When we boil water, does the generated steam remain at the same temperature of boiling point until all water is boiled? [closed]

Question

Suppose we are boiling water in such conditions so that the boiling point of water be $\pu{100 ^\circ C}$.

• Does the generated steam have the same temperature of 100 C until all water is boiled?
• And only after complete boiling does the temperature of steam start to rise?
• Does it make a difference if we are boiling with a lid on or not?
• Does only water have a temperature of 100 C and the temperature of steam increases even if all water is not boiled?
• In slightly technical terms, is the heat provided to boil the water used only in changing the phase of water or is it also used in raising the temperature of the vapor formed?
• If the temperature of steam does remain constant until all water is boiled the question arises why, why does it remain constant?
• What is stopping the heat to not raise its temperature?
• What forces it to be only used do drive phase change?

Answer 1

Steam (actually, water vapor) above a rapidly boiling can be hotter than the average temperature of the liquid because the situation is not in equilibrium.

Macroscopically, consider that in nucleate boiling the greater part of water in the container is not boiling; only in certain parts of the vessel has the water been superheated, and then flashes into vapor. "When the temperature difference is [sufficient], isolated bubbles form at nucleation sites and separate from the surface... [at a greater temperature difference] the vapor escapes as jets or columns which subsequently merge into slugs of vapor."

Those regions that formed bubbles were above the boiling point of water. Though the temperature of the bubble drops due to rapid expansion, it must still be above 100 °C or it would condense before reaching the surface (That oscillation of bubbles forming and collapsing can be observed at an incipient boil , i.e., liquid entrainment stage of boiling.). Therefore, the vapor immediately released from the bubble can be hotter than the body of the liquid, which can be verified by putting a thermometer in the spout of a steaming kettle and in the liquid.

On a microscopic level, consider the fastest moving particles, i.e, the hottest, are the ones that escape. Of course, that is on average, since there can be areas in the liquid that are superheated and have not yet boiled.

Note, though, that in film boiling, the Leidenfrost effect maintains a layer of exceedingly hot water vapor against the surface of the container, below the level of the water! Of course, that is not liquid water, but it is below the surface of the water... so the answer to your question depends on the rate of boiling and where in the container you measure the temperature.

Answer 2

In addition to the nonequilibrium factors described well by DeMi8she Pippik, we may also consider the behavior when the water is not pure. Assuming we have set up nucle-ation sites and other factors so that the temperature no longer differs measurably from equilibrium, that equilibrium temperature will, in most practical cases, tend to rise as boiling proceeds and the composition of the liquid inevitably changes.

• With nonvolatile solutes, the frugality and therefore vapor pressure of the water will drop with the composition shifting to more solute. In the case of sugar solutions in water, this effect should be familiar to anyone who cooks candies or jams.

• With volatile solutes such as ethanol, the boiling starts off with predominantly the volatile solute at lower than the boiling point os the water alone, and the temperature rises as the liquid composition shifts towards more of the less volatile water component. Here the manufacturing of distilled liquor comes to mind.