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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?
- 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?
- 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.
Temperature and Solubility
Solid in liquid
Solubility rises as temperature rises.
Gas in liquid
Solubility falls as temperature rises.
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 | 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
Use the salt solubility data (Table 5.4) to answer questions about saturated solutions and temperature.
View Answer →
