States of Matter
Section: 1. States of Matter | Syllabus: Cambridge AS Level Physics 9702
Changes of State Matter can change from one state to another when energy is added or removed. These physical changes are reversible and do not create new substances. Change of State A physical change where a substance transforms from one state of matter to another (solid, liquid, or gas) through the addition or removal of energy.
The Six Changes of State 1. Melting (Solid → Liquid) Energy change: Energy is absorbed (endothermic) What happens: Particles gain enough energy to overcome some of the forces holding them in fixed positions Example: Ice melting to water at 0°C Melting point: The temperature at which a solid changes to a liquid 2.
Freezing (Liquid → Solid) Energy change: Energy is released (exothermic) What happens: Particles lose energy and forces of attraction pull them into fixed positions Example: Water freezing to ice at 0°C Freezing point: The temperature at which a liquid changes to a solid (same as melting point) 3.
Boiling (Liquid → Gas) Energy change: Energy is absorbed (endothermic) What happens: Particles gain enough energy to completely overcome all forces of attraction Example: Water boiling to steam at 100°C Boiling point: The temperature at which a liquid changes to a gas throughout the entire liquid Special feature: Bubbles form throughout the liquid 4.
Condensation (Gas → Liquid) Energy change: Energy is released (exothermic) What happens: Particles lose energy and forces of attraction pull them closer together Example: Steam condensing to water droplets on a cold surface 5.
Evaporation (Liquid → Gas) Energy change: Energy is absorbed (endothermic) What happens: Particles at the surface gain enough energy to escape into the gas phase Example: Water evaporating from a puddle at room temperature Key difference from boiling: Only occurs at the surface and can happen at any temperature 6.
Sublimation (Solid → Gas) Energy change: Energy is absorbed (endothermic) What happens: Particles gain enough energy to go directly from fixed positions to complete freedom Example: Solid carbon dioxide (dry ice) subliming at -78°C, iodine crystals Special feature: Skips the liquid phase entirely Important The reverse process of sublimation (gas → solid) is called deposition or reverse sublimation .
An example is frost forming directly from water vapor on cold surfaces. Heating and Cooling Curves When a substance is heated or cooled, its temperature changes in a characteristic way that can be shown on a graph.
Key Features of Heating Curves Sloping sections: Temperature increases as energy is used to increase particle kinetic energy Flat sections (plateaus): Temperature stays constant during state changes Why plateaus occur: Energy is used to overcome forces between particles, not to increase temperature Key Concept During a change of state, temperature remains constant even though energy is being added or removed.
This is because the energy is used to break or form bonds between particles rather than changing their kinetic energy. Evaporation vs. Boiling Property Evaporation Boiling Where it occurs Only at the surface Throughout the entire liquid Temperature Can occur at any temperature Occurs at a specific temperature (boiling point) Speed Slow process Fast/rapid process Bubbles No bubbles form Bubbles form throughout the liquid Energy source Surroundings (causes cooling) Usually external heat source required Particles involved Only the fastest particles escape All particles have enough energy Factors Affecting Rate of Evaporation Several factors can speed up or slow down the rate of evaporation: 1.
Temperature Higher temperature means more particles have enough kinetic energy to escape from the surface, increasing the rate of evaporation. 2. Surface Area A larger surface area means more particles are at the surface and can escape, increasing the rate of evaporation.
3. Air Movement (Wind) Moving air removes evaporated particles from above the liquid surface, maintaining a concentration gradient and increasing evaporation rate. 4. Humidity Lower humidity means fewer water molecules in the air, so less condensation occurs back into the liquid, resulting in faster net evaporation.
Exam Tip When asked about factors affecting evaporation, explain the factor AND why it affects the rate in terms of particle behavior. Cooling by Evaporation Evaporation causes cooling because the fastest-moving particles (with the highest kinetic energy) escape from the liquid surface.
When high-energy particles leave, the average kinetic energy of remaining particles decreases Since temperature is proportional to average kinetic energy, the temperature drops This is why sweating cools us down - water evaporates from skin, taking energy with it It's also why alcohol on skin feels cold - it evaporates quickly, removing energy rapidly Energy Changes During State Changes Change of State Energy Transfer Type of Process Melting Energy absorbed from surroundings Endothermic Freezing Energy released to surroundings Exothermic B…
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