# Why the temperature remains constant at the boiling point?

• Tahmeed
In summary, the reason why all liquids vaporize at their boiling point instead of gradually increasing in temperature is due to the equilibrium between atoms transitioning from liquid to gas and gas to liquid. Any slight increase in temperature will lead to more atoms leaving the liquid, which cools it down and maintains the boiling point. This process is more rapid at boiling point because bubbles can form throughout the liquid, rather than just at the surface. Additionally, the amount of liquid present does not affect this process, as the system is not necessarily in equilibrium unless the relative humidity is at 100%.
Tahmeed
Why all liquids vaporize on boiling point instead of the temperature gradually increasing along with vapor pressure and more liquid atoms evaporating? Does it have anything to do with formation of vapor bubbles?

To a good approximation, the system is in equilibrium between atoms going from liquid to gas and atoms going from gas to liquid. If the temperature would rise a tiny bit, we would quickly have more atoms leaving the liquid, taking away energy, and cooling it down to the boiling point again. Same with the opposite direction.

Why would you expect a dependency on the amount of liquid present?

The rate of evaporation does increase gradually as you raise the temperature. Vaporization is much more rapid at boiling since bubbles can form throughout the liquid, whereas below the boiling point, vaporization only happens at the surface.

To contradict mfb, there's no reason to believe the system is close to equilibrium. If the liquid is slowly evaporating, then it's not in equilibrium. In case of water, there is equilibrium when the relative humidity is 100%, and water no longer evaporates. Anything less than 100% relative humidity is not equilibrium.

This thread may help explain the difference between evaporation and boiling:

You have 100% relative humidity (actually, 100% water vapor) above the surface if your liquid is at boiling temperature. If you keep adding heat there is a net migration from liquid to gas, but that is tiny compared to the overall equilibrium reaction.

Oh I see you were answering the question in the title. I didn't even notice that question, so I was just answering the question in the body.

## 1. Why does the temperature remain constant at the boiling point?

At the boiling point, a substance is undergoing a phase change from liquid to gas. During this process, the energy being added to the substance is used to break the intermolecular bonds and convert the liquid molecules into gas molecules. As a result, the temperature remains constant until all of the liquid has been converted to gas.

## 2. How does atmospheric pressure affect the boiling point?

The boiling point of a substance is directly related to the atmospheric pressure. As the atmospheric pressure increases, the boiling point also increases. This is because higher pressure requires more energy to overcome the intermolecular bonds and cause the substance to change into a gas.

## 3. Can the boiling point change?

Yes, the boiling point can change depending on the surrounding conditions. As mentioned before, atmospheric pressure can affect the boiling point. Additionally, the composition of the substance can also impact the boiling point. For example, adding salt to water increases the boiling point.

## 4. Why is the boiling point different for different substances?

The boiling point is different for different substances because it is determined by the strength of the intermolecular forces within the substance. Substances with stronger intermolecular forces, such as water, will have a higher boiling point compared to substances with weaker intermolecular forces.

## 5. How do impurities affect the boiling point?

Introducing impurities into a substance can affect its boiling point. This is because the impurities disrupt the intermolecular bonds and make it easier for the substance to change into a gas. As a result, the boiling point is lowered compared to a pure substance.

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