Summary of "[중② 1단원] 3강. 끓는점, 녹는점, 어는점￨그래프 해석￨혼합물의 끓는점🌡"

States of matter and phase changes

States of matter: solids, liquids, gases
- Examples: ice (solid), liquid water (liquid), water vapor (gas)
Common phase changes and names:
- Melting: solid → liquid
- Freezing / solidification: liquid → solid
- Vaporization / boiling: liquid → gas
- Condensation / liquefaction: gas → liquid
- Sublimation: solid ↔ gas (direct transition)

Particle-model explanation

Heating a substance increases particle motion and average separation; cooling decreases motion and separation. Phase changes correspond to changes in particle spacing and motion (for example, melting increases particle mobility and spacing compared with the solid state).

For pure substances, temperature stays constant while a phase change occurs (this appears as a plateau on a temperature vs. time graph).

Temperature behavior during phase changes (pure substances)

For a pure substance, the temperature remains constant during a phase change (plateau on a temperature-vs-time graph).
Melting point = temperature where solid → liquid (equal to the freezing point for a pure substance).
Boiling point = temperature where liquid → gas (equal to the condensation point for a pure substance).
These characteristic temperatures are intrinsic to each pure substance and are specified at a given external pressure (commonly 1 atm).
Heating and cooling curves are symmetric for pure substances (melting point = freezing point; boiling point = condensation point).

Other factors: - Amount of substance and heating intensity do not change the melting/freezing/boiling temperatures for a pure substance; they only change how long it takes to reach those temperatures (more mass or weaker heating → longer time). - External pressure affects boiling point: higher pressure → higher boiling point; lower pressure → lower boiling point. Reported boiling points are usually at 1 atm.

Mixtures

Mixtures do not show sharp, constant-temperature plateaus during phase changes; instead phase change occurs over a temperature range.
Boiling point elevation: the boiling point of a solution (e.g., salt water) is higher than that of the pure solvent.
Freezing point depression: the freezing point of a solution is lower than that of the pure solvent.
Practical example: adding CaCl2 (or salt) lowers water’s freezing point and is used for de-icing roads.

Virtual experiment — heating ice (example procedure and graph sections)

Start with ice at a subzero temperature (example: −20 °C).
Heat and record temperature vs. time:
- Section A — solid-only region: temperature rises until the melting point (0 °C for water).
- Section B — melting plateau: temperature remains constant at the melting point while solid and liquid coexist.
- Section C — liquid-only region: after complete melting, temperature rises again until the boiling point (100 °C for water at 1 atm).
- Section D — boiling plateau: temperature remains constant at the boiling point while liquid and gas coexist; after complete vaporization, vapor temperature increases further.

Cooling (reverse) curve: 1. Start from vapor and cool: at the condensation temperature the temperature plateaus while gas → liquid occurs. 2. Continue cooling to the freezing temperature: temperature plateaus again while liquid → solid occurs.

How to read temperature-vs-time (or heat-vs-time) graphs

Sloped segments indicate temperature change while the sample is in a single phase (solid, liquid, or gas).
Flat segments (plateaus) indicate phase transitions; the plateau temperature equals the characteristic phase‑change temperature (melting point, boiling point, or freezing point) for a pure substance.
For mixtures, plateaus are absent or not flat — phase change occurs over a temperature range rather than at a single temperature.

Numeric examples (selected values at 1 atm unless noted)

Water: melting/freezing point 0 °C; boiling point 100 °C
Ethanol: boiling point ≈ 78.3 °C
Iron: melting point ≈ 1538 °C
Gallium: melting point ≈ 30 °C
Mercury: melting point ≈ −39 °C
Salt water (example from demonstration): observed boiling ≈ 105 °C (higher than pure water); freezing began around −8 °C (lower than pure water)

Key takeaways

Melting point, freezing point, and boiling point are intrinsic properties of pure substances and are useful for identification.
These characteristic temperatures are independent of sample size and heating rate but depend on external pressure.
Mixtures alter characteristic temperatures (boiling point elevation, freezing point depression) and generally do not show sharp phase-change plateaus.
Reading temperature-vs-time or heat-vs-time graphs lets you identify which phases are present and detect phase-change plateaus.

Speakers and examples cited

Unnamed instructor / video narrator (lesson-style presentation)
Examples/substances mentioned: water, ethanol, iron, gallium, mercury, salt (salt water), calcium chloride (CaCl2)