Acids and bases are two fundamental categories of substances in chemistry, defined by their properties and their behavior in water.
### Acids
**Acids** are substances that:
1. **Release Hydrogen Ions (H⁺)**: In an aqueous solution, acids release hydrogen ions (H⁺). For example, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻ ions.
2. **Taste Sour**: Many acids have a sour taste, like citric acid in lemons and acetic acid in vinegar.
3. **React with Metals**: Acids often react with metals to produce hydrogen gas. For example, HCl reacts with zinc to form zinc chloride and hydrogen gas.
4. **Change Litmus Paper Red**: Acids turn blue litmus paper red.
5. **Have a pH Less Than 7**: The pH scale measures how acidic or basic a solution is. Acids have a pH less than 7.
### Bases
**Bases** are substances that:
1. **Release Hydroxide Ions (OH⁻)**: In an aqueous solution, bases release hydroxide ions (OH⁻). For example, sodium hydroxide (NaOH) dissociates in water to form Na⁺ and OH⁻ ions.
2. **Taste Bitter**: Bases tend to have a bitter taste, though tasting chemicals is generally unsafe.
3. **Feel Slippery**: Bases often feel slippery to the touch, like soap.
4. **Change Litmus Paper Blue**: Bases turn red litmus paper blue.
5. **Have a pH Greater Than 7**: Bases have a pH greater than 7.
### The pH Scale
The **pH scale** ranges from 0 to 14 and measures the acidity or basicity of a solution:
- **pH 7**: Neutral (pure water)
- **pH < 7**: Acidic (the lower the pH, the stronger the acid)
- **pH > 7**: Basic or alkaline (the higher the pH, the stronger the base)
### Acid-Base Theories
There are several theories to explain the behavior of acids and bases:
1. **Arrhenius Theory**:
- **Acids** produce H⁺ ions in water.
- **Bases** produce OH⁻ ions in water.
2. **Brønsted-Lowry Theory**:
- **Acids** are proton donors (H⁺ donors).
- **Bases** are proton acceptors (H⁺ acceptors).
3. **Lewis Theory**:
- **Acids** are electron pair acceptors.
- **Bases** are electron pair donors..