I'm finally branching out, and adding a little bit of chemistry to my science tutor video series. This short (6-minute) science tutor video covers the established definitions of acids and bases. I've also put in a quick tutorial on the pH scale and how it indicates acidity.
You probably have a clear idea about acids and bases, even without a science tutor. If you don't have an exact definition of an acid, you probably still know one when you see it. For example, you know that sour foods like lemons and vinegar are acids. You probably know that batteries often contain acid.
For a cheap and interesting way to evaluate the acidity of a substance, check this out.
Scientists, science tutors and even chemists have never come up with an exact definition of acids and bases, but rather they look at various things and acid or a base may do in different circumstances.
For example, the chemist Svante Arrhenius first suggested that an acid is a substance that releases hydrogen ions when dissolved in water. A base releases hydroxide ions. This observation brings up the point that water itself will often "dissolve" in water. When the H2O molecule splits, it releases both a hydrogen ion (H+) and a hydroxide ion (OH-).
Essentially, according to the Arrhenius model, water is both an acid and a base!
Later chemists took the view that an acid or a base can be defined by what happens on the submolecular level. As Arrhenius observed, an acid releases a hydrogen ion in a chemical reaction. However, chemists argue that a hydrogen ion is really just a proton. Therefore, an acid is a proton donor and a base is a proton acceptor.
Some chemists prefer the inverse of this interpretation. They argue that the acid really doesn't "donate" a proton to the reaction, but instead it accepts two electrons. In other words, the base is an electron donor, while the acid is an electron acceptor.
As a science tutor, I prefer the proton interpretation. The movement of protons plays an important role in photosynthesis and the creation of ATP. By defining acid an base in terms of protons, you can link chemistry, physics, and biology in one fell swoop.
The proton interpretation is also useful for understanding the pH scale. After all, pH simply rates the concentration of hydrogen ions (i.e., protons) in a substance. Water;s pH is 7, and this is considered neutral. As the numbers decrease, the concentration of protons increases, and the substance becomes more acidic. Thus, a pH lower than 7 indicates and acid, while a pH higher than 7 indicates a base.