Chemistry · Lesson

Colloids, suspensions and dispersed systems

Distinguish small dissolved species from dispersed droplets, particles or bubbles. Identify continuous and dispersed phases while treating size, scattering and stability as useful evidence with limits.

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By the end, you can

Identify the continuous medium and dispersed material.
Describe colloidal scale as approximate rather than a rigid universal cutoff.
Avoid using cloudiness or settling as a single absolute classification test.

A dispersion has two roles to identify

In an emulsion, liquid droplets are dispersed in another liquid. In a foam, gas bubbles are dispersed in a liquid, solid or gel. In an aerosol, dispersed liquid or solid entities occur in a gas. The continuous medium and dispersed material define the description more clearly than a familiar everyday name.

A molecular or ionic solution disperses relatively small species throughout the medium at the relevant scale. A colloidal dispersion can instead contain larger entities, including aggregates or single large macromolecules. Not every colloidal entity is a collection of many smaller molecules.

Name what is dispersed and what surrounds it

Description	Dispersed entities	Continuous medium	Useful distinction
Molecular or ionic solution	Small dissolved molecules or ions	The solution medium	No separate dispersed droplet phase in the stated description
Emulsion	Liquid droplets	Another liquid	Droplets retain a distinct liquid phase
Foam	Gas bubbles	Liquid, solid or gel	A bubble is not an individual gas molecule
Aerosol	Liquid droplets or solid entities	Gas	Identify both the dispersed material and gas medium
Colloidal macromolecular dispersion	Large individual macromolecules may be present	Appropriate continuous medium	Colloidal entities need not all be aggregates

Phase-based descriptions. Size and stability need separate evidence; the names do not guarantee one size range or lifetime.

Size helps describe the system, with context

A common colloidal dimensional range is roughly 1 nm to 1 µm in at least one direction. The limits are approximate; fibres, films and networks need not have all dimensions in that interval. Emulsion droplets or aerosol entities can also exceed the usual range without losing their phase-based classification.

For an original unit example, a stated dimension of 1.00 × 10² nm is 1.00 × 10⁻⁷ m, or 0.100 µm. This is within the usual approximate scale. The size label describes a dimension rather than proving a particular composition, shape or lifetime.

Light scattering is evidence, not a complete diagnosis

Dispersed entities can scatter light and make a beam’s path visible, often called the Tyndall effect in introductory comparisons. This can help distinguish some dispersions from small-species solutions under appropriate observation conditions. Particle size, refractive contrast, concentration and wavelength affect what is visible.

A coloured molecular solution can absorb light, and scattering alone is not an exclusive test of one classification. Transparency or cloudiness does not replace evidence about entities and phases. Describe what was observed and what additional information supports the proposed interpretation.

A slow change is not a guarantee of permanence

Some colloidal dispersions remain apparently uniform over an observation period, yet can aggregate, sediment or cream under other conditions or over longer times. Stabilization can slow separation; kinetic persistence does not necessarily mean the dispersed system is the lowest-energy bulk equilibrium.

Larger suspended entities often settle more readily in familiar settings, but density differences, viscosity and stabilizing effects matter. Record the observation time and conditions. Avoid absolute rules such as colloids never settle or every cloudy mixture is a suspension.

Worked examples

Convert a stated colloidal dimension

Convert 1.00 × 10² nm to metres and micrometres.

1 nm = 10⁻⁹ m, so 1.00 × 10² nm = 1.00 × 10⁻⁷ m.
1 µm = 10⁻⁶ m, so the same dimension is 0.100 µm.
This lies within the usual approximate colloidal dimensional range, without establishing composition or stability.

1.00 × 10⁻⁷ m = 0.100 µm.

Try it yourself

In a liquid foam, what is dispersed and what is continuous?
Show the answer and reasoning
Gas bubbles dispersed in liquid.
The bubbles are separate gas regions, not single molecular solutes.
Must every colloidal entity be an aggregate of many smaller molecules?
Show the answer and reasoning
No.
A sufficiently large individual macromolecule can be a colloidal entity.
Does visible light scattering alone identify one unique dispersion type?
Show the answer and reasoning
No.
Scattering depends on multiple properties and needs supporting entity/phase evidence.
Does a colloid that remains uniform during one observation period have to remain so forever?
Show the answer and reasoning
No.
Aggregation, sedimentation or creaming can occur under different conditions or over longer times.

What to carry forward

Identify dispersed entities and the continuous medium.
Colloidal dimensional limits are approximate and context dependent.
Scattering supports an interpretation but is not a complete diagnosis.
State observation time and conditions when describing stability.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.