Water Temperature Significance in Natural Water
Aside from the temperature variations and thermal stratifications of water, temperature also influences dissolved gases, viscosity, reaction rates and solubility of salts.
At a given atmospheric pressure the concentration of a saturated solution of a slightly soluble gas that does not unite chemically with the water is very nearly directly proportional to the temperature of the water. Altitude must also be considered in order to take atmospheric pressure into account.
For example, the solubility of oxygen in water is increased by lowering the temperature of the water. The solubility increases about 40 percent as fresh water cools from 25° C to freezing. In other words, the solubility of atmospheric gases increases with a decrease in temperature.
As the temperature of water increases, the viscosity decreases. Actually, viscosity changes about 3.5 percent with each degree of temperature, although the relationship is not uniform. This causes silt in water to sink twice as fast at 23 ° C. than it sinks at 0° C.
Thus, warmer water carries less silt than colder water. The lower viscosity also allows the water to flow a little faster. Roughly, the velocity increases about 0.5 percent faster for each 1° C rise between 4° and 20° C.
An increase in temperature produces an increase in reaction rates. The generalization commonly made is that a 10° C. change in temperature changes the rate of reaction by a factor of about 2. The term “reaction” is meant to include biochemical processes as well as inorganic reactions. However, the generalization is only a very rough approximation.
Also, as the temperature increases the solubility of salts and compounds increases. Solubility is defined in terms of equilibrium. It is a function of equilibrium constant for the solution of a specific component at a specific temperature. It represents the total amount of a solute species that can be retained permanently in solution at a given temperature and pressure.
The factors that control the equilibrium of the major and minor constituents of natural water are beyond the scope of this discussion. But since solubility is used frequently in describing the behavior of individual constituents, be aware that increased temperature increases solubility.
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