BOILING vs EVAPORATION                      [home]   [topic]

A lot of confusion exists about boiling and boiling points and evaporation.

• Many covalent molecules easily change from liquid to gas - they evaporate. If they affect the nose, they smell. Perfumes smell, alcohol smells, wine smells. These have many covalent molecules. The term given to these molecules is VOLATILE.

• Evaporation is NOT boiling.

• All gases exert a pressure, a force on an area of surface of solids and liquids as they strike them, even the gases of a perfume.

• Boiling occurs when the PRESSURE of the evaporating gas EQUALS THE PRESSURE OF THE SURROUNDING GAS.

  At this pressure, the liquid abruptly changes to gas anywhere in the liquid at a rate decided by the energy ( heat ) input necessary to break the intermolecular bonds of the liquid.

If the liquid and surrounding gas are placed in a gas tight container, then the boiling point of the liquid is raised as it requires more energy to evaporate - to lift its gas pressure to the surrounding pressure. This is the principle of the PRESSURE COOKER. Foods in the boiling water are raised to higher temperature than otherwise so cook faster.

If the liquid is surrounded by gas at a lowered pressure, then the liquid evaporates far more easily, the boiling point is lower.

At Earth's sea level, at normal weather pressures, the air exerts enough pressure to raise a column of water some 10m in an evacuated tube and mercury some 760mm. This is 1013 hectoPascals ( or 101.3kPa or 1.013 x 10⁵ Nm^-2 ) of pressure. Evaporating water molecules ( water vapour ) reach this pressure when heated to 100⁰C, the boiling point temperature of water at sea level.

This point drops 1⁰C every 300m. If we go to the top of Everest, evaporation pressure equals atmospheric pressure at only 70⁰C - not enough to coagulate - cook - an egg!!

MECHANISM of EVAPORATION

• A liquid consists of molecules or atoms loosely held ( bonded ) together.

• The molecules have a spread of kinetic energies.

• The average kinetic energy totaled over the number of molecules is a direct measure of the HEAT contained in the jar of liquid.

• As there is a range, it follows that some molecules near the surface have enough kinetic energy to overcome the forces holding them in a liquid format.

• These escape - they evaporate.

• If they affect the nose, they can be smelt.

PS
vapour - individual molecules in gaseous condition above the liquid from which they are derived. There are always some vapourising from the liquid and recondensing into the liquid.

gas - individual molecules in gaseous condition with NO derivative liquid.