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# Boiling Point: Boiling Point, Factors Affecting the Boiling Point, Calculation of Boiling Point, Boiling Point of Water, Practice Problems & Frequently Asked Questions

Do you expect any difference in quicking in plains and hill stations? Yes. Cooking in a hill station takes more time. This gave rise to the invention of pressure cookers to the happiness of cooking lovers. .

When it comes to pressure cookers, have you ever wondered how they work?

In practically every kitchen, pressure cooking is the most popular method of cooking. Water heated inside a pressure cooker eventually boils into steam. It cooks the food using steam pressure. Since it is closed vessel, when heated, the temperature within the cooking pot rises, trapping the vapours that rise from the liquid water and increasing the pressure inside the pressure cooker, substantially speeding up the cooking process. Cooking, as we know, entails raising the temperature of food, which causes chemical reactions such as breaking down tough tissues in meat or softening the starch in vegetables. The best thing about pressure cooking is that it saves cooking time while simultaneously keeping nutrients intact.

• Boiling point
• Factors affecting boiling point
• Calculation of boiling point
• Boiling point of water
• Practice problems

## Boiling point:

The temperature at which a liquid's vapour pressure matches the atmospheric pressure is known as the boiling point of the substance.

The average kinetic energy of a liquid increases with temperature, as do the rates at which liquid molecules are ejected from the surface into the gas phase. The liquid's molecules finally build up enough kinetic energy to vaporise all of them. The liquid starts to boil at this stage, and the vapour pressure reaches air pressure. The boiling point of a liquid is the temperature at which this phenomena happens.

## Factors affecting boiling point:

1. Temperature effect on boiling point:

Kinetic energy of molecules is directly dependent on the temperature. Higher the kinetic energy of the molecules, higher will be those forming vapour and nearing the atmospheric pressure and nearer to their boiling point.. The boiling point of a liquid can be quickly reached by increasing the temperature.

During boiling, a liquid's temperature doesn't change. This is due to the fact that water is heated externally during boiling, which causes water to vaporise (the latent heat of vaporisation).

As a result, water will remain liquid at 100o C if certain parameters are met, such as a pressure of 1 atm or 101.3 KPa during boiling. The temperature will only rise once the water has been completely transformed to vapour.

2. Molecular Interactions in the liquid - effect on boiling point:

Molecules of a liquid interact with each other because of Brownian motion. Molecules inside the liquid are surrounded by molecules on all sides, while molecules at the surface will not have any of its molecules to interact above. So, molecules at the surface experience less interactions than molecules inside the liquid. This allows surface molecules to escape out into the atmosphere. Number of molecules escaping out at a particular time depends on the

1. Force of interactions between molecules and
2. the pressure above the surface (atmospheric pressure)

Liquids differ from each other by the interactive forces between them. Naturally then, liquids having larger interactive forces try to remain in the liquid state, allowing only a fraction of molecules to escape. Some of the molecules in the vapour phase also may condense to liquid form. After some time the number of molecules escaping from liquid and number molecules condensing from vapour become equal, establishing an equilibrium. The equilibrium pressure is known as vapour pressure of the liquid at that particular temperature. Lesser the interactions-higher the molecules escaping as vapours-higher the vapour pressure -lower the boiling point and vice versa.

3. Atmospheric pressure effect on boiling point:

Atmospheric pressure in the case of open vessel exert pressure on the liquid surface suppressing the formation of liquid vapour. In other words, as pressure rises, a substance's boiling point rises as well. A liquid needs more energy to boil when the pressure is higher. The liquid's boiling point rises as a result. Similar to this, a liquid requires less energy to change into its vapour state as the pressure decreases. As a result, we can state that the boiling point decreases as pressure falls.

Higher the atmospheric pressure-lower the formation of vapour-higher the boiling point and vice versa. In hill stations, the atmospheric pressure is lesser than in plains, allowing more liquid molecules to escape and equal the atmospheric pressure at lower temperatures. This makes the liquid boil at lower temperatures in hill stations compared to plains.

We may therefore state that as atmospheric pressure rises, so does the boiling point of water. The boiling point of water also reduces as atmospheric pressure drops. As a result of the lower atmospheric pressure, water will have an extremely low boiling point at high elevations.

Do you know why using a pressure cooker makes cooking simpler? This is due to the pressure within the pressure cooker rising as a result of the steam that has been trapped there. The boiling point rises as a result. Similarly, a substance's boiling point drops as pressure drops.

## Calculation of boiling point:

The heat of vaporisation (the energy necessary to turn a given quantity of a substance from a liquid into a gas at a certain pressure) is involved during the transformation of liquid into its vapours (during boiling).

As a result, if we know the heat of vaporisation and vapour pressure of a liquid at a certain temperature, we may calculate its boiling point by using clausius clapeyron equation

Where TB= Boiling point at the desired pressure

T0= Boiling temperature

R= universal gas constant

Vapourisation of the liquid

P= vapour pressure of the liquid at the desired pressure

P0= vapour pressure corresponding at To

## Boiling point of water:

The term "standard boiling point" refers to a substance's boiling point under standard atmospheric pressure. As a result, we can now state that water boils at 100 degrees Celsius on average.

The typical boiling point of water is 373.15 K on the Kelvin scale.

It is generally believed that water has a high boiling point. This is due to the fact that water molecules contain hydrogen bonds. In other words, a compound's boiling point rises when hydrogen bonds are present.

Any liquid's boiling point will vary depending on its temperature, pressure, and vapour pressure. For instance, a liquid's boiling point is lower in a partial vacuum than it is at atmospheric pressure. A liquid's boiling point will also be greater under high pressure than it would be under atmospheric pressure.

Elevation of Boiling point:

Boiling point of pure liquid is characteristic of the forces between the molecules and hence the liquid. But the boiling point of solutions is a colligative property changing with the concentration of the number of nonvolatile solute particles in it. The dissolved solute particles interact with the solvent liquid reducing the escaping tendency of the solvent molecules and there by increase the boiling point of the solution. The phenomenon is called as elevation of boiling point.

The increase or elevation of boiling point of the solution from that of the pure solvent is referred as ΔTb.

ΔTb = i Kb m: where i is the vant’ Hoff factor related to the number of particles formed from one molecule of the solute, Kb is the ebullioscopic constant unique for each solvent and m is the molality of the solution.

## Practice problems:

Q.1. When non-volatile impurities contaminate a liquid, its boiling point gets.

(A) Elevated
(B) Remains same
(C) Depressed
(D) None of the mentioned

Solution: When non-volatile contaminants pollute a liquid, its boiling point rises. This is a very important colligative characteristic. Salt water, for example, boils at a greater temperature than pure water.

Q.2. A liquid's normal boiling point is the temperature at which it boils when external pressure is

(A) 1 atm
(B) 2 atm
(C) 3 atm
(D) 4 atm

Solution: A liquid's normal boiling point is the temperature at which it boils when the external pressure is one atmosphere. The boiling point of a liquid changes with the surrounding atmospheric pressure.

Q.3. Select the appropriate statement for a given substance.

(A) Molar heat of vapourization$\left({\Delta H}_{vap}\right)$is greater than molar heat of fusion $\left({\Delta H}_{fus}\right)$
(B) Molar heat of fusion$\left({\Delta H}_{fus}\right)$ is greater than molar heat of evaporation $\left({\Delta H}_{vap}\right)$
(C) Molar heat of vapourization$\left({\Delta H}_{vap}\right)$ is equal to molar heat of fusion $\left({\Delta H}_{fus}\right)$
(D) None of the above

Solution: It is important to note that the heat of vaporisation is significantly greater than the heat of fusion for all substances. It takes far more energy to convert a liquid to a gas than it does to convert a solid to a liquid. This is due to the huge particle separation in the gas state. The heats of fusion and vaporisation are proportional to the strength of the intermolecular forces.

Q.4. The molar heat of vaporisation $\left({\Delta H}_{vap}\right)$ and boiling point depend on

(A) Size of molecules
(B) Composition of the liquid compound
(C) Strength of the intermolecular forces
(D) All of these

Solution: The strength of the intermolecular forces affects the molar heat of vaporisation $\left({\Delta H}_{vap}\right)$ and boiling point. The boiling point of a liquid changes with the surrounding atmospheric pressure.

Q1.What happens when salt is added to water?
When salt is added to water, the intermolecular interactions between water molecules change as a result of the breakdown of NaCl into sodium ions (Na+) and chloride ions (Cl). The hydrogen bonding between water molecules is disrupted, and an ion-dipole interaction between sodium ions, chloride ions and a dipole (water) begins. The interaction between ions and dipoles is a relatively strong bond. As a result, more heat is required by water molecules to come out as a gaseous phase and evaporate, resulting in an increase in the boiling point of water.

Also, when the salt is dissolved in water, the water molecules require more energy to generate enough pressure to escape the liquid's boundary. The more salt added, the higher the rise in the boiling point.

Q2.Why is cooking tough in the hills?
High altitude air pressure is lower than low altitude air pressure. The air pressure is highest at sea level. The quantity of air molecules in the air reduces as altitude increases, and as the number of air molecules falls, so does the pressure.

The movement and collision of gas particles in the air causes air pressure. The number of gas-particles decreases as altitude increases, and hence the collision between the gas-particles lowers as well. As a result, the pressure drops.

The boiling temperature of water now lowers as pressure increases. Water boils when the vapour pressure of the water exceeds the air pressure. The boiling temperature of water near the earth's surface is 100o C. However, as altitude increases, air pressure lowers, and so the temperature required to exceed vapour pressure decreases. As a result, the boiling temperature of water lowers at high altitude.

So, we may argue that boiling food takes considerably longer in the hills than in the plains because the atmospheric pressure is lower in the hills than in the plains, so water boils at a temperature lower than 100o C, creating an increase in cooking time.

Q3.What is the purpose of ethylene glycol as an antifreeze?