Comparing Colligative Properties: Which Solution Boils or Freezes More?

Do not just compare molarity or molality. Colligative properties depend on the number of dissolved PARTICLES, not the number of moles you added. So calculate i x m for each solution. Here i is the van't Hoff factor (particles per formula unit) and m is the molality (or molarity for dilute aqueous solutions). The solution with the LARGEST i x m has the highest boiling point, because elevation of boiling point is delta T_b = i x K_b x m. Example: 0.01 M Na2SO4 gives i x m = 3 x 0.01 = 0.03, which beats 0.01 M KNO3 (2 x 0.01 = 0.02). So Na2SO4 wins even though both are 0.01 M.

i is simply the number of particles (ions or molecules) that one formula unit gives when it dissolves, assuming complete dissociation. Count the ions: NaCl -> Na+ + Cl-, so i = 2. Na2SO4 -> 2Na+ + SO4^2-, so i = 3. Ba(OH)2 -> Ba^2+ + 2OH-, so i = 3. K4[Fe(CN)6] -> 4K+ + [Fe(CN)6]^4-, so i = 5. For non-electrolytes like glucose, urea and sucrose, they do NOT split, so i = 1. In NEET, unless told otherwise, assume strong electrolytes dissociate 100%.

Two different things! 'Freezes first' can be confusing, so read the question carefully. The solution with the LARGEST i x m has the LOWEST (most negative) freezing point, because depression of freezing point is delta T_f = i x K_f x m. That means more particles push the freezing point down more. Pure solvent freezes highest; the solution with most particles freezes lowest. So the ranking for lowest freezing point is exactly the SAME order as highest boiling point.

The one with the bigger i wins, because i multiplies the concentration. If you compare 0.1 M NaCl (i=2) and 0.1 M MgCl2 (i=3), then MgCl2 has i x m = 0.3 versus NaCl 0.2. So MgCl2 boils higher and freezes lower. This is the classic NEET trap: students pick equal because the molarity is equal, forgetting to multiply by i.

No, and this is a common mistake. A more concentrated solution of a non-electrolyte can still boil LOWER than a dilute electrolyte if the electrolyte gives more particles. Example: 0.015 M glucose (i=1) gives i x m = 0.015, but 0.01 M Na2SO4 (i=3) gives 0.03. The dilute Na2SO4 boils higher. Always compare i x m, never concentration alone.

If the same mass of solute is dissolved in the same volume, then moles = mass / molar mass, so osmotic pressure pi is proportional to i / M (i = van't Hoff factor, M = molar mass). For equal-mass non-electrolytes (all i = 1), the smallest molar mass gives the largest osmotic pressure. That is why urea (M=60) beats glucose (M=180) which beats sucrose (M=342): order is urea > glucose > sucrose.

Compare i x m. For boiling point delta T_b = i x K_b x m and for freezing point delta T_f = i x K_f x m. Since K_b and K_f are fixed for the same solvent, the winner is decided by the largest i x m. This matters for NEET because at least one question every year asks you to rank or pick a solution.

The electrolyte, because it splits into ions and gives more particles (i is greater than 1). A non-electrolyte like glucose stays as one particle (i = 1), so at the same concentration it has a smaller colligative effect.

Yes. Both boiling elevation and freezing depression grow with i x m. So the solution that boils highest is also the one that freezes lowest. You only compute i x m once.

Strictly, colligative properties use molality. But for dilute aqueous solutions, molarity is approximately equal to molality, so NEET often gives molarity and you can compare directly. Always compare using the same unit for every option in the question.

When a strong electrolyte dissociates fully, i equals the number of ions produced. If association happens (like some acids in benzene forming dimers), i can drop below 1. NEET usually assumes complete dissociation unless a degree of dissociation is given.