MYP Physics Phase Change Notes

Matter Changes State When Energy And Pressure Change

1. A phase or state change occurs when matter changes between the solid, liquid, and gas states.
2. During a phase change, the substance remains the same material, meaning no new substance is formed.
3. Only the arrangement, spacing, and motion of particles change.
4. Phase changes occur because particles gain or lose kinetic energy.

Energy and Particle Motion in Phase Changes

1. Phase changes occur because particles gain or lose kinetic energy.
2. When energy is added to a substance, particles move faster and may overcome attractive forces.
3. When energy is removed, particles slow down and attractive forces pull them closer together.
4. Whether particles can escape or remain close together determines the state of matter.

Evaporation (Liquid → Gas)

1. Evaporation is the process by which particles escape from the surface of a liquid and become a gas.
2. Evaporation occurs only at the surface of a liquid, not throughout the liquid.
3. It can happen at any temperature, as long as some particles have enough energy to escape.
4. Particles in a liquid are constantly moving and colliding with each other.
5. These collisions cause energy to be transferred unevenly between particles.
6. Some particles gain more kinetic energy than others.
7. The most energetic particles at the surface can overcome attractive forces and escape into the gas state.

Why Evaporation Cools A Liquid

1. The particles that escape are above-average energy particles.
2. When they leave, the particles left behind have a lower average kinetic energy, so the temperature of the remaining liquid decreases.
3. This explains everyday observations:
   1. A hot drink cools as water evaporates from its surface.
   2. Sweat cools your skin when it evaporates.
   3. Ocean water is cooled by evaporation from the surface, partly counteracting solar heating.

Evaporation Compared with Boiling

1. Evaporation occurs at the surface, while boiling occurs throughout the liquid.
2. Evaporation happens at any temperature, while boiling occurs at a fixed temperature.
3. Boiling produces bubbles of gas inside the liquid, while evaporation does not.

Condensation (Gas → Liquid)

1. Condensation is the phase change in which a gas changes into a liquid.
2. Condensation occurs when gas particles lose kinetic energy.
3. This energy loss can happen due to cooling or increased pressure.
   1. As gas particles cool, they move more slowly.
   2. Slower-moving particles remain close together for longer periods.
   3. Attractive forces between particles become strong enough to hold them together.
   4. The gas particles form a liquid.

Condensation in the Atmosphere

1. Warm air rises and cools as it moves higher in the atmosphere.
2. Water vapor in the air loses energy as it cools.
3. The water vapor condenses into tiny droplets of liquid water.
4. These droplets form clouds.
5. When droplets grow large enough, they fall as rain.

Condensation Due to Pressure

1. Increasing pressure forces gas particles closer together.
2. When particles are closer, attractive forces become effective.
3. This can cause gases to condense into liquids.

Temperature Is A Measure Of Average Particle Kinetic Energy

1. In the particle model, particles are always moving and colliding.
2. Temperature is linked to the average kinetic energy of the particles:
   1. Higher temperature, particles move faster on average.
   2. Lower temperature, particles move more slowly, collisions are less frequent and less energetic.
3. In a gas, cooling has an important extra effect.
4. When particles move more slowly, they spend longer close to one another during collisions.
5. At short distances, attractive intermolecular forces can pull particles together. If conditions are right, a gas can change into a liquid.

Compressing A Gas (Increasing Pressure)

1. Another way to cause condensation is to increase the pressure.
2. Compressing a gas forces particles closer together, making it easier for attractive forces to hold them as a liquid.
3. Some gases liquefy at relatively modest pressures (for example, propane and butane can be stored as liquids at around 10 atm or less).
4. Air requires extremely high pressure to liquefy (about $10\,000\ \text{atm}$).

Sublimation (Solid → Gas)

1. Sublimation is a phase change where a solid changes directly into a gas.
2. The substance does not become a liquid during this process.
3. Sublimation occurs under specific temperature and pressure conditions.

Pressure Helps Explain Boiling Points And Why Liquids Are Rare In Space

1. The boiling point is the temperature at which boiling occurs for a particular pressure.
2. If the external pressure decreases (for example, at high altitude), a liquid boils at a lower temperature.
3. That is why cooking can be difficult on very high mountains: at the top of Mount Everest, the pressure is about $0.33\ \text{atm}$, so water boils well below $100^\circ\text{C}$.

Linking Condensation And Evaporation To Earth's Energy Balance

1. Condensation and evaporation do more than move water around, they move energy around.
   1. Evaporation absorbs energy from the surface (cooling it).
   2. Condensation releases energy into the surroundings (warming the air where droplets form).
2. These processes help drive weather and circulation.
3. Water vapor also plays a role in Earth's temperature because it is a greenhouse gas.
4. Greenhouse gases allow much of the Sun's energy to reach Earth's surface, but absorb some of the infrared radiation emitted by Earth and re-radiate it back toward the surface.
5. Common greenhouse gases include water vapor ($\mathrm{H_2O}$), carbon dioxide ($\mathrm{CO_2}$), and methane ($\mathrm{CH_4}$).
6. Changes in their concentrations can change the strength of the greenhouse effect.

Understanding Phase Diagrams

1. A phase diagram shows the state of matter at different temperatures and pressures.
2. It includes boundaries between solid, liquid, and gas states.