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š¬ Think about it
Add sugar to a glass of water and keep stirring — at some point no more dissolves. There is a limit. And that limit changes with temperature. What is this limit called, and why does heat matter? Let us find out.
What is solubility and a saturated solution?
Saturated solution
Saturated
Full — not one more grain dissolves
Solution
Solute dissolved in a solvent
↓
Saturated solution
= a solution that can dissolve no more solute
at that temperature
.
Solubility
Step 1 — Fix the solvent
100 g (or 100 mL) of water
↓
Step 2 — Fix the temperature
solubility changes with it, so it must be stated
↓
Step 3 — Keep adding solute
until no more will dissolve
↓
Solubility
= that maximum mass of solute, in g per 100 g of solvent.
- Solubility is the maximum solute that dissolves in 100 g (or 100 mL) of solvent at a given temperature.
- A solution that can dissolve no more solute at that temperature is saturated.
- We mention temperature because solubility changes with it.
How does temperature affect solubility?
How does temperature affect solubility?
Solubility
Changes with temperature
1. Solid in liquid
Rises as temp rises
Example:
Sugar in water
Salt in water
2. Gas in liquid
Falls as temp rises
Example:
Carbon dioxide in soda
- For a solid in a liquid, solubility generally increases with temperature.
- For a gas in a liquid, solubility generally decreases with temperature.
- Solubility is an important property used to separate substances.
In this Activity, we will read the solubility curves of two compounds A and B to compare how their solubility changes with temperature.
Procedure
Take water as the solvent and compounds A and B as solutes. A graph of solubility versus temperature is a solubility curve (Fig. 5.6). The x-axis shows temperature (°C) and the y-axis shows solubility (g per 100 g of water). Use the graph to compare A and B.
Observation (fill in the blanks)
(i) The solubility of compound A at 20 °C is
less than
its solubility at 60 °C.
(ii) The solubility of compound B at 20 °C is less than its solubility at 60 °C.
(iii) The solubility of compound B increases more than that of compound A with an increase in temperature.
(ii) The solubility of compound B at 20 °C is less than its solubility at 60 °C.
(iii) The solubility of compound B increases more than that of compound A with an increase in temperature.
ā Summary
- Read solubility curves.
- Both rise with temperature.
- B rises more than A.
- Cooling will drop out solute.
Table 5.4: Solubility of various salts (in g per 100 g of water) at different temperatures
| Salts | Temperature (°C) | |||||
|---|---|---|---|---|---|---|
| 10 °C | 20 °C | 30 °C | 40 °C | 60 °C | 80 °C | |
| Potassium nitrate | 21 | 32 | 45 | 62 | 106 | 167 |
| Sodium chloride | 36 | 36 | 36.3 | 36.5 | 37 | 37 |
| Potassium chloride | 35 | 35 | 37.4 | 40 | 46 | 54 |
| Ammonium chloride | 24 | 37 | 41 | 41 | 55 | 66 |
Important Points
- Solubility is the most solute that dissolves at a given temperature.
- A saturated solution can hold no more solute at that temperature.
- Heating usually dissolves more solid; cooling drops some out.
ā Test Yourself
-
What is a saturated solution?
View Answer
One that can dissolve no more solute at that temperature. -
How does heating affect a solid's solubility in water?
View Answer
It generally increases. -
How does heating affect a gas's solubility in water?
View Answer
It generally decreases.
Important Definitions
- Solubility — the maximum solute that dissolves in a fixed amount of solvent at a given temperature.
- Saturated solution — a solution that cannot dissolve any more solute at that temperature.
š NCERT Question 13 — Answer the following questions with
Answer the following questions with the help of the data given in Table 5.4.
(i) What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40 °C?
(ii) A student makes a saturated solution of potassium chloride in water at 80 °C and leaves the solution to cool at room temperature (25 °C). What would she observe as the solution cools? Explain.
(iii) What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10 °C to 80 °C.
View Answer →
(i) What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40 °C?
(ii) A student makes a saturated solution of potassium chloride in water at 80 °C and leaves the solution to cool at room temperature (25 °C). What would she observe as the solution cools? Explain.
(iii) What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10 °C to 80 °C.
