Particles, diffusion, osmosis, and the three states — everything you need to understand the physical world around you.
Section 1 — The Particulate Theory of Matter
Matter is anything that has mass and occupies space. All matter is made of tiny particles — and understanding how those particles behave explains almost everything in chemistry and physics.
The Four Main Ideas
- 1All matter is made of particles (atoms, molecules or ions).
- 2Particles are in constant, random motion — they never stop!
- 3There are spaces between the particles — other particles can move into these gaps.
- 4There are forces of attraction between the particles — these hold matter together.
Section 2 — Diffusion
Diffusion occurs in gases and liquids — NOT in solids (particles are fixed in position). It provides evidence that particles can move and that there are spaces between particles.
🔬 Experiment 1 — KMnO₄ in Water
When a purple potassium manganate(VII) crystal is placed in still water, the purple colour slowly spreads throughout. After a few days the colour is evenly distributed — particles moved from high to low concentration.
🔬 Experiment 2 — NH₃ & HCl Gas
Cotton wool soaked in NH₃ and HCl placed at opposite ends of a sealed tube. The gases diffuse towards each other and form a white ring of NH₄Cl — closer to the HCl end because NH₃ molecules are lighter and diffuse faster.
NH₃(g) + HCl(g) → NH₄Cl(s)
Section 3 — Osmosis
Osmosis is a special case of diffusion — it involves only water molecules, and it requires a differentially permeable (semi-permeable) membrane that allows water through but blocks larger solute particles.
🥭 Paw-Paw Experiment
| Beaker Contents | What Happens to Strip | Explanation |
|---|---|---|
| Distilled water | Strip becomes longer and more rigid | Distilled water has MORE water molecules than paw-paw cells. Water moves INTO cells by osmosis. Cells swell → strip lengthens. |
| Concentrated NaCl solution | Strip becomes shorter and softer (floppy) | NaCl solution has FEWER water molecules than paw-paw cells. Water moves OUT of cells by osmosis. Cells shrink → strip shortens. |
Practical Uses of Osmosis
🐌 Controlling Garden Pests
Salt sprinkled on slugs dissolves in moisture around their body, forming a concentrated solution. Water moves out of the slug's body by osmosis — they dehydrate and die.
🥩 Food Preservation
Salt and sugar draw water out of food cells AND microorganisms by osmosis — without water, bacteria and fungi cannot grow. Examples: salt fish, salt pork, crystallised fruit, guava jelly.
Diffusion vs Osmosis — Comparison
| Feature | Diffusion | Osmosis |
|---|---|---|
| Particles involved | Any particles | Water molecules only |
| Membrane required? | NO — occurs freely | YES — differentially permeable |
| Direction of movement | High → low concentration | High water content → low water content |
| Example | KMnO₄ spreading in water | Paw-paw in concentrated solution |
Section 4 — The Three States of Matter
The same particles can exist in three different states depending on temperature and pressure. The state determines how the particles are arranged and how they move.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed shape — particles in fixed positions | Takes shape of container; surface always horizontal | Takes shape AND volume of entire container |
| Volume | Fixed volume | Fixed volume | Variable — expands to fill container |
| Density | Usually HIGH | Usually lower than solid | Very LOW |
| Compressibility | Very difficult to compress | Can be compressed slightly | Very easy to compress |
| Particle arrangement | Closely packed, regular pattern | Randomly arranged, small spaces | Randomly arranged, large spaces |
| Forces of attraction | VERY STRONG | Weaker than solid | VERY WEAK |
| Kinetic energy | Very small — vibrate in fixed position | More than solid — move slowly past each other | Large — move freely and rapidly |
Section 5 — Changes of State
Matter can change from one state to another by adding (heating) or removing (cooling) energy. These are physical changes — the chemical composition does not change.
| Change of State | Process Name | Energy Change | What Happens to Particles |
|---|---|---|---|
| Solid → Liquid | Melting | Energy ADDED | Particles gain KE, vibrate more vigorously, overcome forces of attraction, move more freely |
| Liquid → Gas | Evaporation / Boiling | Energy ADDED | Particles gain KE, overcome forces of attraction, escape to form gas |
| Gas → Liquid | Condensation | Energy REMOVED | Particles lose KE, slow down, forces of attraction pull them closer |
| Liquid → Solid | Freezing | Energy REMOVED | Particles lose KE, slow down further, forces of attraction become very strong |
| Solid → Gas (direct) | Sublimation | Energy ADDED | Particles go directly from solid to gas, bypassing liquid state |
| Gas → Solid (direct) | Deposition | Energy REMOVED | Particles go directly from gas to solid, bypassing liquid state |
Evaporation vs Boiling
| Feature | Evaporation | Boiling |
|---|---|---|
| Temperature | Can occur at any temperature | Only at a specific temperature (boiling point) |
| Where it occurs | At the surface of the liquid only | Throughout the entire liquid AND at its surface |
| Cooling effect | Takes energy from liquid — cools it | External heat source needed to maintain boiling |
Section 6 — Heating & Cooling Curves
A heating curve plots temperature against time as a substance is heated from solid → liquid → gas. The flat horizontal sections are where changes of state occur.
Reading a Heating Curve (e.g. Water)
- 1Rising slope (first): Substance is a SOLID, temperature rising as KE increases.
- 2First flat section (0°C for water): MELTING — solid and liquid coexist. Energy breaks forces of attraction, not raising temperature.
- 3Rising slope (middle): Substance is a LIQUID, temperature rising.
- 4Second flat section (100°C for water): BOILING — liquid and gas coexist. Energy used to completely overcome forces of attraction.
- 5Rising slope (last): Substance is a GAS, temperature rising.
Section 7 — Resources & Simulations
Section 8 — CSEC Practice Questions
(A) water molecules are attracted to KMnO₄ (B) particles are in constant random motion (C) KMnO₄ is an ionic compound (D) temperature increases diffusion rate
1 The purple colour spreading is caused by KMnO₄ particles moving from the crystal (high concentration) outward through the water (low concentration).
2 This movement is possible because particles are in constant random motion — they move around and gradually spread out.
3 Options A, C, and D are either irrelevant or not what this specific observation demonstrates.
1 NH₃ has a relative molecular mass of 17; HCl has a relative molecular mass of 36.5. NH₃ molecules are therefore lighter.
2 Lighter particles move faster at the same temperature. NH₃ molecules diffuse faster and travel a greater distance in the same time.
3 The two gases meet closer to the HCl end (where HCl started), because NH₃ travelled most of the distance.
1 The concentrated sucrose solution has a lower concentration of water molecules than the potato cell contents.
2 Water molecules move by osmosis from the potato cells (higher water concentration) through the differentially permeable cell membranes into the sucrose solution (lower water concentration).
3 As water leaves the cells, they lose their rigidity. The cells shrink (become plasmolysed), so the potato strip decreases in length and becomes soft/floppy.
a Oxygen gas: Gas particles are far apart with large spaces between them. When pressure is applied, the particles can be pushed closer together — the spaces between them decrease. Therefore it is easy to compress.
b Iron rod: In a solid, particles are already packed closely together with very little space between them. There are no significant spaces for particles to move into, so compression is extremely difficult.
c Constant temperature during melting: The energy added goes entirely towards overcoming the forces of attraction between particles — breaking the bonds that hold them in fixed positions. It does NOT increase kinetic energy, so temperature remains constant until melting is complete.
a First flat section = melting point = 12°C. Second flat section = boiling point = 65°C.
b At 40°C, the temperature is between the melting point (12°C) and boiling point (65°C), so the substance is in the liquid state.
c Substances that sublime: Iodine (I₂) and Carbon dioxide (CO₂) / dry ice. Also acceptable: ammonium chloride, naphthalene.