The most important substance on earth is water. Water has so many different uses that water becomes very valuable in the chemical community. For instance, for most living things to survive, water is necessary; for cooking, water is necessary, and for keeping mechanical engines cool, water is necessary. Well, in chemistry, water is also very important because it is necessary for many reactions to take place. An example of this is seen in salt. Table salt (NaCl) when put into water dissolves into its ions, Na+ and Cl-. Due to the salt dissolving, the Na+ ions and the Cl- ions can react with other substances by chemical reactions.
In the example above, NaCl, is dissolved in water. In this example, water is called the solvent. The solvent can be any substance that allows a substance to be dissolved in it. Also in this reaction, NaCl, is called the solute, or more basically put, the substance that gets dissolved. Although water is not the only substance that can be used as a solvent, it is the main substance used, so the scientific community came up with a term that is used to describe a situation when water is used as a solvent. The term is aqueous solution. Using the example above, the NaCl is an aqueous solution
The Water Molecule has many important properties that allow for substances to be dissolved in them. Water acts much like a magnet, because one side of the water molecule is negatively charged, and the other side is positively charged. The hydrogen side of water is the positively charged side, and the oxygen side of water is the negatively charged side. This characteristic of being polar is when a substance has a positively charged side and a negatively charged side. Water is a polar molecule.
Due to the polar nature of water, NaCl can be broken up into its ions. For example, NaCl breaks up into Na+ and Cl-, which means that the hydrogen side of water (+) attracts the Cl- ions, and the Na+ ions are attracted to the oxygen side of water. This is how solid substances are dissolved into water, and this is why water is so valuable to the chemistry world.
Not all substances are polar. Some are nonpolar. Benzene is a common nonpolar solvent. Because of nonpolar and polar substances there is a rule that students and chemists use a like, to describe whether substances should dissolve. The rule is likes dissolve likes. This means that if a chemist is trying to dissolve a polar solute, a polar solvent must be used, and likewise, if a chemist is trying to dissolve a nonpolar solute, a nonpolar solvent must be used. This works about ninety-five percent of the time, but like all rules there are always exceptions.
Another useful characteristic of solutions is the ability to conduct electricity. To determine if a solution has the ability to conduct electricity, an electrical conductivity apparatus is used. An electrical conductivity apparatus is basically a battery and light bulb setup which lights up when electricity is conducted through the solution.
Some solutions conduct electricity better than others do, and to allow scientists to understand how well substances conduct electricity, they came up with a scaling system. The highest level on this system is when solutions conduct electricity very well. This is called a strong electrolyte. The middle level, or when solutions conduct electricity, but not very well, is called a weak electrolyte. The lowest level, or when a solution does not conduct electricity at all is called a non-electrolyte. These levels are purely subjective and are open to interpretation by each scientist. To determine the level of conductivity that a solution has, a scientist uses a electrical conductivity apparatus and uses the brightness of the light bulb to determine the level of which the substance should be. (The brighter the light bulb is, the more conductive the solution is, and the stronger the electrolyte is.)
As stated above, the most conductive solutions are called strong electrolytes. There are three common ways in which strong electrolytes can be created. The first way to create a strong electrolyte is to dissolve salts in a solvent by breaking the salt up into its ions. (An example of this is the NaCl example above.) The second way involves acids. Arrhenius discovered that acids give off H+ ions and allow for a good flow of electricity through a solution. The third class was also proposed by Arrhenius. Arrhenius discovered that bases give off OH- ions and OH- ions also allow for a good flow of electricity through the solution.
Weak Electrolytes have two major classes of which they can be made from. The first class is called weak acids. A weak acid is much like a strong acid because it produces H+ ions, but the difference is that a weak acid only dissociates partially. In the terms of the common man, only partially dissociating means that only a small percentage of the acid gives off their H+ ions. For example, if HC2H3O2 were put in water, maybe 25% of the HC2H3O2 break up to H+ and C2H3O2- and the other 75% stays together. The second class of weak electrolytes are weak bases. Weak bases are exactly like weak acids except they release OH- ions instead of H+ ions.
Non-Electrolytes are substances that will dissolve, but do not produce any ions. For example, sugar will dissolve in water, but will not produce ions. Thus there is nothing to transfer electricity through the solution. That is why non-electrolytes do not conduct electricity.
In Chemistry, many reactions take place because two solutions are mixed together, but to do any mathematical calculations to determine what the final solution will be composed of or how much of the reactants there were, the Chemist needs to know a few things. The first thing that the Chemist needs to know is what type of reaction is taking place. This allows the Chemist to correctly determine what the products will be. The second piece of information that is necessary is the amount of chemicals that are reacting. There are many ways to express how much of something that is reacting, but the most common way is through Molarity. Molarity is defined as:
Moles of Solute / L of solution
The units on molarity is mol/L or more simply written M.
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Unit 1 - Section 6