All matter is made up of tiny particles called atoms and molecules. These particles are incredibly small (a water molecule is about 0.0000000003 m or 3 × 10-10 m across), too small to see with the naked eye or even an ordinary microscope.
States of Matter
Matter exists in three main states — solid, liquid, and gas — based on the arrangement and movement of particles.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | No fixed shape | No fixed shape |
| Volume | Fixed | Fixed | No fixed volume |
| Particle arrangement | Very close, orderly | Close, random | Far apart, random |
| Particle movement | Vibrate in place | Slide past each other | Move rapidly in all directions |
| Compressibility | Very low | Very low | High |
Diffusion
Diffusion is the mixing of particles of one substance into another due to random particle motion. It occurs in gases, liquids, and even slowly in solids.
- Diffusion is faster in gases than liquids (particles move faster in gases).
- Diffusion increases with temperature (particles have more kinetic energy → move faster).
- Evidence: smell of a perfume spreading across a room; a drop of ink spreading in water.
Brownian Motion
When pollen grains (or smoke particles) are observed under a microscope in water (or air), they show a zig-zag random motion called Brownian motion. This is caused by the bombardment of the pollen/smoke particles by the surrounding invisible water/air molecules. This is direct evidence for the particulate nature of matter.
Interconversion of States
Matter changes state when energy (heat) is added or removed:
Melting: Solid → Liquid. The temperature at which this occurs is the melting point. Heat energy breaks intermolecular bonds but temperature stays constant during the change — this absorbed heat is called latent heat of fusion.
Boiling/Vaporisation: Liquid → Gas. Occurs at the boiling point. Absorbed heat is the latent heat of vaporisation.
Evaporation: Liquid → Gas, but occurs below the boiling point at the surface. Faster with higher temperature, larger surface area, lower humidity, and breeze.
Condensation: Gas → Liquid (reverse of boiling/evaporation). Morning dew is water vapour condensing on cool surfaces.
Freezing: Liquid → Solid. Opposite of melting.
Sublimation: Solid → Gas directly (no liquid stage). Examples: dry ice (solid CO2), camphor, iodine crystals, naphthalene.
Deposition: Gas → Solid directly. Example: frost formation on windowpanes.
Key Values (for water at standard pressure)
- Melting point: 0 °C
- Boiling point: 100 °C
- Latent heat of fusion: 334 J/g
- Latent heat of vaporisation: 2260 J/g
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Diffusion in air — When perfume is sprayed in one corner of a room, molecules diffuse (spread) from the region of high concentration (near the bottle) to low concentration (rest of room). After a few minutes, the scent is detectable throughout the room even without a fan, proving gases diffuse on their own.
Diffusion in water — A drop of blue ink placed carefully in still water in a glass gradually spreads until the water is uniformly light blue. Ink molecules diffuse from regions of high concentration to low concentration. This is slower than in air because liquid particles move more slowly.
Brownian motion — Robert Brown (1827) observed pollen grains in water through a microscope. The grains moved erratically in all directions — not because the grains were alive, but because countless invisible water molecules were bombarding them from all sides. When bombardment is unequal, there is a net force in a direction, causing visible movement.
Latent heat during melting — Ice at 0 °C is placed in a heated container. Temperature stays at 0 °C until all ice melts, even though heat is continuously supplied. This energy (334 J per gram) breaks bonds between water molecules without raising temperature. This heat is the latent heat of fusion.
Evaporation causes cooling — When you step out of a swimming pool, you feel cold even on a warm day. Water evaporates from your skin. The most energetic molecules escape as vapour, leaving less energetic (cooler) molecules behind. This is why sweat cools the body.
Sublimation of dry ice — Solid CO2 (dry ice) placed in air directly converts to CO2 gas at –78 °C without forming liquid. It is used in theatres for fog effects. Camphor balls also sublimate — they shrink over time without leaving a liquid residue.
States and particle distance — 1 gram of water (liquid, about 1 cm3) when converted completely to steam (gas) at 100 °C occupies approximately 1671 cm3 — about 1671 times more volume. This shows that gas particles are about 10-20 times farther apart than liquid particles on average.
Key Formulas / Relationships
- Diffusion rate: temperature↑ → diffusion rate↑
- During melting/boiling, temperature stays constant despite heat input (latent heat).
- Gas volume >> liquid volume for same mass.
Common mistakes
Students think that temperature rises during melting or boiling — it does NOT. Heat is used to change state (latent heat), not to raise temperature. Also, evaporation differs from boiling: evaporation occurs at any temperature at the surface; boiling occurs throughout the liquid at the boiling point.
Summary
All matter is made of tiny moving particles. Solids, liquids, and gases differ in particle arrangement and movement. Diffusion (spontaneous mixing) and Brownian motion are evidence for the particulate nature of matter. Heating and cooling cause state changes; latent heat is absorbed/released during these changes without temperature change.