Which of the following is the best definition of diffusion?

Options

A. Movement of molecules from an area of their higher concentration to an area of their lower concentration
B. Movement of water across a semi-permeable membrane from an area of high water concentration to an area of lower water concentration
C. Movement of molecules from an area of their lower concentration to an area of their higher concentration
D. Movement of water across a semipermeable membrane from an area of low water concentration to an area of higher water concentration

Explanation

To determine the best definition of diffusion, let's analyze the options provided step by step:

### Step-by-Step Solution:

1. **Understanding Diffusion**:
- Diffusion is a process that involves the movement of molecules. It is essential to recognize that diffusion occurs due to the concentration gradient.

2. **Evaluating the First Option**:
- The first option states: "Movement of molecules from an area of their high concentration to an area of their low concentration."
- This accurately describes diffusion, as it highlights the natural tendency of molecules to move from areas where they are more concentrated to areas where they are less concentrated.

3. **Evaluating the Second Option**:
- The second option states: "Movement of water across a semi-permeable membrane."
- This describes osmosis, which is a specific type of diffusion that pertains only to water molecules. Therefore, it is not a general definition of diffusion.

4. **Evaluating the Third Option**:
- The third option states: "Movement of molecules from an area of their low concentration to an area of their high concentration."
- This is incorrect for diffusion, as diffusion does not occur in this direction. Molecules do not move from low to high concentration without the input of energy.

5. **Conclusion**:
- Based on the evaluations, the best definition of diffusion is the first option: "Movement of molecules from an area of their high concentration to an area of their low concentration."

### Final Answer:
The best definition of diffusion is: **"Movement of molecules from an area of their high concentration to an area of their low concentration."**

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