Boiling Point Formula

The boiling point of a substance is the temperature at which the substance changes state from liquid state to gas state. The boiling point of a liquid varies depending on the surrounding atmospheric pressure, but it is usually close to the melting point of the solid form of the same substance. 

How is Boiling Point Determined?

Boiling point of a substance can be determined by using a Boiling Point Apparatus. This method uses the fact that liquids boil when their vapor pressure equals the atmospheric pressure. The apparatus consists of a container of liquid, a heater, and a mercury manometer. First, the liquid is placed in the container and heated until its vapors fill the container. The mercury manometer is then used to measure the atmospheric pressure inside the container. When the mercury column in the manometer reaches the same level as the liquid in the container, this indicates that the vapors have reached atmospheric pressure and the boiling point has been reached.

Boiling point of Water

Boiling point of water is the temperature at which the liquid water becomes water vapor. The boiling point of water is 212 degrees Fahrenheit using Fahrenheit to Celsius conversion (100 degrees Celsius). The boiling point of water is also known as the “saturation temperature.”

Boiling Point Formula of Liquid 

The boiling point of a liquid is the temperature at which the liquid changes state from a liquid to a gas. The boiling point of a liquid is also the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. The boiling point of a liquid is affected by the atmospheric pressure, which is why boiling points are usually given at standard atmospheric pressure.

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure.

The boiling point depends on the pressure of the surrounding atmosphere. When the atmospheric pressure is equal to the vapor pressure of the liquid, the liquid will boil. The boiling point also depends on the purity of the liquid, impurities lower the boiling point of a liquid.

Example: Water boils at 100 degrees Celsius (212 degrees Fahrenheit) at sea level, but only at 93 degrees Celsius (199.4 degrees Fahrenheit) at an altitude of 2,438 meters (8,000 feet).

The formula for calculating the change in the boiling point is:

K_b = RT_b2M / ΔHv

where,
K_b is the boiling point constant
R is the universal gas constant (8.314 J/mol-K)
T_b is the boiling point of the liquid
M is the molar mass of the liquid
ΔH_v is the heat of vaporization for the particular liquid (2260 J/kg: latent heat of vaporization of water)

Properties of Boiling Point

Following are the properties of the boiling point:

• Boiling point of a liquid varies depending on the surrounding pressure. For example, water boils at 100°C (212°F) at sea level, but only at 93°C (199°F) at an altitude of 1,000 m (3,281 ft).
• Boiling point of a liquid can also be affected by the presence of other substances in the liquid. For example, the boiling point of water is lower when salt is added to it.
• Boiling point of a liquid is a measure of the liquid’s ability to change state from a liquid to a gas. The higher the boiling point, the greater the liquid’s ability to resist changes in state.
• Boiling point of a liquid is also a measure of the liquid’s ability to vaporize. The higher the boiling point, the greater the liquid’s ability to resist vaporization.
• Boiling point of a liquid is a function of the liquid’s vapor pressure. The vapor pressure is the pressure exerted by the vapor of the liquid on the walls of the container.
• Boiling point of a liquid is also a function of the liquid’s volatility. The volatility of a liquid is a measure of the liquid’s ability to vaporize.
• Boiling point of a liquid is affected by the surrounding pressure. The higher the surrounding pressure, the higher the boiling point.

Factors Affecting the Boiling Point of Liquid

The boiling point of a liquid is the temperature at which it changes from a liquid to a gas. The factors that affect the boiling point of a liquid are its pressure, temperature, and purity.

• Purity: The higher the purity of a liquid, the higher its boiling point. This is because impurities lower the vapor pressure of a liquid, making it easier for the liquid to boil.
• Pressure: The boiling point of liquid increases as the pressure on the liquid increases. This is because the vapor pressure of a liquid increases as the pressure on the liquid increases.
• Temperature: The boiling point of a liquid also increases as the temperature around the liquid increases.

How to Find the Boiling Point of a Substance?

To determine the boiling point of a substance, we need to heat the substance until it reaches its boiling point. The boiling point of a substance is defined as the temperature at which the liquid changes its state to gas form. At this temperature, the vapor pressure of the liquid is equal to the atmospheric pressure.

We use a thermometer to measure the temperature of the substance as it is heated. As the substance approaches its boiling point, we will notice that the rate of heating will begin to slow down. This is because the heat is being used to convert the liquid into a gas, rather than increase the temperature of the liquid.

Once the substance reaches its boiling point, it will begin to boil. You can then remove the thermometer and checked the temperature to get the boiling point.

Solved Examples on Boiling Point Formula

Example 1: Find the K_b of LiCl if 2 g of LiCl is dissolved in 500 g of water which boils at 100.6°C. 

Solution: 

Given,
Mass of LiCl = 2 g
Molar mass of LiCl = 42 g/mol
Mass of water = 500 g
Molar mass of water = 18 g/mol
Boiling point of solution = 100.6°C

R = 8.314 J/mol-K (Universal gas constant)
T_b = 100.6 + 273 = 373.6 K (Boiling point in Kelvin) 
V = 500 / 1000 = 0.5 L (Volume in litres) 
M = (2 / 42) + (500 / 18) = 25 mol/L (Molality) 
H_v= 2260 J/kg  (latent heat of vaporization of water)

K_b = RT_b2M / ΔHv

      = 8.314 × 373.6 × 0.5 × 25 / 2260

      = 68.7 molal

Example 2: Find the boiling point of 1 molar solution of sucrose.

Solution:

Given,
Molarity of solution (M) = 1 mol/L
Boiling point elevation (ΔT_b) = 100°C (the Kb for sucrose is 100°C/m) 
R = 8.314 J/mol-K (universal gas constant)

T_b = 100 + 273 = 373 K (boiling point in Kelvin)

K_b = RT_b2M / ΔHv

      = 8.314 × 373 × 2 × 1 / 2260

      = 2.74 molal

Example 3: Find the K_b of the glucose if the solution of glucose in water boils at 103°C when the concentration of the solution is 0.5 molar.

Solution:

Given,
Concentration of solution (c) = 0.5 molar
Boiling point elevation (ΔT_b) = 103°C (the Kb for sucrose is 100°C/m) 

R = 8.314 J/mol-K (universal gas constant)

T_b = 103 + 273 = 376 K (boiling point in Kelvin)

K_b = RT_b2M / ΔH_v

      = 8.314 × 376 × 2 × 0.5 / 2260

      = 1.38 molal

FAQs on Boiling Point

Question 1: What is the boiling point of the liquid?

Answer:

The boiling point of a liquid is the temperature at which the liquid changes state from a liquid to a gas.

Question 2: What factors affect the boiling point of a liquid?

Answer:

The boiling point of a liquid is affected by the pressure of the surrounding atmosphere, the purity of the liquid, and the presence of any dissolved solids.

Question 3: How does the boiling point of a liquid change with altitude?

Answer:

The boiling point of a liquid decreases with the rise in altitude. This is because the atmospheric pressure is lower at higher altitudes, and the boiling point of a liquid is inversely affected by the pressure of the surrounding atmosphere.

Question 4: How does the boiling point of a liquid change with the addition of solids?

Answer:

The boiling point of liquid increases with the addition of solids. This is because the solids take up space in the liquid, and the liquid must heat up to a higher temperature in order to vaporize.

Question 5: What is the boiling point of alcohol?

Answer:

The boiling point of alcohol is 78.37 degrees Celsius (172 degrees Fahrenheit).

Question 6: What is the boiling point of mercury?

Answer:

The boiling point of mercury is 356.7 degrees Celsius (673 degrees Fahrenheit.