Solutions, Colloids, and Dispersed Systems
Think about the last time you stirred sugar into your cup of tea. The sugar seemed to vanish, yet the tea tasted sweet throughout — top to bottom, edge to edge. That, my friend, is a Solution at work!
Now imagine muddy water after a rain — you can literally see the dirt particles, and if you leave the glass undisturbed for a few minutes, the mud settles down. That is a Suspension.
And milk? It looks like a solution, but it isn’t quite one — it belongs to a fascinating middle world called a Colloid. In this section, we are going to understand these three pillars of mixture chemistry in a way that will stay with you for life.
Solution
What is a Solution?
A Solution is a homogeneous mixture of two or more substances. The word ‘homogeneous’ is key here — it means uniform throughout. No matter which part of the solution you examine, the composition will be identical. Solutions can exist in all three physical states: solid, liquid, or even gas.
The two fundamental components of any solution are:
- Solvent: The medium that does the dissolving. It is present in the larger amount. In saltwater, water is the solvent.
- Solute: The substance that gets dissolved. Present in a smaller amount. In saltwater, salt is the solute.
A simple mnemonic: Think of the Solvent as the ‘host’ and the Solute as the ‘guest’. The guest (solute) mixes into the host’s (solvent’s) house so completely that you can’t even tell they were ever separate!
Key Properties of a Solution
What makes a solution stand apart from other mixtures? The following properties define it precisely:
- Homogeneous Nature: Composition is uniform throughout. You cannot distinguish solute from solvent visually.
- Particle Size < 1 nm: The solute particles are incredibly tiny — less than 1 nanometre in diameter. This is why you cannot see them.
- Transparent: True solutions are transparent because particles are too small to scatter light.
- Stability: Solute particles do not settle over time. You can store a solution indefinitely — the solute will not fall to the bottom.
- Non-filterable: The solute and solvent cannot be separated by ordinary filtration because particles are too small to be caught by a filter.
- No Tyndall Effect: Pass a beam of light through a true solution — you will not see the beam. Particles are too small to scatter light.
- Colligative Properties: These are properties that depend on the number of solute particles, not on the nature of the solute.
- They include Boiling Point Elevation, Freezing Point Depression, Vapour Pressure Lowering, and Osmotic Pressure.
- Adding salt to water raises its boiling point — this is a Colligative Property in your kitchen!
- Electrical Conductivity: Electrolyte solutions (e.g., saltwater) conduct electricity due to ions. Non-electrolyte solutions (e.g., sugar water) do not, since sugar does not ionise.
Types of Solution
Solutions are not just limited to liquids. They can be classified in multiple ways:
Based on Physical State: Solid Solution (e.g., alloys like brass), Liquid Solution (e.g., saltwater, vinegar), and Gaseous Solution (e.g., air, which is a mixture of gases).
| Solvent Phase | Solute Phase | Example |
| Solid | Solid | Alloys (e.g., Brass) |
| Solid | Liquid | Mercury in Gold (Amalgam) |
| Solid | Gas | Hydrogen in Palladium |
| Liquid | Solid | Sugar in Water |
| Liquid | Liquid | Alcohol in Water |
| Liquid | Gas | Carbonated Water |
| Gas | Solid | Camphor in Air |
| Gas | Liquid | Water Vapour in Air |
| Gas | Gas | Air (mixture of gases) |
Based on Concentration, solutions are classified as: Dilute (low solute), Concentrated (high solute), Saturated (maximum solute at given temperature), Unsaturated (more solute can dissolve), and Supersaturated (contains more solute than can typically dissolve — achieved under special conditions, like honey at room temperature).
Concentration of a Solution — How ‘Strong’ is Your Solution?
Concentration tells us how much solute is dissolved in a given quantity of solvent or solution. Think of it as measuring the ‘strength’ of your tea — a little tea leaves give you dilute tea; a lot gives you strong, concentrated tea.
For scientific purposes, we measure concentration in several precise ways:
| Method | Formula / Definition | Unit |
| Molarity (M) | Moles of solute / Volume of solution (L) | mol/L (Molar) |
| Molality (m) | Moles of solute / Mass of solvent (kg) | mol/kg |
| Normality (N) | Gram equivalents of solute / Volume of solution (L) | equivalents/L |
| Mass Percent | (Mass of solute / Mass of solution) × 100 | % |
| Volume Percent | (Volume of solute / Volume of solution) × 100 | % |
| Mass-Volume Percent | (Mass of solute in g / Volume of solution in mL) × 100 | % |
| Parts Per Million (ppm) | (Mass of solute / Mass of solution) × 10⁶ | ppm |
| Mole Fraction | Moles of component / Total moles of all components | Dimensionless |
An important note: Molarity changes with temperature (because volume changes), but Molality does not change with temperature (because it uses mass, which is temperature-independent). For precise lab work involving temperature variations, Molality is preferred.
Concentration units — especially Molarity, Molality, and ppm — are very important to remember.
The concept of ppm is especially relevant in environmental chemistry (pollutant levels in air/water). Colligative properties explain why seawater freezes at a lower temperature than fresh water — a geographical phenomenon.
Colloids — The In-Between World
Here is a beautiful paradox: milk looks homogeneous — it appears uniform and smooth. Yet, if you look at it under a powerful microscope, you will find tiny fat droplets dispersed in water. It is neither a true solution nor a simple suspension.
This is the world of Colloids — a state of matter that occupies the fascinating middle ground, and nature has used it brilliantly in almost everything we eat, breathe, and apply on our skin.
What is a Colloid?
A Colloid is a heterogeneous mixture in which one substance (the Dispersed Phase) is distributed evenly throughout another substance (the Dispersion Medium).
The key distinction is particle size: colloidal particles range from 1 nm to 1000 nm — larger than solution particles but smaller than suspension particles.
Unlike solutions, colloidal particles are large enough to scatter light (Tyndall Effect) but small enough to not settle under gravity. This gives colloids their unique, almost magical stability.
Properties of Colloids
- Heterogeneous at Microscopic Level: Colloids appear homogeneous to the naked eye but reveal their two-phase nature under a microscope.
- Particle Size (1 nm to 1000 nm): Particles are visible under an electron microscope but not the naked eye.
- Tyndall Effect: When a beam of light passes through a colloid, the path of the beam becomes visible due to scattering by colloidal particles. This is why you can see the beam of a torch in a foggy night — fog is a colloid! In a true solution, no such scattering occurs.
- Stability (Brownian Motion): Colloidal particles are in constant, random, zigzag motion due to collisions with molecules of the dispersion medium. This Brownian Motion keeps them suspended perpetually — they do not settle.
- Non-filterable: Cannot be filtered through ordinary filter paper, but can be separated by Ultrafiltration or Centrifugation.
Types of Colloids — Based on Physical States
| Dispersed Phase | Dispersion Medium | Colloid Type | Example |
| Solid | Solid | Solid Sol | Ruby glass, Gemstones |
| Solid | Liquid | Sol | Paints, Muddy water |
| Solid | Gas | Aerosol | Smoke, Dust in air |
| Liquid | Solid | Gel | Jelly, Cheese, Butter |
| Liquid | Liquid | Emulsion | Milk, Mayonnaise |
| Liquid | Gas | Aerosol | Mist, Fog, Clouds |
| Gas | Solid | Solid Foam | Pumice stone, Foam rubber |
| Gas | Liquid | Foam | Shaving cream, Whipped cream |
Types Based on Interaction with Medium
- Lyophilic Colloids (Liquid-loving): The dispersed phase has a strong affinity for the dispersion medium. These are stable and easily prepared. Examples: Gelatin, Starch, Gum in water.
- Lyophobic Colloids (Liquid-hating): Little or no affinity for the dispersion medium. Less stable; need stabilising agents. Examples: Gold sol, Fe(OH)₃ sol.
Types Based on Particle Size
- Multimolecular Colloids: Formed by aggregation of many small molecules. E.g., Sulfur sol, Gold sol.
- Macromolecular Colloids: Formed by dispersing macromolecules in a medium. E.g., Proteins, Starch, Cellulose.
- Associated Colloids (Micelles): Substances that behave as normal electrolytes at low concentrations but form micelles at higher concentrations. E.g., Soap solution, Detergent solution. This is how soap cleans — the Micelle traps grease in its hydrophobic core and is washed away by water!
Emulsions — A Special Type of Colloid
An Emulsion is a colloid where two immiscible liquids (liquids that do not mix, like oil and water) are combined such that one is dispersed as tiny droplets in the other.
Think of oil and vinegar salad dressing — left undisturbed, they separate; shaken together with an emulsifier, they form an emulsion.
Emulsifiers stabilise emulsions by reducing surface tension between the two liquids, preventing droplets from merging (coalescing). Natural emulsifiers include Lecithin (from egg yolk) and Proteins; Artificial ones include Detergents and Soaps.
- Oil-in-Water Emulsion: Oil droplets dispersed in water. Examples: Milk (fat droplets in water), Mayonnaise.
- Water-in-Oil Emulsion: Water droplets dispersed in oil. Examples: Butter, Cream.
Emulsions find applications in the Food industry (ice cream, salad dressings), Pharmaceuticals (cod liver oil), Cosmetics (creams, lotions), Paints, and even Photography (photographic film emulsions).
Suspension — The Visible Rebel
Have you ever shaken a bottle of Himalayan shilajit or an Ayurvedic syrup and noticed the instruction ‘shake well before use’? That is because these are Suspensions — the particles settle at the bottom when left undisturbed.
Unlike solutions and colloids, Suspensions make no attempt to hide — their particles are visible to the naked eye, and given half a chance, they will sink to the bottom.
What is a Suspension?
A Suspension is a heterogeneous mixture where solid particles are dispersed in a liquid or gas. The particles are larger than 1000 nm — large enough to be seen, large enough to settle.
If you leave a suspension undisturbed, gravity wins, and the particles sink. They can, however, be separated by simple filtration.
Key Characteristics of Suspensions
- Heterogeneous: Components are not uniformly distributed.
- Particle Size > 1000 nm: Particles are visible to the naked eye.
- Settling: Particles settle at the bottom when left undisturbed.
- Opaque or Cloudy: Large particles block light, making suspensions non-transparent.
- Filterable: Particles can be easily separated by filtration.
- No Tyndall Effect: Particles are too large and irregular to scatter light effectively in the Tyndall manner.
- Unstable: Suspensions are inherently unstable; particles settle over time.
Types of Suspension
Based on particle size: Coarse Suspension (large particles, settle rapidly — e.g., sand in water) and Fine Suspension (smaller particles, settle slowly — e.g., chalk powder in water).
Based on stability: Stable Suspension (remains dispersed for extended periods using stabilisers — e.g., blood, which has natural protein stabilisers) and Unstable Suspension (settles quickly — e.g., freshly mixed clay in water).
Based on particle behaviour: Flocculated Suspension (particles form loose clumps/flocs that settle quickly but redisperse easily — e.g., sewage sludge) and Deflocculated Suspension (particles remain individual, settle slowly to form a compact sediment that is hard to redisperse — e.g., paints, inks).
Based on medium: Aqueous Suspension (water as medium — e.g., muddy water) and Non-Aqueous Suspension (oil or organic solvent as medium — e.g., pigments in oil-based paints).
The Master Comparison — Solutions, Colloids, and Suspensions
Now let us look at these three types of mixtures side by side.
| Feature | Solution | Colloid | Suspension |
| Particle Size | < 1 nm | 1 nm – 1000 nm | > 1000 nm |
| Nature of Mixture | Homogeneous | Apparently Homogeneous | Heterogeneous |
| Visibility of Particles | Not visible | Visible under microscope | Visible to naked eye |
| Settling | Does not settle | Does not settle | Settles on standing |
| Filtration | Cannot be filtered | Cannot be filtered | Can be filtered |
| Tyndall Effect | No | Yes | No |
| Stability | Very stable | Moderately stable | Unstable |
| Example | Saltwater, Sugar water | Milk, Fog, Blood | Muddy water, Sand in water |