Temperature, Kinetic Energy, and Changes of State
Quick Notes:
- Temperature in Kelvin (K) measures the average kinetic energy (Ek) of particles in a substance.
- Higher temperature = faster particle motion.
- Kelvin (K) is the SI unit of temperature.
- 0 K = absolute zero (particles have no kinetic energy).
- Conversion between Celsius and Kelvin:
- K = °C + 273
- °C = K − 273
- During a change of state, temperature stays constant even though heat is added or removed – energy is used to break or form intermolecular forces.
- Observable changes:
- Melting: solid softens, begins to turn to liquid.
- Boiling: bubbling, rapid vapor formation.
- Condensation: fogging, droplets forming.
- Freezing: liquid solidifies, hardens.
Full Notes:
What Is Temperature, Really?
Temperature isn’t just a number – it tells us about the average kinetic energy (Ek) of the particles in a substance.
When something feels hot, its particles are moving faster. When it feels cold, they’re moving slower.
Kinetic energy (Ek) increases as temperature increases.
At absolute zero (0 K), particle motion stops completely — this is the theoretical lowest temperature possible.
The Kelvin Temperature Scale
Kelvin (K) is the SI unit of temperature used in science.
It has the same step size as degrees Celsius (°C), but it starts from absolute zero instead of the freezing point of water. This enables us to link the temperature of a substance to its kinetic energy.
Converting between Celsius and Kelvin:
- To convert °C to K: K = °C + 273
- To convert K to °C: °C = K − 273
For Example: 25 °C = 298 K; 100 K = −173 °C
Temperature and Changes of State
When substances change state (e.g., melting, boiling), we often add or remove energy but temperature stays the same during the change.
That’s because the energy is being used to break or form intermolecular forces, not to speed up particles.
What You’ll Observe:
- Melting: Solid softens and becomes liquid – temperature stays constant during the actual melting.
- Boiling: Liquid forms bubbles of gas – temperature plateaus while boiling.
- Condensation: Gas turns to liquid, often seen as fog or dew.
- Freezing: Liquid hardens into solid — no change in temperature while it freezes.
Heating Curves
If you were to plot temperature over time while heating ice:
The flat parts of the graph show changes of state – energy is still going in, but it’s not raising temperature.
The energy is being used to break the bonds holding particles together instead of making them move faster.
Summary
- Temperature measures average kinetic energy of particles.
- Kelvin is the SI unit of temperature and starts at absolute zero.
- During state changes, temperature remains constant as energy is used to break or form intermolecular forces.
- Heating curves show flat sections where changes of state occur.
Linked Questions
What is the graphical distribution of kinetic energy values of particles in a sample at a fixed temperature?
The kinetic energies of particles in a substance at a fixed temperature are distributed according to the Maxwell–Boltzmann distribution. Most particles have an energy close to the average, while fewer have very low or very high energies. The curve starts at zero, rises to a peak (the most probable energy), and then tails off gradually. The area under the curve represents all particles in the sample, and the portion above the activation energy corresponds to those able to react during collisions.
What must happen to particles for a chemical reaction to occur?
According to collision theory, particles must collide with sufficient energy to overcome the activation energy barrier and with the correct orientation for bonds to break and new ones to form. Only collisions that meet both these conditions — adequate energy and proper orientation — result in a successful reaction. Increasing temperature, concentration, or using a catalyst raises the number of effective collisions.