Can we keep adding spoon after spoon of sugar to one glass of water forever?
Try it — and at some point the sugar stops disappearing.
Why does the water say "enough"?
What is solubility?
Spoon by spoon into 100 mL of water
1st spoon of sugar
dissolves completely, water stays clear
↓
2nd, 3rd, 4th spoon
still dissolving
↓
5th spoon
stops dissolving and settles at the bottom
↓
The water has reached its limit — that limit is called solubility
There is a
limit
up to which a solute dissolves in a solvent.
That limit is called
solubility
.
Solubility is the
maximum amount of solute
that dissolves in a fixed quantity of solvent (100 mL or 100 g)
at a given temperature
.
Definition
Solubility
is the maximum amount of a solute that dissolves in a fixed quantity of the solvent at a given temperature.
What are saturated and unsaturated solutions?
Saturated solution
Unsaturated solution
Has reached its maximum capacity
Still below the maximum limit
Cannot dissolve any more solute at that temperature
Can still dissolve more solute
Extra solute settles at the bottom
Everything added still disappears
The glass after the 5th spoon of sugar
The glass after the 1st spoon of sugar
If a sugar solution can hold at most about 200 g of sugar, then below that amount the sugar dissolves — this is an
unsaturated solution
.
Beyond that amount it will not dissolve — this is a
saturated solution
.
A saturated solution is like a glass of water whose stomach is full.
Definition
A
saturated solution
is a solution that cannot dissolve any more solute at that temperature. A solution that can still dissolve more is
unsaturated
.
Why does the definition mention a fixed temperature?
Because
solubility changes with temperature
.
The solubility of a
solid solute in a liquid solvent generally increases
with temperature — raise the temperature and more sugar will dissolve.
For
gases dissolved in liquids
, solubility generally
decreases
as temperature increases.
Solubility is an important property used to separate substances from mixtures — this is the idea the next section is built on.
π
Activity 5.2 — Let us represent solubility graphically
In this Activity, we will read a solubility curve and compare how two different compounds behave when the temperature is raised.
Set-up
Water is the solvent. Two compounds, 'A' and 'B', are the solutes, and each has a different solubility. A graph of solubility versus temperature is called a
solubility curve
. On the x-axis is temperature in °C, and on the y-axis is solubility in grams per 100 g of water.
Fill in the blanks
(i) The solubility of compound 'A' in water at 20 °C is ______ (less than / more than / similar to) its solubility at 60 °C.
(ii) The solubility of compound 'B' at 20 °C is ______ its solubility at 60 °C.
(iii) The solubility of ______ increases more than that of ______ with an increase in temperature.
Answer (i)
Look at the
red line
of compound A. It is lower down and has a gentle slope, but it does rise. At 20 °C, A is about 37 g per 100 g of water; at 60 °C it is about 56 g. So the solubility at 20 °C is
less than
at 60 °C.
Answer (ii)
Look at the
blue line
of compound B. At 20 °C it is about 204 g; at 60 °C it is about 287 g. So here too the answer is
less than
.
Answer (iii)
The solubility of
B
increases more than that of
A
. From 0 °C to 80 °C, A goes only from about 28 g to 65 g — a difference of about 37 g. In the same range B goes from about 181 g to 356 g — a difference of about 175 g. The blue line gets steeper as it rises, while the red line stays almost straight.
Careful — the red line only
looks
flat because the y-axis runs up to 400 while all of A's values sit between 30 and 65. Looking flat and being flat are two different things: 37 g to 56 g means A's solubility became 1.5 times larger.
→ If solubility falls when we cool a solution, the extra solute must come out. That is the idea behind crystallization.
β Summary
A solubility curve plots solubility against temperature
For both A and B, solubility rises with temperature
B's solubility rises far more steeply than A's
β Test Yourself
In a sugar solution, which is the solute and which is the solvent?
View Answer
Hide Answer
Sugar is the solute and water is the solvent.
In the formula for concentration, what always goes in the denominator?
View Answer
Hide Answer
The solution — never the solvent alone.
What happens to the solubility of a solid in a liquid when the temperature is raised?
View Answer
Hide Answer
It generally increases. For gases dissolved in liquids it is the opposite — solubility generally decreases.
A spoon of sugar added to a solution settles at the bottom. What kind of solution is it?
View Answer
Hide Answer
A saturated solution — it has already reached its maximum capacity at that temperature.
π Summary - What we have learnt
Solubility
is the maximum solute that dissolves in a fixed amount of solvent at a given temperature.
A
saturated
solution has hit that limit; an
unsaturated
one has not.
Solubility of solids in liquids generally
increases
with temperature; for gases in liquids it
decreases
.
A
solubility curve
shows this, and compound B's curve is far steeper than compound A's.
Recap — Section 5.2 — Solutions
β Test Yourself
10 g of salt is dissolved in 90 g of water. What is the mass by mass percentage of the solution?
View Answer
Hide Answer
10%. Mass of solution = 10 g + 90 g = 100 g, so (10 g ÷ 100 g) × 100 = 10% m/m.
What does an unsaturated solution mean?
View Answer
Hide Answer
A solution that can still dissolve more solute at that temperature — it has not reached its limit yet.
In the formula for % m/v, what goes on top and what goes on the bottom?
View Answer
Hide Answer
Mass of the solute on top, volume of the solution on the bottom, the whole thing multiplied by 100.
Important Definitions
Solute
The substance that dissolves, usually present in the smaller quantity.
Solvent
The substance that dissolves the solute, usually present in the larger quantity.
Solution
The homogeneous mixture formed when a solute dissolves in a solvent.
Concentration
The amount of solute dissolved in a given amount of solvent or solution.
Solubility
The maximum amount of solute that dissolves in a fixed quantity of solvent at a given temperature.
Saturated solution
A solution that cannot dissolve any more solute at that temperature.
Questions at the End of the Chapter — Related to This Section
π NCERT Question 4 — Cake ingredients and a brass alloy
Work out the concentration of each ingredient in a cake recipe, and identify the alloy in a brass utensil.
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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