Which pairing has the smallest contact angle, indicating the strongest wetting: water on glass, water on copper, water on silver, or mercury on glass?

• A. Water on copper: 20 degrees
• B. Water on silver: 90 degrees
• C. Water on glass: less than 1 degree
• D. Mercury on glass: 132 degrees

Answer: (C)

Wetting strength is shown by the contact angle: the smaller the angle, the better the liquid spreads on the surface. Water on glass has the smallest angle—less than 1 degree—so it spreads almost completely across the surface, indicating very strong wetting. Glass is highly hydrophilic because its surface is polar and can form hydrogen bonds with water, increasing adhesion at the interface. In comparison, water on copper wets less well (about 20 degrees), water on silver shows neutral wetting (around 90 degrees), and mercury on glass hardly wets (about 132 degrees). Thus, the pairing with the strongest wetting is water on glass.