# Why boiling occurs at fix temperature?

• Arvind22
In summary, boiling in an open vessel is ordinarily at constant pressure. This is the thermodynamic constraint for constant temperature boiling of a single component system. How could it be anything but fixed? Suppose you boil away half of the water in a pot. What is different about the remaining water from what you started with, that would cause the boiling temperature to be different? How could it be anything but fixed?
Arvind22
Can anyone explain to me why does temperature of water remains constant while boiling in an open vessel or in general sense why phase transition is isothermal in nature?please give me answers from thermodynamic point of view also.

Boiling in an open vessel is ordinarily at constant pressure. This is the thermodynamic constraint for constant temperature boiling of a single component system.

How could it be anything but fixed? Suppose you boil away half of the water in a pot. What is different about the remaining water from what you started with, that would cause the boiling temperature to be different?

russ_watters said:
How could it be anything but fixed?

I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?

mrspeedybob said:
I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?
So, that question boils down to what is a liquid/what is a gas/why are they different states of matter?

They are different because in a liquid the molecules are loosely chemically bonded together and in a gas, they are not. Chemical bonds carry a specific amount of energy and given the same conditions always require the same energy to break.

Arvind22
mrspeedybob said:
I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?
Perhaps i could not phrase my question well so i am writing the exact statement of a textbook which is like this
An equilibrium process in the course of which the system's temperature remains constant is known as an isothermal (constant-temperature) process. An example of an isothermal process is the boiling of pure water in an open vessel: until all the water boils out of the vessel, its temperature remains practically constant (provided atmospheric pressure does not change in the process of boiling).now my question is why in this case temperature remains constant.

What have you learned from the responses you have gotten so far?

Ok guys.i got the point.thank you all of you for helping me.

## 1. What is the scientific explanation for boiling occurring at a fixed temperature?

The scientific explanation for boiling occurring at a fixed temperature is based on the principles of thermodynamics. When a liquid is heated, the molecules gain kinetic energy and move faster. As the temperature increases, the molecules move even faster and eventually break free from the liquid's surface and escape into the air. This process is known as evaporation. At a certain temperature, known as the boiling point, the vapor pressure of the liquid becomes equal to the atmospheric pressure, causing the liquid to rapidly vaporize and turn into a gas.

## 2. Why does the boiling point of a substance vary with altitude?

The boiling point of a substance varies with altitude due to changes in atmospheric pressure. As altitude increases, the atmospheric pressure decreases, which means that the vapor pressure required for a liquid to boil also decreases. This results in a lower boiling point. For example, water boils at 100 degrees Celsius at sea level, but at a higher altitude, such as on a mountain, the boiling point will be lower, around 90 degrees Celsius.

## 3. Can the boiling point of a substance be affected by other factors?

Yes, the boiling point of a substance can be affected by other factors such as the presence of impurities, the type of container the liquid is in, and the strength of intermolecular forces between the molecules of the liquid. Impurities can raise the boiling point of a substance, while a stronger container can increase the boiling point due to increased pressure. Stronger intermolecular forces can also increase the boiling point by making it more difficult for the molecules to escape into the gas phase.

## 4. Why does water have a higher boiling point than alcohol?

Water has a higher boiling point than alcohol because water molecules have stronger intermolecular forces compared to alcohol molecules. This means that more energy is required to break these forces and turn the liquid into a gas. Additionally, water molecules are smaller and have a higher molecular weight than alcohol molecules, making them more difficult to vaporize.

## 5. Is boiling always a physical change?

Boiling is considered a physical change because the substance does not change chemically during the process. While the molecules are moving from a liquid to a gas state, they are still the same substance. However, if the boiling point is reached due to a chemical reaction, then it can be considered a chemical change. For example, when water boils and turns into steam, it is a physical change. But when ethanol (alcohol) is heated and turns into ethene, it is a chemical change.

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