Basics of Boiling
Boiling occurs through bubble formation, and the bubbles are formed when atoms or molecules of liquids spread out enough to change from its liquid phase to gaseous phase.
The most important factors affecting the boiling point of a liquid are atmospheric pressure, and the vapor pressure of the liquid.
Vapor Pressure and Boiling
During evaporation, the molecules which leaves a liquid creates an upward pressure because they collide with air molecules, and this upward pressure is known as vapor pressure. Intermolecular forces between different molecules are different. Thus, different liquids possess different vapor pressure, and hence, have different boiling point. The liquids with high vapor pressures have lower boiling point, and it can be increased by heating a liquid and causing more molecules to enter the atmosphere. When vapor pressure equals to atmospheric pressure boiling begins as discussed earlier.
Atmospheric Pressure and Boiling
Atmospheric pressure directly affects the boiling point. Atmospheric pressure is defined as the pressure exerted by the weight of air molecules above the liquid in the open system. It can be visualized as air molecules colliding with the surface of the liquid, which creates pressure. This pressure, throughout the liquid, makes the bubble formation difficult, which in turn affects the boiling. As elevation increases, atmospheric pressure decreases because at higher altitudes air is less dense. This decrease in atmospheric pressure, in turn lowers the boiling point. This is the reason that the range of the boiling point of water varies from 70 °C to over 101 °C ; from the highest land point above sea level, i.e., Mount Everest to the deepest point, i.e., the dead sea.
Boiling Point Elevation
When the boiling point of a solution is higher than the boiling point of a pure solvent, boiling point elevation takes place. This is because when solute is added to the solvent, vapor pressure of the solution becomes less than the vapor pressure of pure solvent, and hence, the boiling point of a solution will be greater than the boiling point of the pure solvent. It is a colligative property that means it depends on the number of particles present in a solution and not on the type of particles and their mass. The most common example of this phenomenon is that the boiling point of water is increased by adding salt to the water. Basically, it is the temperature change (rising) of the boiling point of solvent caused by adding a solute, and it is calculated by using a formula, which is described in the following image.
Boiling point of Water
You may have heard that water always boils at 100°C, but this is not completely true. The boiling point of water varies at various locations. It varies from 72°C to 101°C accordingly from the highest point to the lowest point on land. The reason for these variations is the lowering of atmospheric pressure as we travel to the highest point such as mountains from lowest land point, i.e., Dead sea. The following chart shows different boiling temperatures of water.
1) Pressure Cookers
Pressure cooking is the most common method of cooking in almost every kitchen. Also, it is the most common example of boiling in everyday life. Inside a pressure cooker, the water is heated, and eventually it boils into steam. It uses the pressure of steam to cook the food. When heated, the temperature increases inside the cooking pot, which traps the vapors that rises from the liquid water, which further increases the pressure inside the pressure cooker that significantly speeds up the cooking process. As we know that the cooking involves raising the temperature of food, which in turn triggers the chemical reactions such as breaking down the tough tissues in meat, or soften the starch in vegetables. The best part of pressure cooking is that it reduces the cooking time and also keeps the nutrients intact.
2) Cooking with Salt
Salt is a wonderful ingredient in our kitchen. It not only adds flavor to our food but also raises the water’s boiling point. This is an example of boiling point elevation. The chemistry behind this is that when salt, i.e., sodium chloride, is added to water, it dissociates into sodium and chloride ions. These ions alter the intermolecular forces between water molecules. Also, even without charged solute, adding any solute to water raises its temperature because of boiling point elevation. The more salt you add, the more you are raising the boiling point because it depends on the number of particles formed in the solution as it is a colligative property.
3) Sugar Refining
When the sugarcane juice is extracted, it must be refined to obtain crystalline sugar. Cane juice or syrup is boiled at some stages, and the temperature at which it boils depends on the sugar concentration. The important step of the sugar crystallization is the pan boiling in vacuum pans of the boiling house. The main function of this step is to produce and develop optimum sized sugar crystals from the syrup. It is accomplished by boiling the clarified mother liquor (which is the left over solution after crystallization) in specially designed heat exchangers known as vacuum pans, and the mother liquor is concentrated by boiling under vacuum of 25-26″ of mercury using exhaust vapors. Vacuum pans contain a large closed kettle with steam heated pipes.
Antifreeze is basically an additive that, when added to water-based fluid, reduces the freezing point of the mixture. Ethylene glycol is the most common antifreeze, which is used in the cooling of a car radiator during the winter season because it reduces the freezing point of water and in the summer, it serves as a coolant. Its role in an automobile is to absorb heat from the engine. When the temperature of the coolant increases to its boiling point, the system boils over, and therefore, a coolant is effective. It elevates the boiling point of the fluids, and it prevents boil overs.
5) Boiling Milk
Have you ever wondered that why does milk boil after over heating but water does not? Well, the answer to this question is that water is a simple liquid, which does not contain any solids, whereas milk is a compound, which contains fat in emulsion form, protein in a colloidal state, and lactose as true solution. When milk is heated, the fat, which is lighter than water, is collected on the surface along with protein in form of cream, and when milk is overheated, the water vapors expand, which builds up pressure and lifts the creamy layer up, and eventually, milk spills out. Although the boiling points of both milk and water are somewhat closer, the boiling point of milk is slightly higher because of boiling point elevation. As discussed above when solute is also present with pure solvent (here in case of milk, fats, proteins in milk act as a solute), the boiling point of solution increases.
6) Storage of Chemicals
The knowledge of the boiling point of a chemical is very important for its storage and transport. It is never advised to store or transport a liquid at a temperature close to or above its boiling point as boiling may cause its leakage, leading to severe consequences. The special care must be taken in case of flammable and combustible liquids.
7) Poor Cup of Tea at Mountains
You may have heard that it is hard to make decent cup of tea or coffee at mountains. This is because as we move towards higher altitude, the atmospheric pressure is lower as compared to sea level, and hence, water boils at a temperature lower than 100-degree celsius. Thus, water vapors evaporates at faster rate, which leads to poor cup of tea at high altitudes.
8) High Altitude Cooking
Mostly, 3000 feet above sea level is considered as high altitude, but at 2000 feet above sea level, the boiling temperature of water is 208 °F instead of 212°F. Above 2500 feet, the air has less oxygen and atmospheric pressure, so cooking takes longer time because the moisture present in the food evaporates The decreases in atmospheric pressure affects food cooking in following ways:
As high altitude areas are more prone to low humidity , moisture in food evaporates quickly.
Due to low atmospheric pressure, boiling takes place at low temperatures.
The leavening gases in bread and cakes expand more quickly.
Meat and poultry products contain muscle, connective tissues, fats, and bone. The muscles contain high water content, and due to this high water content, meat is more susceptible to drying out while cooking at high altitudes. Approximately, it takes one-fourth more time for cooking at high altitudes as compared to sea level.
Egg cooking is also affected, especially those cooked in boiling water such as poached and hard cooked eggs because of low boiling temperature.