Colligative Properties
Dissolved particles change freezing and boiling points.
About this topic
Colligative properties depend on the number of dissolved solute particles, not their identity. They include vapour pressure lowering, boiling point elevation (ΔTb = i·Kb·m), freezing point depression (ΔTf = i·Kf·m) and osmotic pressure. The van 't Hoff factor i counts particles per formula unit: about 1 for sugar, 2 for NaCl and 3 for CaCl2.
Solute particles interfere with solvent molecules escaping into the vapour and with forming an ordered solid, so the solution boils higher and freezes lower than the pure solvent. Here m is molality, moles of solute per kilogram of solvent, and Kf for water is 1.86 degrees Celsius per molal.
Worked example: dissolving 58.4 g (1.00 mol) of NaCl in 1.00 kg of water gives molality 1.00 m with i = 2, so the freezing point drops by about 2 x 1.86 x 1.00 = 3.7 degrees, to minus 3.7 degrees Celsius. This explains salt on icy roads, antifreeze in car engines (which also raises the boiling point to prevent overheating), and making ice cream with ice and salt. Doctors also use osmotic pressure when preparing saline for intravenous solutions.
A misconception is that the properties depend on the type of solute; at a given concentration, one mole of ions has the same effect as another. Also, ionic solids give more particles than molecular ones at the same molality.
Key ideas
- Freezing point depression
- Boiling point elevation
- Depends on particle count
Quick check
Use cases: where Colligative Properties helps
- Homework: review the key idea behind Colligative Properties before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Colligative Properties without notes.
- Visual learning: play with the interactive lab to see how Colligative Properties changes when you adjust its inputs.
- Teaching a friend: explain Colligative Properties in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Colligative Properties?
Dissolved particles change freezing and boiling points.
What will I learn in Colligative Properties?
Freezing point depression. Boiling point elevation. Depends on particle count.
Is Colligative Properties part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Colligative Properties lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Colligative Properties?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
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