Class 8 › Science › Chapter 7
Particulate Nature of Matter
Matter is made of tiny particles. Interparticle spaces, attractions and thermal energy explain the properties of solids, liquids and gases, and how particles move.
1. What Is Matter Composed Of?
- Chalk activity: break and grind chalk into finer and finer powder. Every speck is still chalk, and grinding is a physical change (no new substance forms; only the size reduces).
- Constituent particle: the basic unit that makes up a larger piece of a substance. If grinding went on, we would reach units that cannot be broken down further. Sand, clay, chalk and sugar are all made of huge numbers of these particles.
- Sugar in water: after stirring, you cannot see sugar, but the top layer tastes sweet, so it is present. Sugar breaks up into its constituent particles, each tiny grain having millions of them.
- Particles are so small that they cannot be seen even with an ordinary microscope.
- Interparticle spaces: the sugar particles occupy the empty spaces between the water particles. These spaces contain nothing at all (not air).
- Heritage: Acharya Kanad (Vaisheshika Sutras) first spoke of the Parmanu (atom): tiny, indivisible, eternal particles that make up matter.
2. What Decides the States of Matter?
- Particles are held together by interparticle attractions (attractive forces). Their strength depends on the nature of the substance and on the interparticle distance: even a slight increase in distance reduces the force drastically.
- The strength of these forces decides whether a substance is a solid, liquid or gas.
- The distance depends on the particles' thermal (heat) energy. So thermal energy ultimately determines the state.
3. Solid State
- Solids have a definite shape and volume. Particles are tightly packed with very strong attractions, held in fixed positions. They can only vibrate (oscillate) to and fro about their positions.
- On heating: vibrations become more vigorous; at a certain stage particles leave their positions, attractions weaken and the solid becomes a liquid.
- Melting point: the minimum temperature at which a solid melts to a liquid at atmospheric pressure.
- Weak interparticle attractions give a low melting point; strong attractions give a high melting point.
- In a liquid the particles are generally a little farther apart than in a solid. Ice is an exception: its particles are farther apart than in water.
| Solid | Melting point |
|---|---|
| Ice | 0 °C |
| Urea | 133 °C |
| Iron | 1538 °C |
4. Liquid State
- No fixed shape (takes the shape of its container) but fixed volume (200 mL of water stays 200 mL in containers of different shapes). Particles move freely but only within a limited space.
- Attractions are slightly weaker than in solids but still strong enough to keep particles close. Your finger can move through water (temporarily displacing it) but not through a solid.
- Boiling point: the temperature at which a liquid boils and turns to vapour at atmospheric pressure. Vapour forms fast, throughout the liquid (bubbles).
- Evaporation: vapour formation at all temperatures below the boiling point; slower and only at the surface (why spilled water dries up).
5. Gaseous State
- Gases have no fixed shape or volume. They fill the whole space available (smoke from an incense stick or iodine vapour spreads through the entire jar).
- Particles move freely in all directions; interparticle attractions are negligible.
- The visible smoke particles are constantly hit by invisible gas particles, which makes their motion visible.
- Fluids: liquids and gases both flow and do not keep a fixed shape.
6. Interparticle Spacing in the Three States
Particle arrangement: closely packed (solid), a little loose (liquid), far apart (gas). Schematic.
- Gases are compressible: pushing a syringe plunger (thumb over the opening) reduces the volume of air since the particles move closer; release it and the plunger moves back. Gas particles have a lot of space between them.
- Water is practically incompressible.
- Sugar in water: the water level rises on adding sugar but falls somewhat after it dissolves, so the solution's volume is less than the sum of the volumes of water and sugar. This shows there are spaces between the water particles which the sugar particles occupy.
- Sand does not dissolve; it settles and occupies space, so the total volume increases.
- Even in solids, some space is left between closely packed particles (and it is empty, not air).
- "Particle" has different meanings: in Suspended Particulate Matter (SPM) in air pollution, "particles" are tiny dust specks, which are far larger than constituent particles and are themselves made of atoms and molecules.
7. How Do Particles Move?
Evidence that particles are in constant motion
Potassium permanganate in water: pink streaks spread out, then the entire water turns uniform pink. Water particles pull particles out of the grain and then hit them, spreading them throughout.
Hot, room-temperature and ice-cold water: it spreads fastest in hot water and slowest in cold water. So particles move faster when heat is provided.
Incense stick in a corner: the fragrance reaches the whole room because air particles keep hitting the fragrance particles and spread them (also how perfume reaches us).
- Why sand is insoluble: its particles are held so strongly that water particles cannot pull them out.
- Soap and oil (Ever heard of): soap particles surround oil particles on the cloth; one end attaches to oil, the other mixes with water, lifting the oil off.
- Atoms and molecules (A step further): the particles of matter are atoms and molecules. Iron is made of iron atoms. Atoms of hydrogen, oxygen and sulfur cannot exist alone, so atoms of the same element join to form a molecule (two hydrogen atoms form one hydrogen molecule; a water molecule has two hydrogen atoms and one oxygen atom).
8. Comparing the Three States
| Solid | Liquid | Gas | |
|---|---|---|---|
| Shape | Fixed | Not fixed | Not fixed |
| Volume | Fixed | Fixed | Not fixed |
| Interparticle spacing | Minimum | A little more than solids | Maximum |
| Packing | Closely packed | A little loosely packed | Particles free |
| Interparticle attraction | Maximum | Slightly weaker than solids | Minimum (negligible) |
| Particle movement | Only vibrations | Within a limited space | In all the available space |
| Thermal energy | Low | Medium | High |
- At the melting point the thermal energy is used to overcome attractions so the solid becomes liquid; in gases the particles have enough energy to overcome the attractions entirely.
9. Quick Revision
Key idea
Three points
Matter is made of tiny particles; there is empty space between them; they attract each other and are always moving.
Remember
Effect of heat
Heating gives particles more energy: they vibrate or move faster and the attractions weaken. This leads to melting (solid to liquid) and boiling or evaporation (liquid to gas).
Exam tip
Explain any property
Link each property to three things: spacing, attraction and movement of particles. For example, gases mix easily because particles move freely and attractions are negligible.
Common mistakes:
• Thinking the space between particles contains air (it contains nothing).
• Saying liquids have no fixed volume (they do; only the shape is not fixed).
• Saying particles in solids do not move at all (they vibrate about fixed positions).
• Confusing boiling (throughout the liquid, at the boiling point) with evaporation (surface only, at any temperature).
• Confusing grinding (physical change; particles stay the same) with a chemical change.
• Calling dust particles in air the "constituent particles" of matter.
• Forgetting ice is an exception: its particles are farther apart than in water.
• Thinking the space between particles contains air (it contains nothing).
• Saying liquids have no fixed volume (they do; only the shape is not fixed).
• Saying particles in solids do not move at all (they vibrate about fixed positions).
• Confusing boiling (throughout the liquid, at the boiling point) with evaporation (surface only, at any temperature).
• Confusing grinding (physical change; particles stay the same) with a chemical change.
• Calling dust particles in air the "constituent particles" of matter.
• Forgetting ice is an exception: its particles are farther apart than in water.