Solutions, colloids and suspensions Notes

How Do Solutions, Colloids and Suspensions Differ?

1. Consider mixing salt in water versus sand in water:
   1. Saltwater looks clear and uniform.
   2. Sand in water looks cloudy, and the sand settles at the bottom.
2. These differences come from particle size, visibility, and stability of the mixture.
3. Chemists classify such mixtures as solutions, colloids, or suspensions.

Key Idea: Particle Size and Stability

1. Solutions → very small particles (ions or molecules), fully dissolved.
2. Colloids → medium-sized particles, dispersed but not dissolved.
3. Suspensions → large particles, which can be seen and will settle.

Summary Table - Key Differences Between Solutions, Colloids, and Suspensions

Solutions – True Homogeneous Mixtures

1. Solute → the substance being dissolved (usually in smaller amount).
2. Solvent → the substance doing the dissolving (usually in larger amount).

Key Characteristics of Solutions

1. Particle size: < 1 nm (molecules or ions).
2. Visibility: Individual particles cannot be seen, even with a light microscope.
3. Uniformity: Completely uniform throughout – no lumps, layers, or floating bits.
4. Stability: The solute never settles out on standing.
5. Light scattering: Does not scatter light → no visible beam.

Colloids – In Between Solutions and Suspensions

Colloids are:

1. Heterogeneous at the microscopic level (non-uniform if you zoom in),
2. But appear uniform to the naked eye.
3. Particle size: approximately 1–1000 nm.

Key Characteristics of Colloids

1. Visibility: Particles are too small to see individually with the naked eye, but can be seen with a microscope.
2. Stability: Particles do not settle out under normal conditions.
3. Filtration: Colloidal particles pass through ordinary filter paper.
4. Tyndall effect: Colloids scatter light, making a flashlight beam visible as it passes through.

Suspensions – Large Particles That Settle

Over time, these particles settle to the bottom under gravity.

Key Characteristics of Suspensions

1. Particle size: > 1000 nm (can range up to millimetres).
2. Visibility: Particles are easily seen with the naked eye.
3. Non-uniformity: The mixture is clearly non-uniform.
4. Instability: Particles settle on standing → forms a layer at the bottom.
5. Filtration: Particles can be trapped by filter paper.

Why Particle Size Matters

Particle size affects:

1. Visibility – whether we can see particles with our eyes.
2. Stability – whether particles settle out.
3. Behaviour – whether mixtures can be filtered or show the Tyndall effect.

How Can We Experimentally Distinguish Solutions, Colloids and Suspensions?

Scientists use simple experimental tests to decide what kind of mixture they are dealing with.

Filtration

Method: Pour the mixture through filter paper.

Results:

1. Solution:
   1. No residue on the filter.
   2. Filtrate is clear.
2. Colloid:
   1. No residue on the filter (particles are too small).
   2. Filtrate may appear cloudy or translucent.
3. Suspension:
   1. Solid particles remain on the filter paper.
   2. Filtrate (liquid that passes through) is clearer than the original mixture.

Light Scattering – The Tyndall Effect

Method: Shine a narrow beam of light through the mixture in a dark room.

Results:

1. Solution:
   1. Light passes through without scattering.
   2. The beam is not visible from the side.
2. Colloid:
   1. Light is scattered by the particles.
   2. The beam becomes visible as a glowing path → Tyndall effect.
3. Suspension:
   1. Large particles may block or reflect light, making the mixture look cloudy.
   2. However, the beam does not show a uniform glowing path as in a colloid.

Placeholder (diagram):
A light beam passing through three tubes labelled “solution”, “colloid”, and “suspension”. The beam is only clearly visible in the colloid tube (Tyndall effect).

Settling and Sedimentation

Method: Leave the mixture undisturbed and observe over time.

Results:

1. Solution:
   1. No visible change.
   2. Particles remain evenly distributed.
2. Colloid:
   1. Particles remain suspended and do not settle.
   2. The mixture stays cloudy or translucent.
3. Suspension:
   1. Heavy particles settle to the bottom, forming a sediment.
   2. The upper liquid becomes clearer.

Why Do Their Properties Matter?

Understanding solutions, colloids and suspensions helps explain the behaviour of many everyday products.

1. Solutions:
   1. Solutions are uniform, so every sip of a drink or every dose of saline has the same composition.
   2. Dissolved substances can be absorbed easily by the body (e.g. in medical solutions).
2. Colloids:
   1. Colloids stay mixed for a long time, so products like milk, creams, and paints don’t separate quickly.
   2. Their ability to scatter light can be useful (fog, smoke detection) or a problem (haze in drinks).
3. Suspensions:
   1. Suspensions can be separated by filtration or settling, which is useful in water treatment.
   2. Some medicines are designed as suspensions so that the solid drug is released or absorbed gradually.
   3. In food, suspension behaviour affects texture and appearance.