2.4 Solution

This topic introduces solutions — homogeneous mixtures in which one substance is dissolved in another. It covers the parts of a solution, particle size, and the three physical states in which solutions can exist.

Definition

A solution is a homogeneous mixture of two or more substances in which one substance is dissolved in the other.

Simple explanation: A solution is a mixture that looks the same throughout — you cannot see the different parts.

Key terms:

• Solute — the substance that is dissolved.
• Solvent — the substance in which the solute is dissolved.

Main Idea

Homogeneous means that no particles or parts of different substances can be seen. When one substance dissolves, the solution looks exactly the same.

• particles are microscopic, less than 1 nm in diameter
• the mixture is very stable
• the solute does not separate from the solvent by itself

Simple explanation: If you stir salt into water, the salt disappears (dissolves) and the water looks clear. That's a solution — uniform and stable.

Core Concept

Every solution has two parts:

• Solute — the substance being dissolved (present in smaller amount usually)
• Solvent — the substance doing the dissolving (present in larger amount usually)

• Salt = **solute**
• Water = **solvent**

More than one solute may be present in a solution.

Detailed Explanation

Examples of Solutions

Salt solution: salt is the solute; water is the solvent.

Soft drinks: water is the solvent; other substances like sugar, salts, and CO₂ are solutes.

Air: Nitrogen gas is the solvent; oxygen, carbon dioxide, and trace gases are solutes.

Classification Based on Physical States

On the basis of physical states of solvent and solute, solutions can be categorized as solid, liquid, and gaseous solutions. Generally, solutions are found in three physical states depending on the physical state of the solvent:

• Air → gaseous solution
• Sea water → liquid solution
• Alloy → solid solution

1. Gaseous Solutions

In gaseous solutions, the solvent is a gas and the solute can be a gas, liquid, or solid.

• Mixture of nitrogen and hydrogen used in Haber's process (ammonia formation)
• Mixture of ammonia and carbon dioxide used for urea preparation
• Fog, clouds, and mist — liquid water (solute) dissolved in air (solvent)
• Smoke — carbon particles (solute) in gaseous air (solvent)

2. Liquid Solutions

In liquid solutions, the solvent is a liquid.

• Carbonated drinks — solvent is liquid water, solute is gaseous carbon dioxide
• Rectified spirit — produced by fermentation of sugar cane
• Vinegar — acetic acid dissolved in water (liquid in liquid)
• Brine — solid salt dissolved in water
• Sugar syrup — solid sugar dissolved in water

3. Solid Solutions

In solid solutions, the solvent is a solid.

• Hydrogen gas on nickel metal surface (used in ghee industry) — hydrogen gas is solute, nickel catalyst is solvent
• Amalgam — solution of any metal (solid) in liquid mercury
• Alloys — formed by mixing different metals (brass, bronze, steel)

Intermediate Level

Why Solutions Are Stable

• The solute particles are very small (less than 1 nm).
• They are evenly distributed throughout the solvent.
• They do not settle down over time.
• They cannot be separated by simple filtration.

Particle Size Comparison

Note: Colloids and suspensions are covered in Section 2.5.

Real-Life Importance

• Sea water — a liquid solution of many salts
• Alloys — solid solutions used in industry (brass, bronze, steel)
• Soft drinks — liquid solutions of sugars, salts, CO₂
• Air — gaseous solution essential for life

Important Points

• - Solution: homogeneous mixture of two or more substances.
• - Solute: substance that dissolves.
• - Solvent: substance in which the solute dissolves.
• - Homogeneous: same composition throughout; no visible particles.
• - Particle size in solution: less than 1 nm.
• - Stability: solutions are stable; solute does not separate by itself.
• - Three states of solutions: gaseous, liquid, solid (based on solvent state).
• - Examples:
• - Gaseous: air, fog, smoke, Haber's process mixture
• - Liquid: carbonated drinks, vinegar, brine, sugar syrup
• - Solid: alloys, amalgam, hydrogen on nickel

Remember

• - Solution = homogeneous mixture.
• - Solute = dissolved substance.
• - Solvent = dissolving substance.
• - Particle size < 1 nm.
• - Solutions are stable — no settling.
• - Three types: gaseous, liquid, solid solutions.
• - In air, nitrogen is the solvent; oxygen, CO₂, trace gases are solutes.
• - In soft drinks, water is solvent; sugar, salts, CO₂ are solutes.

Common Mistakes

• 1. Thinking the solute is always solid. Solutes can be gas (CO₂ in drinks), liquid (acetic acid in water), or solid (salt in water).
• 2. Thinking the solvent is always liquid. Solvents can be gas (nitrogen in air) or solid (nickel in hydrogen–nickel solution).
• 3. Confusing solutions with suspensions. Solutions are homogeneous and clear; suspensions have visible particles.
• 4. Thinking solute particles are visible. In a solution, particles are too small to see (less than 1 nm).
• 5. Thinking solutions are unstable. Solutions are very stable — the solute doesn't settle out.

Quick Revision

• Solution = homogeneous mixture of two or more substances.
• Solute = substance dissolved; Solvent = substance that dissolves.
• Particles are microscopic (< 1 nm).
• Solutions are stable; solute does not separate.
• Three states of solutions: gaseous, liquid, solid.
• Gaseous: air, fog, clouds, mist, smoke, Haber's process mixture.
• Liquid: carbonated drinks, rectified spirit, vinegar, brine, sugar syrup.
• Solid: hydrogen on nickel, amalgam, alloys (brass, bronze, steel).
• In air, N₂ is solvent; O₂, CO₂, trace gases are solutes.
• In soft drinks, water is solvent; sugar, salts, CO₂ are solutes.

60-Second Revision

• Solution = homogeneous mixture.
• Solute = dissolved; Solvent = dissolving.
• Particle size < 1 nm; stable; solute doesn't settle.
• Three types: gaseous, liquid, solid.
• Gaseous: air, fog, smoke.
• Liquid: carbonated drinks, brine, vinegar.
• Solid: alloys, amalgam, hydrogen on nickel.
• Warning: solutes and solvents can be solid, liquid, or gas — not always one type.