Q.[Note: two or more options may be correct] Colligative properties are observed when _____________.

Colligative properties are characteristics of solutions that depend only on the number of solute particles, not on their chemical identity. These properties arise when a non-volatile solute is dissolved in a volatile solvent.

1. Key Idea: Colligative properties are observed when a non-volatile solute alters the properties of a volatile solvent. The solubility of gases, as in option (iii), is described by Henry's Law, $P = k_H x$, and does not typically lead to colligative properties of the solvent.
2. Options (i) and (ii) describe the dissolution of a non-volatile solid or a non-volatile liquid, respectively, in a volatile liquid. In both cases, the solute is non-volatile, and the solvent is volatile. This is the fundamental condition for observing colligative properties such as vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. …

Colligative properties are fascinating characteristics of solutions that depend solely on the number of solute particles present in a given amount of solvent, and not on the identity or chemical nature of these particles. Think of it like this: whether you add sugar or salt to water, the effect on properties like boiling point or freezing point depends primarily on how many particles you've introduced, not what those particles are (though dissociation, like with salt, multiplies the particle count).

The four main colligative properties are:

1. Relative lowering of vapor pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure

For these properties to be observed and quantified using the standard colligative property equations, two fundamental conditions regarding the solute and solvent must be met:

• The solute must be non-volatile: This is crucial because colligative properties like vapor pressure lowering are based on the idea that only the solvent contributes to the vapor pressure above the solution. If the solute were volatile, it would also evaporate and contribute to the total vapor pressure, complicating the simple relationships.
• The solvent must be volatile: Properties like vapor pressure lowering, boiling point elevation, and freezing point depression are inherently linked to the solvent's ability to vaporize or freeze. If the solvent itself is non-volatile, these phase transitions cannot occur, and thus these colligative effects cannot be observed.

Let's analyze each option based on these principles:

1. Analyze option (i): a non volatile solid is dissolved in a volatile liquid.

   • Solute: A non-volatile solid. This perfectly fits the requirement for a non-volatile solute. It will not contribute to the vapor pressure of the solution.
   • Solvent: A volatile liquid. This perfectly fits the requirement for a volatile solvent, allowing for the observation of changes in vapor pressure, boiling point, and freezing point.
   • Conclusion: This is the classic scenario where colligative properties are observed. For example, dissolving sugar (non-volatile solid) in water (volatile liquid).

2. Analyze option (ii): a non volatile liquid is dissolved in another volatile liquid.

   • Solute: A non-volatile liquid. Similar to a non-volatile solid, a non-volatile liquid solute will not contribute to the vapor pressure of the solution.
   • Solvent: A volatile liquid. This allows for the observation of colligative effects.
   • Conclusion: This is also a valid scenario for observing colligative properties. For example, dissolving glycerol (non-volatile liquid) in water (volatile liquid) to make antifreeze.

3. Analyze option (iii): a gas is dissolved in non volatile liquid.

   • Solute: A gas. Gases are inherently volatile. They will contribute to the vapor pressure above the solution (or escape from it), violating the non-volatile solute condition for standard colligative properties. The solubility of gases is typically described by Henry's Law, not the colligative property equations.
   • Solvent: A non-volatile liquid. This is a major issue. If the solvent itself is non-volatile, it cannot have a vapor pressure to be lowered, a boiling point to be elevated, or a freezing point to be depressed.
   • Conclusion: Colligative properties are generally not observed under these conditions in the conventional sense.

4. Analyze option (iv): a volatile liquid is dissolved in another volatile liquid. …

Concept: Colligative Properties

Colligative properties depend only on the number of solute particles, not on their identity.

For a property to be colligative, the solute must be non-volatile (so it does not contribute to vapour pressure) and the solvent must be volatile (so its vapour pressure changes measurably).

Method: Solute–Volatility Check

Steps

1. Identify the solvent — must be volatile (able to evaporate).
2. Identify the solute — must be non-volatile (does not evaporate).
3. If both conditions are satisfied → colligative properties are observed.
4. If the solute is volatile → the property is not purely colligative (vapour pressure changes from both components).

Applying to the options

Here’s a breakdown of the common mistakes students make on this concept and how to avoid each.

🧠 The Core Concept First

Colligative properties depend only on the number of solute particles, not on their identity.

For these properties to be observed, the solute must be non-volatile (so it doesn’t contribute to vapour pressure) and the solvent must be volatile (so it can evaporate and create measurable changes like boiling point elevation or freezing point depression).

Correct answer(s):

• (i) a non volatile solid is dissolved in a volatile liquid.
• (ii) a non volatile liquid is dissolved in another volatile liquid.

✗ Common Mistake #1: Thinking the solvent must be non-volatile

What students do:

They choose option (iii) — “a gas is dissolved in non volatile liquid” — because they think the solvent should not evaporate.

Why it’s wrong:

If the solvent is non-volatile, it doesn’t evaporate significantly. Colligative properties like vapour pressure lowering require the solvent to have a measurable vapour pressure in the first place. A non-volatile solvent has negligible vapour pressure, so no colligative effect is observed.

✓ How to avoid:

Remember: Colligative properties measure changes in solvent behaviour (vapour pressure, boiling point, freezing point). The solvent must be volatile for these changes to be detectable.

✗ Common Mistake #2: Including volatile solutes

What students do:

They select option (iv) — “a volatile liquid is dissolved in another volatile liquid” — thinking any mixture shows colligative effects.

Why it’s wrong:

If both liquids are volatile, both contribute to vapour pressure. The mixture’s vapour pressure is not simply lowered — it follows Raoult’s law for ideal solutions or shows deviations. This is not a colligative property because the identity of the solute matters.

✓ How to avoid:

Colligative properties require the solute to be non-volatile. If the solute evaporates, it changes the vapour pressure in a way that depends on its nature, not just its concentration.

✗ Common Mistake #3: Assuming only solids can be solutes

What students do:

They pick only (i) and ignore (ii), thinking a liquid solute can’t be non-volatile.

Why it’s wrong:

Non-volatility is about vapour pressure, not physical state. Many liquids (e.g., glycerol, ethylene glycol) have very low vapour pressure and behave as non-volatile solutes.

✓ How to avoid:

Focus on volatility, not state of matter. A non-volatile liquid solute works just as well as a solid.

✗ Common Mistake #4: Misreading “non volatile liquid” in option (iii)

What students do: …