States of matter

Learning objectives

• Describe solids, liquids, gases using the particle model.
• Compare properties: shape, volume, density, compressibility, diffusion, and energy.
• Explain changes of state and factors affecting them (temperature, pressure).
• Distinguish evaporation from boiling and outline basic gas-law relations.

What is matter?

• Matter is anything that has mass and occupies space. It is made of particles (atoms/molecules) that are in continuous motion.
• The observable properties of a substance depend on particle arrangement, the strength of intermolecular forces, and kinetic energy of particles.

Particle model: core ideas

• Particles attract each other with intermolecular forces; stronger forces hold particles closer and more orderly.
• All particles move; higher temperature means higher average kinetic energy and faster motion.
• Space between particles varies across states: smallest in solids, largest in gases.

Differences between states of matter

Solids

• Fixed shape and fixed volume.
• Particles are closely packed in an orderly arrangement; vibrate about fixed positions.
• Strong intermolecular forces.
• High density (generally), negligible compressibility.
• Very slow diffusion (practically negligible).
• Examples: ice, iron, wood.

Liquids

• No fixed shape; take the shape of the container. Fixed volume.
• Particles are close but can slide past one another.
• Intermediate intermolecular forces.
• Moderate density, very low compressibility.
• Diffuse slowly (faster than solids but slower than gases).
• Examples: water, oil, alcohol.

Gases

• No fixed shape or volume; fill the entire container.
• Particles are far apart; move randomly and rapidly in all directions.
• Very weak intermolecular forces.
• Low density, highly compressible.
• Diffuse rapidly and mix uniformly.
• Examples: air, oxygen, carbon dioxide.

Beyond the basics

• Plasma: Ionized gas with free electrons and ions; conducts electricity and responds to magnetic fields. Found in stars, neon signs, flames.
• Bose–Einstein condensate (BEC): Matter at ultra-low temperatures where many atoms occupy the lowest energy state, behaving as a single quantum entity.

Change of state

• Melting (fusion): Solid – liquid at melting point; heat increases particle energy, weakening forces so particles can move past each other.
• Freezing (solidification): Liquid – solid; particles lose energy and arrange into an ordered structure.
• Vaporization (boiling): Liquid – gas throughout the liquid at boiling point; forms bubbles; requires heat.
• Condensation: Gas – liquid upon cooling or compression.
• Evaporation: Surface liquid molecules escape to gas below boiling point; occurs at all temperatures.
• Sublimation: Solid <–> gas directly (e.g., dry ice, naphthalene).
• Latent heat: Energy absorbed or released during change of state at constant temperature. For mass $m$, heat $Q$ is $Q = mL$, where $L$ is latent heat (of fusion or vaporization).

Evaporation vs boiling

• Evaporation occurs at the surface, at any temperature, is slower, and causes cooling (high-energy particles escape first).
• Boiling occurs throughout the liquid, at a fixed boiling point, is rapid, and needs continuous heat.

Factors increasing evaporation:

• Higher temperature.
• Larger surface area.
• Lower humidity.
• Faster airflow.
• Lower atmospheric pressure.

Effect of temperature and pressure

• Increasing temperature raises particle kinetic energy, promoting melting, evaporation, and faster diffusion.
• Increasing pressure pushes particles closer:
• Raises boiling point of liquids.
• Can condense gases to liquids.
• Reduces gas volume significantly.

Everyday applications and examples

• Solid ice melting into water and boiling into steam illustrates all three core states.
• Perfume smell spreading quickly demonstrates fast diffusion in gases.
• Clothes drying faster on a windy, hot day illustrates evaporation factors.
• Pressure cookers raise boiling point, cooking food faster.
• LPG cylinders store gas as a liquid under pressure; it vaporizes on release.

Common misconceptions

• “Solids do not have moving particles.” Incorrect; particles vibrate about fixed positions.
• “Evaporation needs the liquid to be hot.” It occurs at all temperatures, though faster when hotter.
• “Gases have no mass.” Gases have mass and exert pressure.
• “Boiling and evaporation are the same.” They differ in location (surface vs bulk) and conditions.

Quick check questions

• Why are gases highly compressible while solids are not?
• List two factors that increase evaporation and explain the particle-level reason.
• State one practical use of increasing pressure to change the state of matter.
• How does diffusion rate compare among solids, liquids, and gases, and why?
• What happens to the boiling point of water at high altitudes and why?

Revision table

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