How does boiling occur

Assume constant temperature. The boiling point on Mt. The molecules leaving a liquid through evaporation create an upward pressure as they collide with air molecules.

This upward push is called the vapor pressure. Different substances have different vapor pressures and therefore different boiling points. This is due to differing intermolecular forces between molecules. Video: Vapor Pressure and Bioling youtu. The vapor pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere. As a result, liquids with high vapor pressures have lower boiling points.

Vapor pressure can be increased by heating a liquid and causing more molecules to enter the atmosphere. At the point where the vapor pressure is equal to the atmospheric pressure boiling will begin. In effect, without any external pressure the liquid molecules will be able to spread out and change from a liquid to a gaseous phase.

The gas, as bubbles in the liquid, will rise to the surface and be released into the atmosphere. Gaithersburg High School. How Does boiling Occur? Figure 1: Formation of bubbles in boiling water. This seems reasonable in open containers because when pressure exceeds 1 atm, then bubbles can form.

However, consider a closed container at lets say 40 degrees celsius. This is too cold for boiling, but the vapor pressure of the liquid must be equal to the pressure exerted by the vapor system will move until this equilibrium is reached.

Why doesn't this water boil? The pressures are the same so bubbles can form and boiling can occur. My fundamental misunderstanding of boiling at the molecular level leads to more related questions:.

When heat is added, why does the temperature rise until boiling point and then all the energy goes towards breaking bonds? In melting water, the vapor pressure of the water and solid are equal so they can both coexist. If this is the case, isn't this technically the triple point?

This can't be of course because melting point is not equal to triple point. Boiling will occur when the equilibrium vapor pressure at the temperature of the liquid is equal to the total pressure of the system and heat is being provided either externally, or by the sensible heat of the liquid itself.

The gas phase in contact with the liquid can be comprised of pure vapor in which case the total pressure is essentially equal to the equilibrium vapor pressure , or, it can be comprised of a mixture of vapor and air in which case the total pressure essentially equal to the sum of the equilibrium vapor pressure of the boiling substance plus the partial pressure of the air.

At 40 C, it is still possible for water to boil in a container if the gas is evacuated to a pressure below the equilibrium vapor pressure at 40 C a non-equilibrium situation. Boiling can continue until the total pressure in the head space rises to a value equal to the equilibrium vapor pressure. As long as the partial pressure of the substance in the vapor and the total pressure if air is present is less than the equilibrium vapor pressure of water at the liquid temperature, boiling will continue.

The heat of vaporization can be supplied by the liquid itself, so the water temperature will be dropping assuming heat is not being directly supplied. Once the equilibrium vapor pressure at the liquid temperature falls below the total pressure, boiling will stop.

Once the partial pressure of the vapor in the gas phase becomes equal to the equilibrium vapor pressure, evaporation will stop. With regard to the question about the melting point and the triple point, the melting point is very close to the triple point. At the triple point, pure water is present in all three phases no air present in the gas phase and the total pressure is equal to the equilibrium vapor pressure of the liquid and solid.

At the melting point, air is present in the gas phase at 1 atm. The difference between the melting point and the triple point is only about 0. I think the question is really pertinent. The temperature at which a liquid boils is affected by atmospheric pressure. The higher the altitude, the lower the temperature at which boiling occurs and the lower the liquid's boiling point. When quoting a boiling point, scientists also quote the air pressure. All standard boiling points are measured at sea level where the air pressure is an average of 1 atmosphere or Different liquids have different boiling points depending on the strength of bonding between the particles and the mass of the particles.

The heavier the particles in the liquid, and the stronger the bonding, the higher the boiling point will be. Boiling point of pure and impure substances For pure substances, the temperature at which boiling and condensing occurs is quite sharp. For impure substances, boiling and condensing occurs more gradually over a range of temperatures. This is one way that chemists can measure the purity of a substance. A pure substance will boil at a specific temperature, the more impure a substance the more its boiling point will vary over a range of temperature.

Evaporation When a liquid is heated, its particles eventually gain enough energy to overcome the forces holding them loosely in place in the liquid and become free, fast moving individual particles in the gas state. Evaporation occurs when particles in a liquid pass directly into the gas phase, at a temperature below the boiling point of the liquid.