 Nuclear power plants require large amounts of water to generate steam for the turbines and to cool the equipment. They will usually be situated near bodies of water to use that water as a coolant, returning the warmer water back to the lake or river. This increases the overall temperature of the water, which lowers the quantity of dissolved oxygen, affecting the survival of fish and other organisms. The solubility of a substance is the amount of that substance that is required to form a saturated solution in a given amount of solvent at a specified temperature. Solubility is often measured as the grams of solute per 100 g of solvent. The solubility of sodium chloride in water is 36.0 g per 100 g water at 20°C. The temperature must be specified because solubility varies with temperature. For gases, the pressure must also be specified. Solubility is specific for a particular solvent. We will consider solubility of material in water as solvent. The solubility of the majority of solid substances increases as the temperature increases. However, the effect is difficult to predict and varies widely from one solute to another. The temperature dependence of solubility can be visualized with the help of a solubility curve , a graph of the solubility vs. temperature (see figure below ).
Notice how the temperature dependence of NaCl is fairly flat, meaning that an increase in temperature has relatively little effect on the solubility of NaCl. The curve for KNO 3 , on the other hand, is very steep and so an increase in temperature dramatically increases the solubility of KNO 3 . Several substances – HCl, NH 3 , and SO 2 – have solubility that decreases as temperature increases. They are all gases at standard pressure. When a solvent with a gas dissolved in it is heated, the kinetic energy of both the solvent and solute increases. As the kinetic energy of the gaseous solute increases, its molecules have a greater tendency to escape the attraction of the solvent molecules and return to the gas phase. Therefore, the solubility of a gas decreases as the temperature increases. Solubility curves can be used to determine if a given solution is saturated or unsaturated. Suppose that 80 g of KNO 3 is added to 100 g of water at 30°C. According to the solubility curve, approximately 48 g of KNO 3 will dissolve at 30°C. This means that the solution will be saturated since 48 g is less than 80 g. We can also determine that there will be 80 - 48 = 32 g of undissolved KNO 3 remaining at the bottom of the container. Now suppose that this saturated solution is heated to 60°C. According to the curve, the solubility of KNO 3 at 60°C is about 107 g. Now the solution is unsaturated since it contains only the original 80 g of dissolved solute. Now suppose the solution is cooled all the way down to 0°C. The solubility at 0°C is about 14 g, meaning that 80 - 14 = 66 g of the KNO 3 will recrystallize.
Glossary
Licenses and AttributionsCC licensed content, Shared previouslyAs the temperature increases, the solubility of a gas decrease as shown by the downward trend in the graph. More gas is present in a solution with a lower temperature compared to a solution with a higher temperature. Increased temperature causes an increase in kinetic energy. Temperature. Basically, solubility increases with temperature. Hence under high temperature the molecules are able to move freely and make the gas molecules to dissolve freely. The higher kinetic energy causes more motion in the molecules which break intermolecular bonds and escape from solution.
The solubility of solutes is dependent on temperature. When a solid dissolves in a liquid, a change in the physical state of the solid analogous to melting takes place. Heat is required to break the bonds holding the molecules in the solid together. At the same time, heat is given off during the formation of new solute -- solvent bonds. Figure: Temperature dependent solubilities of three salts in water. The use of first-aid instant cold packs is an application of this solubility principle. A salt such as ammonium nitrate is dissolved in water after a sharp blow breaks the containers for each. \[NH_4NO_{3(s)} \rightarrow NH_{4(aq)}^+ + NO^-_{3(aq)}\] The dissolving reaction is endothermic and requires heat. Therefore the heat is drawn from the surroundings and the pack feels cold.
Charles Ophardt (Professor Emeritus, Elmhurst College); Virtual Chembook
A saturated solution is at equilibrium (rate of dissolution is equal to rate of crystallization) with some equilibrium constant $K_1$. If you change the temperature of the system at equilibrium, you will observe a different equilibrium constant $K_2$. Whether the equilibrium constant increase or decreases is described by the Van't Hoff equation: $$\ln \left(\frac{K_2}{K_1}\right) = \frac{-\Delta H}{R}\left(\frac{1}{T_2}-\frac{1}{T_1}\right)$$ So it depends on the sign of the enthalpy of reaction, in this case the enthalpy of dissolution.
The statement about solubility is not always true, and the explanation leaves out something. A lot of things change when you increase the temperature. The solvent gains kinetic energy, the solute gains kinetic energy, and the solid gains kinetic energy. How this influences the solubility depends on the specific system, and is hard to predict.
Now that we introduced the Van't Hoff equation, we know it must have to do with the enthalpy of dissolution. Usually, you expect the interactions in the solid that need to be broken to be stronger than the gains from solvating the solute. In these cases, it is apparently the opposite. |
Why does the solubility of a decrease with increase in temperature?
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