Q1(c). Wetting of glass

Answer in brief

Water–glass adhesion favours spreading, while mercury cohesion dominates its adhesion to clean glass.

Key points

• Wetting is described by the contact angle θ measured through the liquid. Young' s equation is γ_(SV)=γ_(SL)+γ_(LV)cos θ.
• Water interacts strongly with polar surface hydroxyl groups on clean glass. The resulting small contact angle allows water to spread and gives a concave meniscus.
• Mercury has strong cohesive metallic interactions and a high liquid surface tension.
• Wetting depends on interfacial energies and surface cleanliness, not simply on whether the substance is a liquid.