The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water. When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them.
This process represents a physical change known as dissociation. Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes. 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.
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.
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 increase.
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. Let's explore why this happens.
The phenomenon that polar solvents dissolve ionic and polar solutes, nonpolar solvents dissolve nonpolar solutes, and polar solvents don't dissolve nonpolar solutes and vice-versa is often summed up by the phrase "Like dissolves like.
As mentioned earlier, polar solvents are good at dissolving polar solutes. To explain this, we'll describe the process that occurs when table salt sodium chloride dissolves in water.
As we learned in The Mole , water is a polar molecule with partial positive charge on each hydrogen atom and partial negative charge on the oxygen atom. This polarity is shown in the following figure:. Figure In some cases, the attraction of water molecules for the polar solute isn't strong enough to pull the solute molecules apart. As a result, some polar solutes don't dissolve in water. Ionic solids like sodium chloride, by definition, contain cations and anions.
As a result, when an ionic solid such as sodium chloride is placed into water, we see the following take place:. When sodium chloride is placed into water, the partial positive charges on the hydrogen atoms in water are attracted to the negatively charged chloride ions.
Likewise, the partial negative charges on the oxygen atoms in water are attracted to the positively charged sodium ions. Because the attractions of the water molecules for the sodium and chloride ions are greater than the forces holding the crystal together, the salt dissolves. Check out a few examples in the Table below. We want to focus on solutions where the solvent is water.
An aqueous solution is water that contains one or more dissolved substances. The dissolved substances in an aqueous solution may be solids, gases, or other liquids.
Some examples are listed in the Table above. Other examples include vinegar acetic acid in water , alcoholic beverages ethanol in water , and liquid cough medicines various drugs in water. Figure 2. Typical solution — clear and stable. Ben Mills Wikimedia : Benjah-bmm In order to be a true solution, a mixture must be stable.
When sugar is fully dissolved into water, it can stand for an indefinite amount of time and the sugar will not settle out of the solution. Why is solvation considered a surface phenomenon? How do hydration and solvation differ? How do you calculate percent dissociation? What is solvation?
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