We have observed that some chemicals dissolve in water, are soluble, and some do not, are insoluble. For chemicals that are soluble there is a limit to how much of the substance will dissolve. This limit is known as solubility. Solubility is the maximum concentration for a solution with a given solute at a given temperature. Every substance has a unique solubility. Solubility can be expressed in any units of concentration, but is commonly expressed in grams of solute per 100 ml of water. For example, at 25.0 0C, the solubility of sodium chloride is 35.0 grams per 100 ml of water. That means that you can dissolve up to 35.0 grams of sodium chloride in 100 ml of water. If you add more than 35.0 grams the solid will simply not dissolve and the system will reach a condition known as dynamic equilibrium. This condition occurs when the rate of dissolving becomes equal to the rate of crystallization. Dissolving never stops, it simply is nullified by the reverse process, crystallization, occurring at the same rate.
Solubility and Temperature:
As we observed in our lab, the amount of solute that will dissolve in water changes with temperature. As the temperature increases, the solubility increases for most solids. For this reason the temperature is always given along with the solubility. Most solubility tables are given for mixtures at 25.0 0C because this is close to room temperature.
A graph that show the relationship between solubility and temperature is called a solubility curve. A solubility curve with many common salts is shown below:
Solubility of Gases:
When something is heated it will favor the gas phase, therefore, heating solutions with gaseous solutes will decrease the solubility. For example, carbonated water can dissolve more carbon dioxide at lower temperatures. The following is a solubility curve for gases: