Colligative properties of dilute solutions depend entirely on what primary factor?

Introduction / Context:
Colligative properties—such as boiling-point elevation, freezing-point depression, vapour-pressure lowering, and osmotic pressure—are invaluable for determining molar masses and understanding solution behavior in food, pharma, and chemical processes.

Given Data / Assumptions:

• Dilute, ideally behaving solutions.
• Non-electrolytes compared with electrolytes (which dissociate into more particles).
• Solvent amount held constant when comparing effects.

Concept / Approach:
Colligative effects depend solely on the number of solute particles relative to the amount of solvent, not on the chemical identity of those particles. Hence, two different non-electrolytes at the same molality cause the same freezing-point depression. Electrolytes produce larger effects when they dissociate into multiple ions, effectively increasing the particle count (van ’t Hoff factor i).

Step-by-Step Solution:

Verification / Alternative check:
Experimental plots of ΔT_f versus molality for many solutes collapse when corrected by i, confirming particle-number control.

Why Other Options Are Wrong:

• Chemical composition/constitution/bond strength: influence non-colligative properties but not the magnitude of ideal colligative effects at equal particle counts.
• None of these: incorrect because particle number is the defining factor.

Common Pitfalls:
Using molarity instead of molality when temperature changes; remember that molality is temperature-independent and preferred for colligative calculations.

Final Answer:
number of solute molecules (or particles) present per amount of solvent