Colligative Properties

Core Concepts

In this tutorial, you will learn about 3 colligative properties: vapor pressure lowering, boiling point elevation and freezing point depression! You will also learn background information to help understand the content.

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Concepts related to Colligative Properties

Before reading about this topic, first make sure you read and understand this section in order to fully understand the tutorial!

Raoult’s Law

François-Marie Raoult was a french chemist born in Fournes who created “Raoult’s law”; this law focuses on the depression of freezing points and vapor pressure of solutions.

P_(solution) = χ_(solvent) × Pº_(solvent)

P_(solution) – vapor pressure of solution
χ_(solvent) – mole fraction of solvent
Pº_(solvent) – vapor pressure of pure solvent

Henry‘s Law

William Henry was an English chemist born in Manchester who created “Henry’s Law”. According to this law, the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. An exception to this law is when the gas and solution participate in chemical reactions with each other; because of this, Henry’s Law is also know as the “limiting law”.

C = k × P_(gas)

C – concentration of dissolved gas
k – Henry’s Law constant
P_(gas) – partial pressure of gas

Molarity vs. Molality

Molarity (M): molar concentration of a solution. Formula is: M = (mols of solute)/(liters of solution)
Molality (m): ratio of moles of substance to a kilogram of solvent. Formula is: m = (mols solute)/ (kg of solvent)

Solubility

Solubility is the ability of a substance (solid, liquid, or gas) to completely dissolve in a solvent. There are many physical and chemical factors that have an effect on solubility.

For example, pH and solubility have an inverse relationship; if the pH of a substance decreases, the solubility will increase. Additionally, size and solubility have a direct relationship; the smaller the particles, the faster the substance will dissolve due to increased surface area exposure to the solvent. Next, the structure of both the solvent and solute can play a role in how they interact. They should have the same structural properties, such as hydrophobicity, in order to mix together and not repel one another. Also, temperature will have a different effect depending on the structure of the solute. If a solid, the higher the temperature, the faster it will dissolve; however, in the cases of gases, the lower the temperature, and higher the pressure, the faster it will dissolve.

What Are Colligative Properties?

“Colligative” comes from the Latin word “colligatus” which means bound together.

These properties have physical changes that are affected by the concentration of a substance; in other words, they are dependent on the molarity or molality of solute molecules or ions. Colligative properties rely exclusively on the number of dissolved particles and do not depend on the unique identity of the particles.

Colligative properties can be divided into multiple sections, which can be seen throughout the graph below: vapor pressure lowering, boiling point elevation, and freezing point depression. Let’s go through each of them in detail!

colligative properties table example - boiling point elevation

Vapor Pressure Lowering

When an ionic compound is added to a solvent, it dissociates; when this happens, the boiling point of the solution increases and the freezing point decreases. In the case of a substance like sugar, it would not dissociate but still, the boiling point increases and the freezing point decreases.

P_solution = χ_solvent × P°_solvent

Boiling Point Elevation

Boiling point elevation is directly related to vapor pressure lowering; it has a presence in every solute in all solutions. As another compound, a non-volatile solute, is added to a solution, a pure solute, the boiling point will increase. When this happens, the substance liquid phase or gas phase is equivalent in terms of energy.

ΔT_b= i × K_b × m

ΔT_b = change in boiling point
i = Von’t Hoff coefficient
K_b = molal boiling point constant
m = molality

Freezing Point Depression

In this property, liquid solvents and solid solvents are at equilibrium; however, before this happens, it is necessary that a lower temperature is reached so freezing can happen. An example of this property is ice cream, since when sugar is dissolved with water, a universal solvent, the freezing point is lowered and it interferes with crystallization due to the modification of water; sugar particles are interfering with the attractive forces in the solvent.

∆T_F = K_F × m

∆T_F= change in freezing point
K_F= molal freezing point constant
m= molality