Freezing Point Depression
ΔTf = Kf·m·i. Colligative property: solute lowers freezing point. Antifreeze, cryoscopy, molecular weight determination. Water Kf = 1.86°C·kg/mol.
Why This Chemistry Calculation Matters
Why: Antifreeze in cars, ice cream freezing, molecular weight by cryoscopy. Colligative: depends on particle count.
How: ΔTf = Kf × m × i. Kf = cryoscopic constant. Water 1.86°C·kg/mol. i = van't Hoff (NaCl→2, sucrose→1).
- ●Water Kf = 1.86°C·kg/mol.
- ●Antifreeze: ethylene glycol lowers FP.
- ●Cryoscopy: measure Kf, ΔTf to find M.
- ●Salt on roads: depresses ice melting point.
Freezing Point Depression Examples
🧂 Salt Water
Sodium chloride in water
🚗 Antifreeze
Ethylene glycol in water
🍦 Ice Cream
Sugar solution freezing point
🛣️ Road De-icing
Calcium chloride solution
⚖️ Molar Mass Determination
Find molar mass from depression
🧪 Benzene Solution
Non-aqueous solvent example
Calculate Freezing Point Depression
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
ΔTf = Kf·m·i. Colligative, particle-count dependent.
— IUPAC
Water Kf = 1.86°C·kg/mol.
— CRC
Antifreeze: ethylene glycol + water.
— Applied chem
Cryoscopy: molar mass from ΔTf.
— Analytical
What is Freezing Point Depression?
Freezing point depression is a colligative property that describes how adding a nonvolatile solute to a solvent lowers the freezing point of the solution compared to the pure solvent. This phenomenon is independent of the nature of the solute and depends only on the number of solute particles.
Kf = cryoscopic constant, m = molality, i = van't Hoff factor
Common Solvents and Kf Values
| Solvent | Formula | Freezing Point (°C) | Kf (°C·kg/mol) |
|---|---|---|---|
| Water | H_{2}O | 0 | 1.86 |
| Benzene | C₆H₆ | 5.5 | 5.12 |
| Ethanol | C_{2}H₅ ext{OH} | -114.6 | 1.99 |
| Chloroform | CHCl_{3} | -63.5 | 4.68 |
| Ether | (C_{2}H₅)_{2}O | -116.2 | 1.79 |
| Nitrobenzene | C₆H₅NO_{2} | 5.7 | 6.9 |
| Acetic acid | CH_{3} ext{COOH} | 16.6 | 3.9 |
| Camphor | C_{1}_{0}H_{1}₆O | 179.8 | 40 |
Van't Hoff Factor
The van't Hoff factor (i) accounts for the number of particles a solute dissociates into when dissolved. For nonelectrolytes like sucrose, i = 1. For strong electrolytes like NaCl, i approaches the number of ions formed.
Nonelectrolytes
i = 1.0
Examples: Sucrose, glucose, ethylene glycol
Strong Electrolytes
i ≈ number of ions
NaCl: i ≈ 1.9, CaCl₂: i ≈ 2.7
How Does Freezing Point Depression Work?
When a solute is added to a solvent, it disrupts the formation of the solid crystal lattice. The solution requires a lower temperature to freeze because the solute particles interfere with the orderly arrangement needed for freezing.
🔬 Colligative Properties
Key Characteristics
• Depends on number of particles
• Independent of solute identity
• Proportional to molality
Applications
• Road de-icing (salt)
• Antifreeze (ethylene glycol)
• Molar mass determination
Practical Applications
Freezing point depression has numerous practical applications in everyday life and scientific research.
Road De-icing
Salt (NaCl) or calcium chloride lowers the freezing point of water, preventing ice formation on roads.
Antifreeze
Ethylene glycol in car radiators prevents water from freezing in cold temperatures.
Ice Cream
Sugar lowers the freezing point, keeping ice cream soft and scoopable at serving temperatures.