Whether you are sipping lemonade, scrubbing a sink, or digesting lunch, you are using acids and bases. They show up in your stomach, in your soap, in your soil, and across countless chemical reactions. Understanding them is also the key to understanding the pH scale.
Let’s break down what acids and bases are, how to tell them apart, how strong they are, and what happens when they meet.
What Are Acids and Bases?
Chemists define acids and bases in more than one way. The two most common are:
1. Arrhenius Definition (classic but simple):
- Acid: produces H⁺ (hydrogen ions) in water
- Base: produces OH⁻ (hydroxide ions) in water
Example:
HCl → H⁺ + Cl⁻ (acid)
NaOH → Na⁺ + OH⁻ (base)
2. Brønsted–Lowry Definition (broader and more flexible):
- Acid: donates a proton (H⁺)
- Base: accepts a proton
This definition works even outside of water, which makes it especially useful in biochemistry and gas reactions.
Properties of Acids vs Bases
| Property | Acids | Bases |
| Taste (do not test) | Sour (lemon juice) | Bitter (baking soda) |
| Feel | Can burn skin | Slippery, soapy |
| pH Range | 0–6.9 | 7.1–14 |
| Ion Released | H⁺ | OH⁻ |
| Litmus Paper | Red | Blue |
| Reaction with Metals | Produces hydrogen gas | Usually no reaction |
| Neutralize Each Other | Yes, form salt and water | Yes, form salt and water |
The pH Scale
The pH scale measures the concentration of hydrogen ions in a solution.
- Scale runs from 0 to 14
- 0 = very acidic (battery acid)
- 7 = neutral (pure water)
- 14 = very basic (drain cleaner)
Key fact: every step on the pH scale is a 10× difference in hydrogen ion concentration.
- pH 3 is 10 times more acidic than pH 4.
- pH 2 is 100 times more acidic than pH 4.
Strong vs Weak Acids and Bases
The strength of an acid or base is not about danger or concentration. It is about how completely it ionizes in water.
- Strong Acids: ionize completely (HCl, HNO₃, H₂SO₄)
- Strong Bases: ionize completely (NaOH, KOH, Ca(OH)₂)
- Weak Acids and Bases: ionize only partially (acetic acid, ammonia)
Neutralization Reactions
When an acid and base meet, they react to form a salt and water.
General Reaction:
Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H₂O
This type of reaction is important in:
- Antacids for treating heartburn
- Water treatment systems
- Farming and soil balance
Indicators: How We Measure pH
Chemists often use indicators to test acidity or basicity.
- Litmus paper: turns red in acids, blue in bases
- Phenolphthalein: colorless in acids, pink in bases
- Universal Indicator: shows a full color range across the pH scale
Recap Chart
| Concept | Acid | Base |
| Ion Produced | H⁺ | OH⁻ |
| pH Range | < 7 | > 7 |
| Litmus Paper | Red | Blue |
| Strength | Based on % ionization | Based on % ionization |
| Reaction Together | Neutralization → salt + water | Neutralization → salt + water |
Thought to Take With You
Acids and bases are like opposites in a story. They are powerful on their own, but when they meet, they create balance.
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