Unit IV: Acids & Bases
From Sour to Slippery: The Chemistry of Protons.
This tutorial is the third chapter of Unit IV. It builds on our knowledge of Solution Stoichiometry.
← Go back to Unit IV, Chapter 2: Solution Stoichiometry.
The Problem: Sour, Bitter, and Slippery
You already know about acids and bases from everyday life. Lemons and vinegar are sour (acids), while soap and baking soda are bitter and feel slippery (bases). But what makes them chemically different? For centuries, scientists struggled to define them.
The first major step was the Arrhenius theory, which said acids produce H⁺ ions in water and bases produce OH⁻ ions. This was a good start, but it was too limited. It only worked for reactions in water and couldn't explain why a substance like ammonia (NH₃), which has no OH⁻, acts as a base.
The Breakthrough: Brønsted-Lowry Theory
In 1923, a more powerful definition emerged. The Brønsted-Lowry theory defines acids and bases not by what they produce, but by what they do with a proton (a hydrogen ion, H⁺).
- An Acid is a proton (H⁺) donor.
- A Base is a proton (H⁺) acceptor.
This simple definition is incredibly powerful. It explains why ammonia is a base (it accepts a proton from water) and works for reactions in any solvent. When an acid donates a proton, what's left is its conjugate base. When a base accepts a proton, it becomes its conjugate acid.
Interactive Conjugate Pair Finder
Drag each chemical species from the equation into the correct box below.
Reactants
Products
Measuring Acidity: The pH Scale
How do we measure how acidic or basic a solution is? We use the pH scale. It's a logarithmic scale that measures the concentration of H⁺ ions in a solution. The scale typically runs from 0 to 14.
- pH < 7 is acidic.
- pH = 7 is neutral.
- pH > 7 is basic (or alkaline).
Because it's logarithmic, a change of 1 pH unit means a 10-fold change in acidity. A solution with pH 2 is 10 times more acidic than pH 3, and 100 times more acidic than pH 4!
Use the slider below to see the "Aha!" moment: as H⁺ concentration goes up, pH goes down, and vice-versa. Notice the color change and how common items fit on the scale.
Strength vs. Concentration
Not all acids are created equal. A 1M solution of Hydrochloric Acid (HCl) is far more dangerous than a 1M solution of Acetic Acid (vinegar). Why? The difference is strength.
- Strong Acids/Bases: Dissociate (break apart) completely in water. Every single molecule releases its proton (for acids) or accepts one (for bases).
- Weak Acids/Bases: Only dissociate partially. In a solution, most of the molecules remain intact, and only a small fraction are involved in the acid-base reaction.
This is why pH is so important. It measures the actual concentration of H⁺ ions, which is a direct result of both the acid's initial concentration and its strength.
You've Mastered Acids & Bases. What's Next?
You now understand the properties of acids and bases and how to measure their strength. The next step is to see how they react with each other in a process called neutralization, which is the basis for the lab technique of Titration.