Scientists have found that all the substances of same weight cannot be dissolved in the same quantity of water at a particular temperature. “The solubility of a substance at a given temperature is defined as the quantity of the substance that can be dissolved in 100 parts of the solvent at that temperature to give a saturated solution”. It means, solubility of a substance at a particular temperature is fixed. Different substances have different solubility at the same temperature.
At 30°C, 25gm of copper sulphate (CuSO4) dissolved in 100gms of water forms a saturated solution. The solubility of copper sulphate is 25gm. Similarly, the solubility of potassium nitrate (KNO3) is 46 gm, sodium chloride (NaCl) is 36gm, sugar is 220gm and sodium nitrate (NaNO3) is 95gm. The solubility of different substance differs according to the temperature can be understood from below table:
S.N | Name of substance | Temp°C | Solubility gm/100gm water |
1. 2. 3. 4. 5. | Copper sulphate Potassium nitrate Sodium chloride Sugar Sodium nitrate | 30 30 30 30 30 | 25 46 36 220 95 |
The solubility of a substance can be obtained by using the following relationship:
Solubility = 100 × Wt. of solute (gm)
Wt. of solvent (gm)
Example: Find the solubility of sugar if 5.1 gm of it when completely dissolved in 2.5gm of water at 20°C forms saturated.
Solution:
Here,
Wt. of solute = 5.1 gm
Wt. of solvent = 2.5 gm
Temperature = 20°C
Solubility =?
We have,
Solubility = 100 × Wt. of solute (gm)
Wt. of solvent (gm)
= 100×5.1
2.5
= 204
Solubility of sugar at 20°C is 204.
Solubility Curves:
Scientists have found that the solubility of a particular substance is not same at different temperatures. The capacity of solvent to dissolve a substance depends upon the temperature. Therefore, solubility of a substance also depends upon the temperature. Solubility differs according to the change in temperature. This can be understood from solubility curves.
The solubility of a substance at different temperature when plotted on a graph, we can get a line or curve called solubility curve. In solubility curve, the temperature is plotted on X-axis and solubility against that temperature on Y-axis.
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| Fig: solubility curve |
A curve obtained by plotting the solubility of a substance at different temperatures against these temperature is known as solubility curve.
It is observed that in most cases the solubility rises with the temperature. This rise in solubility varies for different substances. It the curve shows the break, it suggests formation of a new hydrate.
Information from solubility curves:
The solubility curves give us valuable information. These are listed below:
1. At a glance, we know the solubility at different temperature.
2. Solubility of a substance increases with temperature.
3. Solubility of different substances can be compared.
4. If two or more solutes are present, we know which one will crystallize first.
5. Temperature of formation of hydrate is indicated.
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