Why Do Salts Dissolve More (or Less) in Hot Water?
How Temperature Affects the Solubility of Salts in Water
The solubility of salts in water is generally influenced by temperature—most salts dissolve more readily in warm water than in cold. However, this increase in solubility with temperature is not consistent across all salts. The reason lies in the unique chemical nature of each salt and how its ions interact with water molecules.
When a salt dissolves in water, the positively charged part of the salt is attracted to the negatively charged oxygen atoms in water, while the negatively charged part of the salt is drawn to the positively charged hydrogen atoms. These interactions help break the salt into individual ions, allowing it to dissolve. Since each salt has different inter-ionic forces and bonding characteristics, its solubility responds differently to temperature changes.
To clearly visualize how solubility varies, solubility curves are often used. These curves show how the solubility of different salts increases or decreases with rising temperature, helping students and chemists better understand this important concept in solution chemistry.
Solubility of Potassium Nitrate (KNO₃) in Water
Based on the solubility curve shown in the figure, potassium nitrate (KNO₃) is only slightly soluble in cold water but becomes highly soluble in hot or boiling water. The solubility graph plots temperature (°C) on the x-axis and solubility in grams per 100 mL of water (g/100 mL) on the y-axis.
At 0°C, the solubility of KNO₃ is approximately 13 g/100 mL, indicating limited dissolving capacity in cold conditions. As the temperature increases, the solubility rises steadily, reaching a high value of 240 g/100 mL at 100°C. This sharp increase demonstrates that KNO₃ is highly temperature-sensitive, making it an ideal example for studying the effect of temperature on salt solubility.
Solubility of Sodium Chloride (NaCl) in Water
According to the solubility curve shown in the figure, sodium chloride (NaCl) is quite soluble in cold water. However, unlike salts such as potassium nitrate, its solubility does not significantly increase with temperature.
In the solubility graph, temperature (°C) is plotted on the x-axis, while solubility in grams per 100 mL of water (g/100 mL) is shown on the y-axis. The curve shows that at 0°C, the solubility of NaCl is approximately 35.65 g/100 mL, and it rises only slightly to about 38.99 g/100 mL at 100°C.
This minimal change indicates that NaCl has nearly constant solubility across a wide temperature range, which distinguishes it from many other salts in terms of temperature sensitivity.
Solubility of Calcium Sulphate (CaSO₄) in Water
The solubility of calcium sulphate (CaSO₄) exhibits a unique pattern compared to most other salts. According to the solubility curve shown in the figure, its solubility initially increases with temperature, but beyond a certain point, it begins to decrease—a behavior that sets it apart from salts like potassium nitrate or sodium chloride.
The solubility graph displays temperature in degrees Celsius on the x-axis and solubility in grams per 100 mL of water (g/100 mL) on the y-axis. At 0°C, the solubility of CaSO₄ is approximately 0.223 g/100 mL. It gradually increases to 0.265 g/100 mL at 40°C, then it starts to decline, reaching 0.205 g/100 mL at 100°C.
This unusual trend is explained by the exothermic nature of calcium sulphate’s hydration process. Since exothermic processes release heat, increasing the temperature actually opposes the dissolution of the salt, resulting in reduced solubility at higher temperatures.
