What are Acids and Bases? • First off, we will watch an introduction video, made by a film-producing genius, in order to be reminded briefly about what acids and bases actually are. I am sorry if this video ruins your life. • http://www.youtube.com/results?search_query=Acids+and+Bases+Have+two+different+faces&aq=f • Is it starting to come back to you?
Physical Properties of Acids and Bases Acids Bases • Acids are corrosive – this means that they can corrode things, or break them down. • Acids have a pH that is below 7 – we will talk much more about pH later. • Acids taste sour, and can be found in fruits and juices. • Acids are good conductors of electricity, we will learn why soon. • Bases are also corrosive • Bases have a pH that is above 7 – note that a pH of 7 is neutral (water) • Bases taste bitter, and are present in things like soap • Bases are very slippery to the touch • Bases are also good conductors of electricity
Chemical Properties of Acids • Acids turn litmus paper red. When you are checking whether or not a compound is acidic, you use blue litmus paper. • Bases turn litmus paper blue. When you are checking whether or not a compound is basic, you use red litmus paper. • Acids react with metals to form hydrogen gas. The metals are dissolved and form an ionic compound in solution during this process.
Arrhenius Model of Acids and Bases • In 1883, a Swedish man named Svante Arrhenius proposed a simple definition of acids and bases that seemed to explain a great deal of the tendencies of acids. • Arrhenius proposed that an acid is any substance that dissolves in water to produce hydrogen ions, H+. • Arrhenius also proposed that a base is any substance that dissolved in water to produce OH- ions.
Is Water Acid or Base? Water is always dissociating and recombining in a state of chemical equilibrium. At any one time, all three of the following species are present in a solution: H2O < - - > H+ + OH- Water is a neutral substance, meaning that it is not an acid or a base. Soon, we will learn that in some ways, it’s both.
Arrhenius Model • This worked really well for some acids and bases. For instance, it explained the acidic properties of HCl and the basic properties of NaOH quite well: • HCl H+ + Cl- • NaOH Na+ + OH-
Arrhenius Goes Wrong • Arrhenius’ model could not explain the basic properties of many substances, however. • For instance, it was well known that a solution of NH3 produced hydroxide ions in water, and yet there is not any oxygen present in an NH3 molecule! • Other molecules also posed a problem for this model, such as CO3-. Therefore, a modified version of acid-base theory was required.