Theoretically which of the following are best lubricants

Options

A. solids
B. liquids
C. gases
D. all have same lubricating capacity

Answer

To determine the best lubricants theoretically, we need to analyze the characteristics of good lubricants and compare the three states of matter: solids, liquids, and gases.

### Step-by-Step Solution:

1. **Understanding Lubricants**:
- Lubricants are substances used to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move.

2. **Characteristics of Good Lubricants**:
- A good lubricant should possess several key characteristics:
- **High Boiling Point**: This ensures that the lubricant remains effective at high temperatures without evaporating.
- **Low Freezing Point**: This allows the lubricant to remain in a liquid state at lower temperatures, preventing it from solidifying.
- **High Viscosity Index**: This indicates that the lubricant maintains its viscosity (thickness) over a wide range of temperatures.
- **Thermal Stability**: The lubricant should be stable at high temperatures and not break down.
- **Hydraulic Stability**: This means the lubricant should maintain its properties under pressure.
- **High Resistance to Oxidation**: This prevents the lubricant from degrading when exposed to air and heat.

3. **Comparison of States of Matter**:
- **Solids**: While some solid lubricants exist (like graphite), they do not possess the necessary properties such as high viscosity index and thermal stability over a wide temperature range.
- **Liquids**: Liquids can easily flow and conform to surfaces, and they can be formulated to have high boiling points, low freezing points, and other desirable properties.
- **Gases**: Gases are generally not effective as lubricants because they cannot maintain a consistent film between surfaces.

4. **Conclusion**:
- Based on the characteristics of good lubricants, liquids are the best lubricants theoretically. They possess all the required properties to effectively reduce friction and wear in mechanical systems.

### Final Answer:
The best lubricants theoretically are **liquids**.