When making a solution, it's handy to know if one thing will dissolve in another. After all, if somebody wants you to make them a liquid solution of one chemical, it won't make you look good if you bring them a beaker of liquid with sludge sitting at the bottom because you chose the wrong solvent.
The best way to tell if something will dissolve is to look at the polarities of the solvent and the solute. If the polarities of the solvent and solute match both are polar or both are nonpolar , then the solute will probably dissolve.
If the polarities of the solvent and solute are different one is polar, one is nonpolar , the solute probably won't dissolve.
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. Some substances are held together by covalent bonds but have properties similar to ionic compounds. For example, sugar molecules are large, complex, and made of several types of atoms held together by covalent bonds.
The bonds between the atoms have small charges on them. As these charges are sufficient to attract water molecules, sugar is soluble in water. Substances dissolved in a liquid form a solution.
This means that the atoms of the dissolved substance are between the molecules of the liquid and cannot be separated by filtration. If you pour a solution of salt and water through filter paper, both the water and the salt will pass through the paper. In this example, the dissolved salt is called the solute, and the water that contains it is called the solvent.
Liquids other than water also act as solvents for some substances. Solutions and Dissolving When solid materials are added to water, several changes can occur. Why We Care: Our fresh water and ocean water too, of course typically contains dissolved minerals from soil, agricultural products such as fertilizer, bacteria and more, and salts from road treatment.
What happens when something dissolves? What is a solution? Ionic solids or salts contain positive and negative ions, which are held together by the strong force of attraction between particles with opposite charges.
When one of these solids dissolves in water, the ions that form the solid are released into solution, where they become associated with the polar solvent molecules.
We can generally assume that salts dissociate into their ions when they dissolve in water. Ionic compounds dissolve in water if the energy given off when the ions interact with water molecules compensates for the energy needed to break the ionic bonds in the solid and the energy required to separate the water molecules so that the ions can be inserted into solution.
Discussions of solubility equilibria are based on the following assumption: When solids dissolve in water, they dissociate to give the elementary particles from which they are formed. Thus, molecular solids dissociate to give individual molecules. When the salt is first added, it dissolves and dissociates rapidly. The conductivity of the solution therefore increases rapidly at first. Once that happens, there is no change in the concentration of these ions with time and the reaction is at equilibrium.
When this system reaches equilibrium it is called a saturated solution , because it contains the maximum concentration of ions that can exist in equilibrium with the solid salt. The amount of salt that must be added to a given volume of solvent to form a saturated solution is called the solubility of the salt. There are a number of patterns in the data obtained from measuring the solubility of different salts.
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