States and State Functions
High-Yield Summary
- Temperature measures the average kinetic energy of a substance's particles — faster motion means higher temperature.
- Three scales: Celsius (anchored to water's 0°C/100°C), Fahrenheit (32°F/212°F), Kelvin (SI unit, starts at absolute zero = −273.15°C).
- Conversions: F = (9/5)C + 32; C = (5/9)(F − 32); K = C + 273.
- Zeroth Law of Thermodynamics: if A and B are each in thermal equilibrium with C, then A and B are in thermal equilibrium with each other — this is what makes a thermometer valid.
Key Terms
- Temperature
- Quantifies the average kinetic energy of the particles in a substance.
- Absolute zero
- 0 K = −273.15°C, the point at which particle motion theoretically stops entirely.
- Zeroth Law of Thermodynamics
- If two systems are each in thermal equilibrium with a third system, they're in thermal equilibrium with each other.
- Thermal equilibrium
- State in which two systems in contact have reached the same temperature and no net heat flows between them.
Temperature Scale Conversions
F = (9/5)C + 32 | C = (5/9)(F − 32) | K = C + 273
- F = temperature in Fahrenheit
- C = temperature in Celsius
- K = temperature in Kelvin
- K = C + 273 is the standard rounded conversion; the precise offset is 273.15.
- Worked example: 37°C body temp → 98.6°F and 310 K.
The Three Temperature Scales
| Scale | Anchor points / use |
|---|---|
| Celsius (°C) | 0°C = water freezes, 100°C = water boils. Worldwide everyday/scientific use. |
| Fahrenheit (°F) | 32°F = water freezes, 212°F = water boils. US non-scientific use. |
| Kelvin (K) | SI unit. 0 K = absolute zero (−273.15°C). Used extensively in scientific research. |
Must-Know Points
- The Zeroth Law is WHY a thermometer works: it's the third system that reaches equilibrium with an object, giving a valid, comparable reading without needing direct contact between two objects.
- Temperature is a state function — it depends only on the current condition of a system, not how it got there.
- Kelvin is always used in gas-law and thermodynamics equations that require absolute temperature (never Celsius or Fahrenheit).
Common MCAT Trap
- Don't plug Celsius or Fahrenheit into equations requiring absolute temperature (e.g., PV = nRT, ΔS = q/T) — convert to Kelvin first.
- Absolute zero is a theoretical limit where particle motion stops — it does NOT mean zero energy in every sense (quantum zero-point energy still exists), but for MCAT purposes treat it as the point of minimum kinetic energy.
Quick Recall
Convert 25°C to Kelvin.
What does the Zeroth Law of Thermodynamics establish?
What does temperature actually measure at the particle level?