Exploring Mixtures and their Separation · Lesson 11 of 13
Colloids
“A colloid looks calm and uniform while billions of tiny particles stay busily dispersed.”
• Describe colloids through particle size and behaviour. • Compare colloids with solutions and suspensions. • Explain why colloidal particles do not settle. • Identify dispersed phase and dispersion medium. • Relate centrifugation to separation of blood components.
Blood cannot be called a true solution because it contains dispersed components, yet it does not behave like muddy water. Its particles are not visible to the naked eye and do not simply settle during ordinary standing. Blood belongs to an intermediate class of mixtures called colloids.
A colloid is a heterogeneous mixture containing particles about 1–1000 nm in diameter that remain dispersed and do not settle under ordinary standing.
Milk, tomato sauce and ice cream are familiar colloids. They often appear uniform because their particles are too small to see individually, but the mixture is heterogeneous at the particle level. This is why appearance alone cannot distinguish a colloid from a solution.
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous but appears uniform | Heterogeneous |
| Particle diameter | Less than 1 nm | About 1–1000 nm | More than 1000 nm |
| Visibility | Not visible | Not visible to the naked eye | Often visible |
| Settling | Does not settle | Does not settle ordinarily | Settles on standing |
| Ordinary filtration | Cannot separate particles | Cannot separate particles | Can generally separate particles |
| Light scattering | No visible path | Visible path | Visible path |
The dispersed phase is the particle-forming, solute-like component distributed through a colloid.
The dispersion medium is the continuous component through which the dispersed phase is distributed.
Problem
Milk looks uniform, does not settle, and scatters light. Why is it a colloid rather than a solution?
- 1.The mixture appears uniform but contains dispersed particles.
- 2.The particles remain spread and do not settle under ordinary standing.
- 3.They scatter light, making a narrow beam visible.
- 4.True solution particles are too small to produce this visible path.
- 5.Therefore, milk is a colloid.
Bridging Science and Society
Donated blood is tested, its group is identified, and it can be separated into useful components such as plasma, platelets, white blood cells and red blood cells. Different patients may require different components rather than whole blood.
Red blood cells form the dense lower layer, while plasma forms the lighter upper layer. Platelets and white blood cells form a thin region between them. The body naturally replaces donated blood over time, allowing carefully managed blood donation to support medical care.
Problem
A mixture has particles around 300 nm, does not settle and scatters light. Classify it.
- 1.A diameter of about 300 nm lies within the 1–1000 nm range.
- 2.Failure to settle supports colloidal behaviour.
- 3.Light scattering also agrees with a colloid.
- 4.Therefore, the mixture is classified as a colloid.
Blood and other body fluids must be handled only by trained people using proper protective procedures. Classroom discussion or models should never use unprotected biological samples.
Quiz
Which description best matches Colloid?
Which description best matches Dispersed Phase?
Which term matches this description: A colloid is a heterogeneous mixture containing particles about 1–1000 nm in diameter that remain dispersed and do not settle under ordinary standing.
Which term matches this description: The dispersed phase is the particle-forming, solute-like component distributed through a colloid.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why can a colloid appear homogeneous even though it is heterogeneous?
- Classify a mixture with particles larger than 1000 nm that settles on standing.
- Identify the dispersed phase and dispersion medium in a general colloid.
- Explain why centrifugation can separate blood components.
Key Takeaways
• Colloids occupy an intermediate particle-size range. • They appear uniform but are heterogeneous. • Their particles do not settle during ordinary standing. • Colloids scatter light and are not separated by ordinary filtration. • Blood components can be separated by centrifugation.