Separation of Solutions and Mixtures

Full Notes

Solutions are homogeneous mixtures, meaning their components are uniformly mixed at the molecular level. Because of this, filtration cannot separate the components of a solution—it only works for separating insoluble solids from liquids.

To separate the parts of a solution, we need to use techniques that exploit differences in intermolecular interactions, such as chromatography and distillation.

Chromatography - What Is Chromatography?

Chromatography is a separation technique used to identify the components of a mixture.

It relies on the distribution of substances between:

• A mobile phase (liquid or gas that carries the sample)
• A stationary phase (solid or liquid on a solid support that does not move)

Different substances interact differently with these phases based on intermolecular forces, leading to separation.

For example, in paper chromatography a sample dissolves in a solvent (the mobile phase) and travels up the paper (stationary phase). The sample spends time adhered to the paper and in the solvent - alternating between the two as it travels up the paper. The relative amounts of time spent in each phase determine how far the sample travels up the paper.

How Separation Occurs

• A component with stronger attraction to the mobile phase (e.g. if it dissolves well in the solvent) will travel further up the paper.
• A component with stronger attraction to the stationary phase (e.g. if it sticks more to the paper) will travel a shorter distance.

This separation is caused by differences in intermolecular forces, such as:

• London dispersion forces (weak attractions between all molecules)
• Dipole–dipole interactions (between polar molecules)
• Hydrogen bonding (strong attraction involving H and electronegative atoms like O or N)

The polarity of both the sample and the solvent plays a major role in determining which components move further. A polar solvent will carry polar components more easily, while non-polar components tend to stay near the baseline.

Paper chromatography

• Stationary phase: Paper plate (for example filter paper)
• Mobile phase: A liquid solvent (usually water)

Process:

• A small sample is spotted onto the paper plate.
• The paper is placed in a solvent, which moves up by capillary action.
• Different components move different distances based on solubility and attraction to the stationary phase.

Thin-Layer Chromatography (TLC)

• Stationary phase: A plate coated with silica or alumina
• Mobile phase: A liquid solvent (e.g., ethanol)

Process:

• A small sample is spotted onto the TLC plate.
• The plate is placed in a solvent, which moves up the plate by capillary action.
• Different components move different distances based on solubility and attraction to the stationary phase.

Column Chromatography (CC)

• Stationary phase: A solid (e.g., silica) packed into a column
• Mobile phase: A liquid solvent flows down the column

Process:

• The sample is added to the top of the column.
• The solvent carries different components at different rates.
• Different substances are collected at different times and identified.

Chromatogram

A chromatogram is the visual result of a chromatography experiment. The position of the separated spots (or bands) indicates the relative solubilities of the components in the given solvent:

• More polar substances are attracted to a polar stationary phase and move more slowly.
• Less polar substances move faster with a nonpolar mobile phase.

Distillation

Distillation separates liquid mixtures based on differences in boiling points, which are determined by:

• The strength of intermolecular forces
• The resulting vapor pressures

Key concepts:

• Substances with weaker intermolecular forces have higher vapor pressures and lower boiling points.
• These components evaporate more easily and can be separated first.

Process:

• Heat the mixture until the most volatile component vaporizes.
• Condense and collect the vapor in a separate container.
• The remaining liquid contains components with stronger intermolecular forces (higher boiling points).

Why Intermolecular Forces Matter

• Different substances in a solution will:
  • Interact differently with surfaces (stationary phase) in chromatography.
  • Vaporize at different temperatures based on their molecular size, polarity, and IMFs.
• Separation methods work because each component behaves differently due to its chemical structure and the types of intermolecular forces it experiences.

Matt’s exam tip

For chromatography, remember that polar substances stick to polar surfaces. If two dyes separate on chromatography paper, the one that traveled less far is likely more polar. For distillation, think about boiling point: weaker IMFs means lower boiling point which will be the compound that comes off first.

Summary

• Components of a liquid solution can't be separated by filtration, but they can be separated using chromatography and distillation, both of which rely on differences in intermolecular forces:
• Chromatography separates based on how substances interact with a mobile phase vs. stationary phase. It's useful for analyzing relative polarity.
• Distillation separates based on differences in vapor pressure and boiling points, determined by the strength of IMFs.
• Understanding these interactions allows chemists to design and interpret separation experiments effectively.