The key difference between flash point and boiling point is that the term flash point is applied for a volatile liquid, whereas the term boiling point can be applied for any liquid.
Flashpoint and boiling point are two terms we use regarding the liquid state of substances. Flashpoint specifically applies for volatile liquids because it is the lowest temperature at which vapour of the volatile liquid can undergo ignition. On the other hand, the boiling point is the temperature at which the vapour pressure of a liquid equals the external pressure surrounding the liquid. Every liquid has a boiling point, but only volatile liquids have a flash point.
CONTENTS
1. Overview and Key Difference
2. What is Flash Point
3. What is Boiling Point
4. Side by Side Comparison – Flash Point vs Boiling Point in Tabular Form
5. Summary
What is Flash Point?
Flash point is the lowest temperature at which vapour of the material will ignite when given an ignition source. We often confuse with fire point and flash point, thinking both are the same. But, fire point gives the lowest temperature at which the vapour of a substance can keep burning when we remove the ignition source, which is completely different from the definition of flash point.
Figure 01: Flaming Cocktails with a Flash Point Lower than Room Temperature
When considering the ignition of vapour, at the flash point, there is enough vapour to induce ignition when we supply an ignition source. A volatile liquid has a unique concentration of flammable vapour, which is necessary for sustaining the combustion in air.
If we are to measure the flash point of a substance, there are two methods: open cup measurement and closed cup measurement. Furthermore, the methods of determining the flash point are specified in many standards.
What is Boiling Point?
Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the external pressure surrounding the liquid. Hence, the boiling point depends on the atmospheric pressure. Here, we can observe a higher boiling point at higher external pressure. Usually, water boils at 1000C. Since the atmospheric pressure is lower at higher altitudes, water will boil between 80 0C – 90 0C. This will cause undercooked meals.
Figure 02: Boiling Water
The boiling of liquid occurs when the temperature of the liquid exceeds its saturation temperature at the corresponding saturation pressure. The saturation temperature is the temperature corresponding to the highest thermal energy the liquid can hold without changing its state to vapour at the given pressure. The saturation temperature is also equivalent to the boiling point of the liquid. Boiling occurs when the thermal energy of the liquid is enough to break the intermolecular bonds. The normal boiling point is the saturation temperature at atmospheric pressure. The boiling point varies only between the triple point and critical point of the liquid.
What is the Difference Between Flash Point and Boiling Point?
Flash point is the lowest temperature at which vapour of the material will ignite when given an ignition source. Boiling point is the temperature at which the vapour pressure of a liquid equals the external pressure surrounding the liquid. So, the key difference between flash point and boiling point is that every liquid has a boiling point, but only volatile liquids have a flash point.
Moreover, at the flash point of a liquid, we can observe ignition above the liquid while at the boiling point, we can observe the formation of bubbles inside the liquid. Hence, this is a notable difference between flash point and boiling point. If we look at their mechanisms, ignition of flammable vapour occurs in the presence of an ignition source at flash point, when there is enough vapour to induce ignition. However, at boiling point, the vapour pressure of a liquid becomes equal to the external pressure surrounding the liquid.
Summary – Flash Point vs Boiling Point
Flash point and boiling point have several important differences between them. The key difference between flash point and boiling point is that the term flash point is applied for a volatile liquid, whereas the term boiling point can be applied for any liquid.
Reference:
1. Helmenstine, Anne Marie. “Definition of Boiling Point in Chemistry.” ThoughtCo, May. 7, 2019, Available here.
2. “Boiling Point.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 20 Feb. 2018, Available here.
Image Courtesy:
1. “Flaming cocktails” By Nik Frey (niksan) – (CC BY 2.5) via Commons Wikimedia
2. “194034” (CC0) via Pexels