Which pairing shows the smallest contact angle: water on silver, water on glass, water on copper, or mercury on glass?

• A. Water on silver: 90 degrees
• B. Water on glass: less than 1 degree
• C. Water on copper: 45 degrees
• D. Mercury on glass: 132 degrees

Answer: (B)

Wettability is shown by the contact angle between a liquid and a solid: a smaller angle means the liquid spreads more on the surface. Water on glass has a contact angle of less than 1 degree, so water nearly completely wets the glass and spreads into a thin film. Clean glass has high surface energy and strong adhesive interactions with water, so it pulls the water across the surface very effectively.

Water on copper shows moderate wetting, with about a 45-degree angle, indicating the surface attracts water but not as strongly as glass. Water on silver is around 90 degrees, meaning the water forms a spherical bead and wets the surface poorly. Mercury on glass has a high angle, about 132 degrees, so mercury beads up and does not wet the glass well due to its high surface tension and weak adhesion to glass.

Therefore, the smallest contact angle is water on glass, reflecting the strongest wetting among the options.