Question
What are the nine possible types of solutions based on the physical state of solute and solvent? Can we have a solid dissolved in a gas, or a gas dissolved in a solid?
(CBSE 12, JEE Main, NEET — solution classification is a direct theory question worth 1 mark)
Solution — Step by Step
A solution has two components: solute (dissolved substance, usually in lesser amount) and solvent (dissolving medium, usually in greater amount). Both can be solid, liquid, or gas — giving combinations.
| Solute | Solvent | Type | Example |
|---|---|---|---|
| Gas | Gas | Gas in gas | Air ( in ) |
| Liquid | Gas | Liquid in gas | Humidity (water vapour in air), mist |
| Solid | Gas | Solid in gas | Smoke (carbon particles in air), camphor vapour in air |
| Gas | Liquid | Gas in liquid | Aerated water ( in water), dissolved in water |
| Liquid | Liquid | Liquid in liquid | Alcohol in water, petrol in kerosene |
| Solid | Liquid | Solid in liquid | Sugar in water, saline solution |
| Gas | Solid | Gas in solid | in palladium |
| Liquid | Solid | Liquid in solid | Amalgam (Hg in Na), hydrated salts |
| Solid | Solid | Solid in solid | Alloys (brass = Cu + Zn), steel (C in Fe) |
The solutions we deal with most often have liquid solvents (especially water). Concentration is expressed as:
- Molarity (M): moles of solute per litre of solution
- Molality (m): moles of solute per kg of solvent
- Mole fraction (): ratio of moles of one component to total moles
- Mass percentage (w/w%): mass of solute per 100 g of solution
Of these, molality is temperature-independent (mass does not change with temperature, but volume does).
- Gas in gas is always a homogeneous mixture (true solution) — gases mix in all proportions
- Alloys are solid solutions: brass (Cu + Zn), bronze (Cu + Sn), stainless steel (Fe + Cr + Ni + C)
- in palladium is the classic example of gas in solid — Pd can absorb up to 900 times its own volume of
- In liquid-liquid solutions, the component present in larger amount is conventionally the solvent
flowchart TD
A["Solution Classification"] --> B["By Solvent State"]
B --> C["Gas solvent"]
B --> D["Liquid solvent"]
B --> E["Solid solvent"]
C --> C1["Air, mist, smoke"]
D --> D1["Soda water, sugar solution, alcohol-water"]
E --> E1["Alloys, amalgam, H₂ in Pd"]
F["Most exam-relevant"] --> D
D --> G["Concentration units: M, m, χ, w/w%"]Why This Works
The classification is based on a simple physical principle: any substance can be dispersed at the molecular level in any other substance, provided the intermolecular interactions allow it. The key condition for a true solution is that the mixing is homogeneous — the composition is uniform throughout at the molecular level. This distinguishes solutions from colloids (1-1000 nm particles) and suspensions (> 1000 nm particles).
Common Mistake
Students often say “gases cannot be solutes in solids.” They can — in palladium is a textbook example. The molecules occupy interstitial sites in the palladium crystal lattice. Similarly, carbon dissolved in iron (steel) is a solid-in-solid solution where C atoms fit into the gaps between Fe atoms. Do not assume that the larger state of matter must be the solvent.
For quick recall in exams: Air (gas-gas), Soda (gas-liquid), Sugar water (solid-liquid), Brass (solid-solid), Amalgam (liquid-solid), H2 in Pd (gas-solid). These six cover the most commonly tested types.