Matter: States and Properties

Matter: States and Properties

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Matter is anything that has mass and occupies space (volume). It is made of tiny particles in constant motion.
• Three common states: solid, liquid, gas. They differ in how strongly particles are held together and how freely they move.
• Density formula you must memorise: ρ = m / V, where ρ (rho) is density in g/cm³, m is mass in grams, V is volume in cm³.
• State changes to know: melting (solid → liquid), freezing (liquid → solid), boiling/evaporation (liquid → gas), condensation (gas → liquid), and sublimation (solid → gas directly, e.g. camphor, iodine).
• High-yield NCEE pointers: (1) gases are the most easily compressed, solids the least; (2) diffusion is fastest in gases, slowest in solids; (3) change of state is a physical change — mass is conserved.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Particle arrangement in each state

In a solid, particles are packed tightly in a fixed, orderly pattern. They vibrate about fixed positions but cannot move from place to place, so a solid keeps a fixed shape and a fixed volume. In a liquid, particles are still close together but have enough energy to slide past one another. A liquid has a fixed volume but takes the shape of its container. In a gas, particles are far apart, move rapidly and randomly in all directions, and only attract each other weakly during collisions. A gas has neither fixed shape nor fixed volume — it fills any container and can be compressed easily.

Changes of state

Adding heat gives particles more kinetic energy, allowing them to overcome the forces holding them together. The main changes are:

Change	From → To	Occurs at
Melting	solid → liquid	melting point
Freezing	liquid → solid	same melting/freezing point
Boiling	liquid → gas (throughout the liquid)	boiling point
Evaporation	liquid → gas (at the surface, below boiling point)	any temperature
Condensation	gas → liquid	boiling point
Sublimation	solid → gas (e.g. camphor, iodine, ammonium chloride)	—

During any change of state, the temperature stays constant while heat is being absorbed (melting, boiling) or released (freezing, condensation), because the energy goes into breaking or forming forces between particles, not into speeding them up.

Density and physical change

Density = mass ÷ volume (ρ = m / V). It is a physical property that identifies a substance. For the same substance, the solid is usually denser than the liquid, which is denser than the gas — a fact that explains why ice floats on water (water is unusual: it is densest at 4 °C). Changes of state do not create a new substance, so they are physical changes; the mass is conserved.

Typical NCEE question patterns

Expect MCQs that: (a) identify the state from a description of particle spacing; (b) pick the correct process name from a diagram of arrows between states; (c) ask which state diffuses fastest or is most compressible; (d) test the difference between evaporation and boiling.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Why properties differ — the kinetic theory

The differences between the three states come from two things: the average kinetic energy of the particles (linked to temperature) and the strength of the forces between them. In a solid, inter-particle forces are strong relative to kinetic energy, locking particles in position. In a liquid, forces are weaker, so particles can slide but still stay in contact. In a gas, kinetic energy dominates; particles are almost free, collide elastically with each other and the container walls, and the collisions with the walls produce gas pressure. This model also explains why heating a gas in a sealed container increases pressure (particles move faster and hit the walls harder and more often) — useful when NCEE questions link gases to pressure and temperature.

Diffusion, evaporation and sublimation — common traps

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration. Because it depends on particle motion, diffusion is fastest in gases, slower in liquids, and extremely slow in solids. A common wrong answer in NCEE is “diffusion does not occur in solids”; in reality, it is just very slow. Evaporation happens only at the surface of a liquid, at any temperature below the boiling point; boiling happens throughout the liquid, only at the boiling point. Sublimation is the direct solid-to-gas change with no liquid stage — the test usually offers a real example (camphor, naphthalene, dry ice, ammonium chloride) and asks for the process name.

Worked micro-example

A metal block has mass 156 g. When placed in a measuring cylinder containing 50 cm³ of water, the water level rises to 68 cm³. Find the density. Volume of block = 68 − 50 = 18 cm³. Density = 156 ÷ 18 = 8.67 g/cm³ — a value close to iron (≈ 7.87 g/cm³), showing how density can identify a substance.

Two practice prompts

1. A student leaves a wet cloth in the sun. Name the process by which the water disappears, and explain why this happens only at the surface and at temperatures below 100 °C.
2. A sealed syringe is half-filled with air and the plunger is pushed in. Use particle theory to explain why the air is harder to compress when the plunger is already pushed in deeply than at the start.

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