which on e of the following characteristics is correct for physical adsorption ?

Options

A. it is very specific
B. Adsorption on solids is irreversible
C. Adsorption decreases with decrease in temperature
D. Generally both enthaply and entropy of adsorption are negative

Answer

To determine the correct characteristic of physical adsorption, let's analyze the properties of physical adsorption step by step.

### Step 1: Understanding Physical Adsorption
Physical adsorption, also known as physisorption, occurs when molecules adhere to a surface through weak Van der Waals forces. This type of adsorption is characterized by its reversibility and is influenced by temperature.

**Hint:** Remember that physical adsorption is primarily due to weak forces and is generally reversible.

### Step 2: Reversibility
One of the key features of physical adsorption is that it is reversible. This means that the adsorbed molecules can easily detach from the surface when conditions change (like temperature or pressure).

**Hint:** Consider how the nature of the forces involved affects the reversibility of the adsorption process.

### Step 3: Specificity
Physical adsorption is not specific to certain molecules. Unlike chemical adsorption, which can be selective, physical adsorption can occur with a variety of molecules.

**Hint:** Think about how the forces involved in physical adsorption differ from those in chemical adsorption regarding specificity.

### Step 4: Temperature Dependence
Physical adsorption is inversely proportional to temperature. As the temperature increases, the kinetic energy of the molecules increases, making it easier for them to overcome the weak forces holding them to the surface, thus reducing the amount of adsorption.

**Hint:** Reflect on how temperature changes the energy dynamics of the adsorbed molecules.

### Step 5: Entropy and Enthalpy
In physical adsorption, both enthalpy and entropy changes are generally negative. The process is exothermic (negative enthalpy) because energy is released when the molecules adhere to the surface. The randomness or disorder (entropy) decreases as the molecules become associated with the surface.

**Hint:** Consider how the arrangement of molecules changes when they move from a gas phase to being adsorbed on a surface.

### Step 6: Evaluating Options
Now, let's evaluate the options based on the characteristics we have discussed:
1. **It is very specific** - Incorrect, as physical adsorption is not specific.
2. **Adsorption on solid is irreversible** - Incorrect, as it is reversible.
3. **Adsorption decreases with decreases in temperature** - Incorrect, as adsorption decreases with increases in temperature.
4. **Both enthalpy and entropy of adsorption are negative** - Correct, as we established that both are typically negative in physical adsorption.

### Conclusion
The correct characteristic of physical adsorption is that both enthalpy and entropy of adsorption are negative.

**Final Answer:** Option 4 is the correct choice.

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