The contact angle of mercury with glass is 135 degrees. This high contact angle indicates mercury's strong cohesive forces and its tendency not to wet surfaces like glass.
Understanding Mercury's Unique Properties
The contact angle is a quantitative measure of the wetting of a solid by a liquid. It is defined as the angle, conventionally measured through the liquid, where a liquid-vapor interface meets a solid surface. A contact angle of 0° indicates complete wetting, while angles greater than 90° suggest non-wetting behavior, and 180° implies complete non-wetting.
Mercury (Hg) exhibits unique characteristics due to its high surface tension and the predominant cohesive forces between its own molecules compared to the adhesive forces with many solid surfaces. This results in a distinctive convex meniscus when mercury is placed in a narrow tube, unlike water, which forms a concave meniscus with glass.
Here are some key properties of mercury:
| Property | Value | Unit |
|---|---|---|
| Contact Angle (with glass) | 135 | degrees (°) |
| Density | 13.6 × 10³ | kg/m³ |
| Surface Tension | 0.49 | N/m |
The large contact angle of 135 degrees with glass is a direct consequence of mercury's strong intermolecular metallic bonds, which are significantly stronger than the attractive forces between mercury and glass.
Implications of the Contact Angle
The specific contact angle of mercury has several practical implications and observable phenomena:
- Convex Meniscus: When mercury is contained in a glass tube, its surface curves downwards at the edges, forming a convex meniscus. This is because the mercury molecules are more attracted to each other than to the glass wall.
- Non-Wetting Behavior: Mercury does not "wet" or spread out on glass surfaces. Instead, it tends to bead up into spherical droplets, minimizing its contact area with the surface.
- Applications in Instruments: Historically, this property made mercury ideal for use in traditional barometers and thermometers. Its non-wetting nature ensures that it does not stick to the glass tube walls, allowing for accurate readings of atmospheric pressure or temperature changes. However, due to its toxicity, mercury is now being phased out in many of these applications in favor of safer alternatives.
Understanding the contact angle helps in predicting how a liquid will behave on a solid surface, which is crucial in various scientific and engineering applications, from material science to fluid dynamics.
