution is cooled
, so the extra solute must come out.
It is used to get a
pure substance
from a homogeneous mixture, and to separate two solids when one is present in small quantity.
CA Maninder Singh is a Chartered Accountant with 16+ years of practical experience and 20+ years of teaching experience. At Teachoo, he simplifies Accounts, Tax and GST with step-by-step examples so students can apply concepts confidently in exams and real life.
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