How to Calculate Elevation of Boiling Point with Formula and Example
Elevation boiling point is an important physical chemistry concept that helps students understand how the properties of solutions change when a solute is added. It is widely used in exam problem solving, laboratory experiments, and even in daily life scenarios. Mastering the topic of elevation of boiling point is essential for students preparing for competitive exams like NEET, JEE, and board examinations.
What is Elevation Boiling Point in Chemistry?
The elevation of boiling point refers to the phenomenon where the boiling point of a pure solvent increases upon the addition of a non-volatile solute. This concept is a classic example of colligative properties. It appears in solution chemistry topics such as vapor pressure lowering, boiling point elevation, and freezing point depression, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The elevation boiling point does not represent a single molecule but is expressed mathematically. Its standard formula is: ΔTb = i × Kb × m, where:
- ΔTb = elevation in boiling point (in °C)
- i = van't Hoff factor (accounts for solute ionization)
- Kb = ebullioscopic constant (depends on the solvent)
- m = molality of the solution (mol/kg solvent)
Preparation and Synthesis Methods
Elevation of boiling point occurs naturally when a non-volatile solute like salt or sugar is dissolved in a pure solvent, such as water or benzene. In school and college laboratories, students often demonstrate this phenomenon by heating pure water and then a salt solution, noting the increase in boiling temperature. Industrially, this property is used in processes requiring precise boiling point control, such as distillation of solutions.
Physical Properties of Elevation Boiling Point
The key physical aspect of elevation boiling point is the rise in boiling temperature relative to the pure solvent. For example, water (Kb = 0.512°C·kg·mol-1) boils at 100°C at 1 atm, but a 1 molal salt solution boils above 100°C. The actual value of boiling point elevation depends on molality, the van’t Hoff factor, and the specific solvent’s Kb value.
Chemical Properties and Reactions
No direct chemical reaction causes elevation boiling point. Instead, dissolving a non-volatile solute lowers the solvent’s vapor pressure due to dilution of surface solvent molecules. The solution now requires a higher temperature to match atmospheric pressure, resulting in the observed boiling point elevation. This principle is closely linked to Raoult's law for ideal solutions.
Frequent Related Errors
- Confusing elevation boiling point with boiling point of pure compounds.
- Incorrectly using molarity instead of molality in calculations.
- Ignoring the van't Hoff factor for electrolytic solutes (like NaCl).
- Applying the formula to volatile solutes, where it does not hold.
- Assuming all solvents have the same Kb value.
Uses of Elevation Boiling Point in Real Life
Elevation boiling point explains several practical phenomena:
- Cooking at High Altitude: At mountains, water boils at lower temperature causing food to cook slower. Adding salt elevates its boiling point.
- Antifreeze Solutions: Car coolants raise the boiling point of water in radiators to prevent overheating.
- Industrial Distillation: Control of solution boiling points in sugar refining and pharmaceutical industries.
Relevance in Competitive Exams
Elevation boiling point is a frequently tested topic in NEET, JEE, and other competitive and board exams. Students should be comfortable with the ΔTb = i × Kb × m formula and able to solve numerical problems involving unknowns such as molar mass determination, van’t Hoff factor, and colligative property calculations.
Relation with Other Chemistry Concepts
Elevation boiling point is closely linked to colligative properties, molality, and ebullioscopic constant (Kb). It also relates to depression of freezing point and Raoult's law, completing your understanding of solution behavior in Physical Chemistry.
Step-by-Step Reaction Example
1. Identify the system: Dissolve 0.1 mol of NaCl in 1 kg water.2. Calculate molality: m = 0.1 mol / 1 kg = 0.1 molal.
3. Use the formula: ΔTb = i × Kb × m
4. For NaCl, i = 2 (since it dissociates into Na+ and Cl-), Kb for water = 0.512°C·kg·mol-1.
5. Plug values: ΔTb = 2 × 0.512 × 0.1 = 0.1024°C.
6. Final Answer: The boiling point of the solution will be 100°C + 0.1024°C = 100.1024°C.
Lab or Experimental Tips
Remember elevation boiling point is only observed with non-volatile solutes—if your solute tends to evaporate, the effect is not predictable by the standard formula. Vedantu educators often suggest drawing the vapor pressure vs temperature graph to understand this effect deeper.
Try This Yourself
- State the relationship between molality and elevation in boiling point.
- Explain how Na2SO4 affects the boiling point and why i ≠ 1.
- Give two examples where boiling point elevation matters in daily life.
Final Wrap-Up
We explored the elevation of boiling point—its physical basis, formula, practical significance, and links with other chemistry concepts. For a deeper dive and video lessons, visit the Vedantu Chemistry section for live classes, topic notes, and exam question practice.
| Solvent | Kb (°C·kg·mol-1) | Boiling Point (°C) |
|---|---|---|
| Water | 0.512 | 100 |
| Benzene | 2.53 | 80.2 |
| Chloroform | 3.63 | 61.2 |
| Acetic Acid | 3.07 | 118 |
| Phenol | 3.04 | 181 |
Quick Links - Colligative Properties | Molality
FAQs on Elevation of Boiling Point in Chemistry
1. What is elevation of boiling point?
Elevation of boiling point is the increase in the boiling point of a solvent when a non-volatile solute is added. This occurs because the solute particles reduce the solvent's vapor pressure, meaning a higher temperature is needed for the vapor pressure to equal atmospheric pressure. It's a colligative property, meaning it depends on the number of solute particles, not their identity.
2. How is boiling point elevation calculated?
Boiling point elevation (ΔTb) is calculated using the formula: ΔTb = Kb × m, where Kb is the ebullioscopic constant (specific to the solvent) and m is the molality of the solution (moles of solute per kilogram of solvent). For solutions with ionic solutes, the van't Hoff factor (i) is included: ΔTb = i × Kb × m
3. Why does adding salt increase the boiling point of water?
Adding salt (NaCl) to water increases its boiling point because the dissolved sodium and chloride ions reduce the water's vapor pressure. These ions disrupt the hydrogen bonding network of water molecules, making it more difficult for water molecules to escape into the gas phase. Therefore, a higher temperature is needed to reach the boiling point.
4. What is the formula for elevation of boiling point?
The basic formula is ΔTb = Kb × m. For ionic compounds, a more accurate calculation involves the van't Hoff factor (i): ΔTb = i × Kb × m. Remember that this formula is most accurate for dilute solutions and non-volatile solutes.
5. What is the role of Kb in boiling point elevation?
Kb, the ebullioscopic constant, is a solvent-specific constant that represents the elevation in boiling point caused by a 1 molal solution of a non-volatile, non-electrolyte solute. It's a measure of how sensitive the solvent's boiling point is to the addition of solute. Each solvent has its own unique Kb value.
6. Is elevation of boiling point a colligative property?
Yes, elevation of boiling point is a colligative property. This means that the magnitude of the boiling point elevation depends solely on the concentration of solute particles (number of particles) in the solution, and not on the identity of the solute particles themselves.
7. What factors affect boiling point elevation?
The primary factors are:
- Molality (m): Higher molality leads to greater elevation.
- Ebullioscopic constant (Kb): Different solvents have different Kb values.
- Van't Hoff factor (i): For ionic compounds, the number of ions produced affects elevation.
- Nature of the solute: The solute should be non-volatile.
8. How does boiling point elevation relate to vapor pressure lowering?
They are directly related. The addition of a non-volatile solute lowers the vapor pressure of the solvent. To reach the boiling point (where vapor pressure equals atmospheric pressure), a higher temperature is required, thus resulting in boiling point elevation.
9. What are some real-world applications of boiling point elevation?
Applications include:
- Antifreeze in cars: Keeps the coolant from boiling over in hot weather.
- Cooking at high altitudes: Water boils at a lower temperature at higher altitudes, so cooking times may need to be adjusted.
- Industrial distillation processes: Used to separate liquids based on their boiling points.
10. Can volatile solutes cause boiling point elevation?
No, significantly less so. The formula for boiling point elevation assumes a non-volatile solute. Volatile solutes themselves have appreciable vapor pressures and can contribute to the overall vapor pressure of the solution, complicating the simple relationship used to calculate boiling point elevation.
11. How is boiling point elevation affected at high altitudes?
At high altitudes, atmospheric pressure is lower. This means the solvent boils at a lower temperature. While the boiling point elevation formula still applies, the overall boiling point of the solution will be lower than at sea level.
12. What is the difference between boiling point elevation and freezing point depression?
Both are colligative properties, but they affect different phase transitions. Boiling point elevation is the increase in boiling point when a solute is added. Freezing point depression is the decrease in freezing point when a solute is added. Both are caused by the solute's effect on the solvent's vapor pressure and the disruption of the solvent's structure.
