When you add a crystal of a solute to an unsaturated solution, the crystal dissolves, becoming part of the solution. An unsaturated solution has the capacity to dissolve more solute, so any solute added, up to the solution’s saturation point, dissolves. Once the solution is saturated, any solute crystals added sink into the solution but don’t dissolve.
A solvent dissolves a solute by overcoming the intermolecular forces holding the substance together. For example, in a crystal of table salt, positively charged sodium ions have a strong attraction to negatively charged chloride ions. The attractive force binds the two types of atoms together so that they form a crystal. Water molecules also have positive and negative charges. When you add salt to water, the water molecules pull the sodium and chlorine apart and surround them, preventing them from rejoining.
Unsaturated and Saturated Solutions
A solvent is unsaturated when it can dissolve additional crystals of solute. In the case of salt and water, it takes several water molecules to surround each ion of sodium and chloride. As long as plenty of free water molecules are available, the solution can dissolve more salt. When it reaches the saturation point, most of the water molecules have already attached themselves to ions. If you add more salt, it settles to the bottom of the container as solid crystals.
A dissolved solute can come out of the solution in a process called precipitation. For example, if you leave a solution of salt and water sitting for several hours, the water eventually evaporates, leaving the salt behind. As water molecules leave the container, fewer are left to surround the salt ions, freeing them to recombine into solid salt. Chemical reactions can also cause precipitation; for example, when you mix a solution of lead nitrate with sodium iodide, a solid lead iodide precipitates from the solution.
Temperature Effects on Solubility
The process of pulling ions loose from salt consumes heat energy from the solution; when the ions dissolve in water, the process releases heat to the solution. If more heat comes out than goes in, then lower temperatures improve the solution’s ability to dissolve crystals of solute and higher temperatures lower the dissolving rate. Because heat is a product of the solution process, adding more product slows the reaction. If more heat goes into the reaction, the relationship with temperature is reversed; increased temperatures allow the solution to dissolve more solute.
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