What is happening when something boils? 

Everyday examples of boiling is, boiling milk, heating water. One would have observed that when we heat water it goes through various stages and at one point bubbles show in water, and water keeps splashing with bubbles bursting, we in layman terms say that water is boiling.  

When we heat a liquid, and its vapor pressure becomes equal to the pressure of the gas above the liquid surface, then we say the liquid is undergoing boiling phenomena, or simply put the liquid is boiling. 

Boiling

Vapor pressure

When we keep a liquid in a container, it will start evaporating and the gas particles of the liquid will exert pressure on the liquid state. After sometime the liquid state and gaseous state will reach equilibrium. The pressure exerted by the vapour at this equilibrium state on the liquid state is known as vapor pressure. 

Properties of Vapor Pressure

Effect of Temperature: With increase of temperature, amount of vapour generation increases inside a closed container. Hence vapor pressure increases. This can be attributed to an increase in kinetic energy of molecules of liquid, and when this energy becomes greater than the intermolecular forces of attraction, they skip into the gaseous phase. It occurs when vapor pressure is less than the liquid pressure. 

Common Mistake: 

People sometimes confuse boiling with evaporation. The mechanism by which water changes from the liquid state to gas or vapour is known as evaporation. Whenever the rate of evaporation crosses the rate of condensation, net evaporation takes place. Evaporation is a natural process unlike boiling which is an unnatural process. In boiling, heat is supplied continuously to liquid. 

Types of Boiling 

Pool boiling: This occurs when bulk fluid is not available. The motion of fluid is due to natural convection currents and buoyancy is the cause for bubble movements. 

Flow boiling: When there is bulk fluid movement then it is known as flow boiling. Here the liquid is forced to move in the heated pipe through some external drive force. For example, a pump. 

Subcooled boiling: When the temperature of the main body of the liquid is lower than the saturation temperature. 

Saturated boiling: When the temperature of the liquid is equal to the saturation temperature. 

Stages of boiling: When we boil water in the container, it goes through various stages, which is difficult to observe. The stages are, natural convection boiling followed by nucleate boiling then transition boiling and film boiling, the whole process is known as pool boiling.  

Note: Boiling passes through different stages depending on temperature difference between saturation temperature and the boiling temperature, ∆T = T_s - T_sat 

Natural convection: Once the liquid reaches a few degrees above the saturation temperature around 2 ℃to 4 ℃ for water, the movement of water is as per natural convection currents. The transfer from container to the water is through a natural convection process. The process A to B shows this. Here the isolated bubbles are formed. 

Nucleate boiling:  In this stage the rate of formation of bubbles increases and nucleation sites increases as well. On the curve it can be noticed between B to C. In this stage large numbers of continuous columns of vapor in the liquid. 

In the A to B zone, heat transfer coefficient increases due to formation of pockets of bubbles on the heating surface and evaporation effects. But the rate of increase of heat flux decreases after point B. 

At C the heat flux is called critical heat flux, which is the maximum heat flux. Beyond point C the heat flux decreases. This is mainly due to formation of a layer of vapour film over the heating surface between container and liquid, which acts as an insulation for the heat to pass through. As we know heat transfer rate through vapor is very low. 

Transition Boiling: When the excess temperature increases past point C, there is a decline in heat flux. This is due to the major portion being covered by vapor film, which acts as an insulation surface. In these zones, one can observe both nucleate and film boiling. 

Film Boiling: After point D the container surface is covered with continuous stable vapor film. At this point, heat flux is minimum and is known as Leidenfrost point. 

Beyond this point there is a steep increase in heat flux. 

Increment of Heat Transfer in Pool Boiling:

• Rate of heat transfer in the nucleate zone depends on the number of activated nucleation sites on the surface and rate of bubble formation at every site. 

• Irregularities over the surface which may be due to roughness and dirt, becomes the site of additional nucleation. 

Flow Boiling:

In this type of boiling, an external source is used to move the water as it undergoes a phase change process. It can be external or internal boiling. 

During external boiling the critical heat flux and the nucleate boiling heat flux increase as velocity increases.Based on the respective proportions of the liquid and vapour phases, two-phase flow in a conduit experiences various flow boiling regimes. Such as Liquid single-phase flow, bubbly flow, slug flow, annular flow, mist flow and vapour single phase flow. 

Application of Boiling 

Antifreeze: Via freeze-point depression, ethylene glycol helps keep the temperature in your vehicle's radiator from freezing. However, you may not be aware that it even raises the boiling point of the fluid. It helps to prevent boil-overs by increasing the boiling point. Many antifreeze brands will list both the amount of boil-over protection and the amount of freeze-up protection given. 

Cooking: Pressure cooker employs application of boiling. It's also worth noting that, contrary to popular belief, applying salt to water would not cause it to boil faster. On the contrary, since the boiling point has already been raised, it would take much longer to boil. 

It is difficult to make a good cup of tea or coffee in the mountains. This is due to the fact that as we ascend in altitude, the air pressure decreases in comparison to sea level, causing water to boil at temperatures colder than 100 degrees Celsius. As a result, water vapour evaporates at a higher rate, resulting in a mediocre cup of tea at high altitudes. 

Practice Question  

Why is the cooking time lower in a pressure cooker compared to normal utensils? 

Answer: In a pressure cooker, water boils at a higher temperature and hence it takes less time since cooking takes place at a higher temperature. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for       

• Bachelors 
• Masters