Solute and Solvent
Solutions are homogeneous mixtures of two or more components. The word homogeneous carries a precise meaning here: the composition and the properties of the mixture are uniform throughout. A spoonful drawn from the top of a sugar solution has the same sweetness, density and refractive index as a spoonful drawn from the bottom. This uniformity is what separates a true solution from a coarse mixture such as sand stirred into water.
Within any solution the components are assigned two roles. The component present in the largest quantity is called the solvent, and a single defining property follows from this: the solvent determines the physical state in which the solution exists. The one or more components present in amounts smaller than the solvent are called solutes. In a glass of glucose dissolved in water, water is the solvent and glucose is the solute; the solution is a liquid because the solvent, water, is a liquid.
| Term | Definition | Defining feature |
|---|---|---|
| Solvent | Component present in the largest quantity | Determines the physical state of the solution |
| Solute | Component(s) present in smaller quantity | Dispersed uniformly through the solvent |
| Solution | The homogeneous mixture as a whole | Uniform composition and properties throughout |
Binary Solutions
The NCERT treatment of solutions restricts itself, for the purpose of classification, to binary solutions — solutions consisting of exactly two components, that is one solute and one solvent. Real solutions can of course hold several solutes at once; sea water carries sodium chloride, magnesium salts, dissolved oxygen and much else in the same body of water. But limiting the discussion to two components makes the state-by-state classification clean and exhaustive, and every type discussed below is a binary solution.
In a binary solution each of the two components may independently be solid, liquid or gas. It is this freedom that generates the full set of solution types, and it is worth pausing on the counting before listing the examples.
Classifying by Physical State
The classification rests on a single combinatorial idea. The solute can be in any one of three physical states — gas, liquid or solid — and the solvent can independently be in any one of the same three states. Pairing each solute state with each solvent state gives $3 \times 3 = 9$ possible combinations. These nine combinations are the nine types of solutions, and they exhaust every binary possibility.
Because the solvent fixes the overall state of the solution, the nine types fall naturally into three larger families: gaseous solutions (gas solvent), liquid solutions (liquid solvent) and solid solutions (solid solvent). The schematic below maps the branching out.
The three solvent states (top row) each accept three solute states, generating the nine binary solution types shown along the bottom.
The Nine Types — Master Table
The table below is the heart of this subtopic. Read each row as a pairing of solute state × solvent state → type → example. The three groupings of rows correspond to the three solvent states, which is also the overall state of the solution. Memorise the examples by anchoring each to its row; NEET questions usually quote an example and ask for its type, or quote a type and ask for the matching example.
| Solute state | Solvent state | Solution family | Common example |
|---|---|---|---|
| Gas | Gas | Gaseous | Mixture of oxygen and nitrogen gases (air) |
| Liquid | Gas | Chloroform mixed with nitrogen gas; humidity in air | |
| Solid | Gas | Camphor in nitrogen gas (camphor in air) | |
| Gas | Liquid | Liquid | Oxygen dissolved in water; soda water ($\ce{CO2}$ in water) |
| Liquid | Liquid | Ethanol dissolved in water | |
| Solid | Liquid | Glucose dissolved in water; sugar in water | |
| Gas | Solid | Solid | Solution of hydrogen in palladium |
| Liquid | Solid | Amalgam of mercury with sodium; mercury in gold | |
| Solid | Solid | Copper dissolved in gold; alloys such as brass (zinc in copper) |
"Liquid" in a row does not mean the solution is liquid
The overall state of the solution is set by the solvent, not the solute. A liquid solute in a gaseous solvent (chloroform vapour in nitrogen) is a gaseous solution; a liquid solute in a solid solvent (sodium amalgam) is a solid solution. Students who read the first column instead of the second misclassify these every time.
Always classify by the solvent's state. The solute's state only labels the row.
Gaseous Solutions
When the solvent is a gas, the solution is gaseous and occupies the top three rows. The cleanest example is air, a homogeneous mixture of oxygen and nitrogen — a gas dissolved in a gas. Because all gases mix freely in all proportions, every gas-in-gas mixture qualifies as a solution. A volatile liquid such as chloroform can evaporate into nitrogen to give a liquid-in-gas solution, and the same is true of water vapour as humidity in air. A volatile solid such as camphor sublimes into the surrounding nitrogen or air to give a solid-in-gas solution.
Teal = gaseous solvent particles. The solute differs in state — gas (purple), liquid vapour (coral squares), sublimed solid (amber triangles) — but the solvent being a gas makes all three gaseous solutions.
Liquid Solutions
Liquid solutions are by far the most important class for this chapter, because vapour pressure, Raoult's law and the colligative properties are all developed for liquid solvents. The three rows here are a gas in a liquid, a liquid in a liquid, and a solid in a liquid. Oxygen dissolved in water — the dissolved oxygen that sustains aquatic life — is the gas-in-liquid case, as is soda water, which is carbon dioxide forced into water: $\ce{CO2(g) + H2O(l) -> CO2(aq)}$. Ethanol in water is the liquid-in-liquid case, and glucose or sugar in water is the solid-in-liquid case.
Liquid-in-liquid mixtures deserve a closer look, because when two liquids are brought together three outcomes are possible. The two liquids may be completely miscible and dissolve in each other in all proportions, partially miscible and dissolve only to a limited extent, or immiscible and not dissolve at all. The table records the standard NCERT and NIOS examples for each.
| Miscibility of two liquids | Meaning | Example |
|---|---|---|
| Completely miscible | Dissolve in each other in all proportions | Alcohol and water; benzene and toluene |
| Partially miscible | Dissolve in each other only to a certain extent | Water and phenol |
| Immiscible | Do not dissolve in each other | Water and benzene; water and toluene |
For two completely miscible liquids, the labelling of solute and solvent follows the general rule: the constituent present in the smaller amount is the solute and the one present in the larger amount is the solvent. The solubility of a liquid in a liquid generally increases as the temperature rises.
Once a solution is classified, the next task is to quantify how much solute it holds. See Concentration Units for molarity, molality, mole fraction and mass percentage.
Solid Solutions
When the solvent is a solid, the solution is a solid solution — a single solid phase with the solute distributed uniformly through the solid lattice. A gas dissolved in a solid is illustrated by hydrogen in palladium, a metal that absorbs hydrogen into its lattice. A liquid in a solid is illustrated by an amalgam, such as the amalgam of mercury with sodium, or mercury dissolved in gold. A solid dissolved in a solid is the realm of alloys: copper dissolved in gold, or brass, which is zinc dissolved in copper, and bronze, which is tin in copper.
Q. Classify each of the following by solute and solvent state, and name the solution family: (a) brass, (b) soda water, (c) camphor in air.
(a) Brass is zinc dissolved in copper — solid solute in a solid solvent, hence a solid solution (an alloy).
(b) Soda water is $\ce{CO2}$ gas dissolved in liquid water — gas solute in a liquid solvent, hence a liquid solution.
(c) Camphor in air is a sublimed solid in a gaseous solvent — solid solute in a gas solvent, hence a gaseous solution.
Aqueous vs Non-aqueous
Within the liquid solutions, a further everyday distinction is drawn by the identity of the solvent. An aqueous solution is one in which water is the solvent — glucose in water, sodium chloride in water, oxygen in water. Water is the most common solvent in chemistry and in biology, and almost all processes in the body occur in some kind of aqueous solution. A non-aqueous solution is one whose solvent is a liquid other than water, such as ethanol, benzene or chloroform.
Both aqueous and non-aqueous solutions are liquid solutions in the nine-type scheme; the distinction is only about which liquid plays the solvent. Recognising it matters because the numerical work later in the chapter — vapour pressure, boiling-point elevation, freezing-point depression — is most often set in an aqueous medium, while questions on Raoult's law and deviations frequently use non-aqueous pairs such as ethanol and acetone.
A solution must be homogeneous
Suspensions and colloids are not true solutions. The nine types apply only to homogeneous mixtures with uniform composition and properties throughout. A heterogeneous mixture — oil floating on water, sand in water — has a visible boundary and is excluded, even though the components are in two states.
No uniform single phase, no solution. Check homogeneity before assigning a type.
Types of Solutions in one screen
- A solution is a homogeneous mixture; the solvent is the major component and fixes the solution's physical state, the solute is the minor component.
- A binary solution has two components; with 3 solute states × 3 solvent states there are exactly nine types.
- Classify by the solvent's state: gas solvent → gaseous solution, liquid solvent → liquid solution, solid solvent → solid solution.
- Key examples: air (gas–gas), camphor in air (solid–gas), soda water (gas–liquid), ethanol in water (liquid–liquid), glucose in water (solid–liquid), $\ce{H2}$ in palladium (gas–solid), sodium amalgam (liquid–solid), brass (solid–solid).
- Aqueous = water as solvent; non-aqueous = any other liquid solvent; both are liquid solutions.