Chemistry · Solutions · NEET
They are 4 properties of a solution that change ONLY because solute particles are present, and the change depends only on how MANY particles you add. It does not matter if the particle is sugar, salt, or urea. Two solutions with the same NUMBER of dissolved particles will show the same change. The word 'colligative' comes from Latin 'colligare' meaning 'to bind together' or 'to collect' — the effect comes from the collection (count) of particles.
When you add a non-volatile solute, the particles spread among the solvent and get in the way of the solvent molecules escaping or freezing. What matters is how many 'blockers' are present, not their identity. So 1 mole of glucose particles and 1 mole of urea particles give the SAME effect, because both give the same number of particles. This is the single most tested idea for NEET.
(1) Relative lowering of vapour pressure, (2) Elevation of boiling point (ΔTb), (3) Depression of freezing point (ΔTf), and (4) Osmotic pressure (π). All four rise when you add more solute particles. NEET loves to ask 'which solution boils highest / freezes lowest / has highest osmotic pressure' — all four answers depend on the same rule: more particles = bigger effect.
Yes. NaCl splits into Na⁺ and Cl⁻, so 1 mole of NaCl gives about 2 moles of particles. Glucose does not split, so 1 mole gives 1 mole of particles. More particles means a larger colligative effect. This 'splitting factor' is called the van't Hoff factor (i). For NaCl i≈2, for Na₂SO₄ i≈3, for glucose/urea i=1.
No. Colour, density, and taste depend on WHAT the substance is. Colligative properties are special — they ignore the identity of the solute and only count particles. That is why they are used to FIND the molar mass of an unknown solute: measure the effect, count back to the number of particles, and get the mass per mole.
Which of the following aqueous solution will exhibit highest boiling point?
If molality of the dilute solution is doubled, the value of molal depression constant (Kf) will be:
The freezing point depression constant (Kf) of benzene is 5.12 K kg mol⁻¹. The freezing point depression for a solution of molality 0.078 m containing a non-electrolyte solute in benzene is (rounded to two decimals):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Molar mass is a fixed property of the substance. But colligative properties are USED to find molar mass: you measure the colligative effect, work out the number of particles, and then calculate the mass of one mole. This is why NCERT links colligative properties to 'determination of molar mass'.
Osmotic pressure. Its value is large even for very dilute solutions, and it is measured near room temperature, so it does not damage sensitive biomolecules like proteins and polymers. NEET has asked this advantage directly.
If the solute is non-volatile, it does not add its own vapour and does not escape. It only blocks the solvent, which cleanly lowers vapour pressure and shifts boiling/freezing points. A volatile solute would add its own vapour and mess up the simple particle-counting rule.
The van't Hoff factor (i) is the number of particles one formula unit gives in solution. For electrolytes that split (NaCl→2, CaCl₂→3), i>1 and the colligative effect is bigger. For non-electrolytes (glucose, urea), i=1. Always include i when comparing solutions in NEET.
