STATES OF MATTER - properties of gases and liquids (fluids) and solids

15. A kinetic particle model for dissolving a solid (the solute) in a solvent to give a solution i.e. describing & explaining what is a solution & how it is formed

INDEX of all my notes on the states of matter

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

15. Dissolving solids and solution formation

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• 3a. WHAT HAPPENS TO PARTICLES WHEN A SOLID DISSOLVES IN A LIQUID SOLVENT?

• What do the words SOLVENT, SOLUTE and SOLUTION mean?

• When a solid (the solute) dissolves in a liquid (the solvent) the resulting mixture is called a solution.

  • In general: solute + solvent == dissolving ==> solution

  • So, the solute is what dissolves in a solvent, a solvent is a liquid that dissolves things and the solution is the result of dissolving something in a solvent.

  • The solid loses all its regular structure and the individual solid particles (molecules or ions) are now completely free from each other and randomly mix with the original liquid particles, and all particles can move around at random.

  • This describes salt dissolving in water, sugar dissolving in tea or wax dissolving in a hydrocarbon solvent like white spirit.

  • It does not usually involve a chemical reaction, so it is generally an example of a physical change.

  • Whatever the changes in volume of the solid + liquid, compared to the final solution, the Law of Conservation of Mass still applies, no loss or gain of the total mass of the system.

  • This means:

  • mass of solid solute + mass of liquid solvent = mass of solution after mixing and dissolving.

  • You cannot create mass or lose mass, but just change the mass of the substances into another form.

  • If the solvent is evaporated, then the solid is reformed e.g. if a salt solution is left out for a long time or gently heated to speed things up, eventually salt crystals form, the process is called crystallisation.

• Homogeneous mixtures
  • A homogeneous mixture (of at least two substances) is a gaseous, liquid or solid mixture that has the same proportions (ratios) of its components throughout a given sample of the material. A homogeneous mixture is uniform in composition throughout the whole sample.
  • Typical examples include solutions e.g. aqueous solutions of soluble salts are homogeneous mixtures.

Key points about the Kinetic Particle Model & Dissolving

Core Explanation

• Particles in solids: Solute particles are closely packed in a fixed lattice, vibrating but not free to move.
• Particles in liquids (solvent): Solvent particles are free to move, sliding past each other with kinetic energy.
• Dissolving process:
  1. Solvent particles collide with solute particles at the surface.
  2. Solvent particles break solute particles away from the lattice.
  3. Solute particles become surrounded by solvent particles (solvation/hydration).
  4. The solute particles spread evenly throughout the solvent due to random motion → forming a solution.

Examples of solutions

• Salt (NaCl) in water: Water molecules surround Na⁺ and Cl⁻ ions, separating them from the lattice.
• Sugar in tea: Water molecules surround sugar molecules, dispersing them evenly.
• Copper sulfate crystals in water: Blue solution forms as Cu²⁺ ions spread throughout.

Typical Exam Board-Specific Requirements

Student Exam Tips about dissolving and solutions

• Draw particle diagrams: Show solute lattice breaking apart, solvent particles surrounding solute.
• Use keywords: "random motion," "collisions," "spread evenly," "solution."
• Link to diffusion: Dissolving is followed by diffusion of solute particles.
• State energy role: Solvent particles must have enough kinetic energy to overcome solute lattice forces.
• Always define solution: A homogeneous mixture where solute particles are evenly distributed.

Typical Misconceptions about dissolving

•  “Particles disappear when dissolved” → Correction: They are still present, just spread out.
•  “Solute particles only sink to the bottom” → Correction: They disperse throughout the solvent.
•  “Dissolving is the same as melting” → Correction: Melting = solid → liquid (same substance); dissolving = solute + solvent → solution.
•  “Only water can dissolve substances” → Correction: Many solvents exist (e.g. ethanol dissolves iodine).
•  “Solute particles stop moving once dissolved” → Correction: They continue moving randomly due to kinetic energy.

Quick Revision Overlay about solution formation

• Solid solute: Fixed lattice.
• Liquid solvent: Free-moving particles.
• Dissolving: Solvent particles collide → break solute particles free → surround them → distribute evenly.
• Result: Homogeneous solution.

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