Explain the terms: Physisorption and Chemisorption.
How does adsorption of a gas on a solid surface vary with pressure?

Answer

### Step-by-Step Text Solution

**1. Define Physisorption and Chemisorption:**

- **Physisorption (Physical Adsorption):**
- Physisorption refers to the process where gas molecules adhere to a solid surface through weak van der Waals forces.
- This type of adsorption is generally reversible, meaning that the adsorbed molecules can be easily removed from the surface without any significant energy change.
- The interaction between the adsorbate (gas) and the adsorbent (solid) is primarily due to physical forces, and it does not involve any chemical bond formation.

- **Chemisorption (Chemical Adsorption):**
- Chemisorption, on the other hand, involves the formation of strong chemical bonds between the adsorbate and the adsorbent.
- This process is usually irreversible, as breaking the chemical bonds requires a significant amount of energy.
- In chemisorption, the interaction is much stronger than in physisorption, leading to a more stable attachment of the gas molecules to the solid surface.

**2. Explain the Variation of Gas Adsorption with Pressure:**

- The adsorption of a gas on a solid surface typically increases with an increase in pressure. This relationship can be described using the adsorption isotherm, which is a model that illustrates how the amount of gas adsorbed varies with pressure at constant temperature.

- According to the Freundlich adsorption isotherm, the relationship can be expressed as:
\[ A = kP^n \]
where:
- \( A \) = amount of gas adsorbed per unit mass of adsorbent
- \( P \) = pressure of the gas
- \( k \) and \( n \) = constants that depend on the nature of the gas and the adsorbent.

- As the pressure increases, the amount of gas adsorbed also increases, indicating a direct proportionality between adsorption and pressure. This means that at higher pressures, more gas molecules are available to interact with the surface of the solid, leading to increased adsorption.