Water Boiling

An open pot of water is boiling on the kitchen stove. Sprinkle some room-temperature table salt (which contains mostly NaCl and some KCl) into the clear boiling water, and the boiling ceases. Isn’t it amazing how the water ceases its boiling as the salt warms up! Can you explain the physics? What is the surprise here?

Answer

The major thermal effect is the raising of the boiling point (i.e., boiling temperature) by the added salt in solution from 100°C (standard conditions at 1 atmosphere) to about 104°C (if the salt is pure NaCl), a significant change that results in a small time lag before boiling begins again if thermal energy input continues.

In contrast, the actual amount of thermal energy needed to raise the temperature of the sprinkled salt itself is minuscule because the specific heat of NaCl is much lower than for water, and the amount of water by weight in the pot is enormous compared to the amount of salt. The actual boiling process is more complicated than the simple, ideal version we have considered here, involving nucleate boiling, transition boiling, etc., in the real situation.  However, the complete analysis produces the same general argument.

One should ask whether different results would occur with sea salt, a mixture of KCl and NaCl and dead organic matter in larger grain sizes than normal table salt. The slow rate of dissolving the larger sea salt grains may delay the reboiling longer than experienced for NaCl.

The relevant physical data are in most chemistry and physics handbooks as well as in some textbooks. The physics pertinent to the idealized case discussed above is part of traditional physics and chemistry courses, but the more detailed complete analysis can be found only in the technical literature.