Q1(e). Superheating of liquids and the Kelvin equation

Answer in brief

A newly formed bubble is so small that the vapour inside it cannot hold it open at Tb, so the liquid overshoots Tb.

Key points

• Boiling needs bubbles of vapour to form inside the liquid and then grow.
• Kelvin equation for a cavity of radius r: p = p* exp(−2γVm/rRT); the vapour pressure inside a cavity is below p* and falls further as r gets smaller.
• To stay open the bubble must also balance surface tension (Laplace): pin = pext + 2γ/r; for water a bubble of r = 1 μm needs 2.2 atm inside.
• At Tb, p* only equals pext, so a tiny bubble collapses; the liquid must be heated above Tb (superheating), and boiling chips supply large cavities to prevent bumping.