Module 06 — Heat and Particles · quiz worth 15 pts
The hardest part is choosing the equation. Start by asking whether temperature or phase is changing.
Temperature changes: use Q = mcΔT.
Temperature stays at the transition point: use Q = mL.
Calculate each stage separately, then add the energy transfers.
PhET Energy Forms and Changes can make energy transfer and storage visible. It is qualitative support, not a replacement for Q calculations.
Ice at 0 °C melts at 0 °C. Which equation is the starting point?
Q = mLf because the phase changes while temperature stays constant.
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| Substance | c (J/kg·K) | Lf (J/kg) | Lv (J/kg) |
|---|---|---|---|
| Water | 4186 | 3.33 × 10⁵ | 2.26 × 10⁶ |
| Lead | 128 | 2.5 × 10⁴ | 8.68 × 10⁵ |
| Copper | 387 | 1.34 × 10⁵ | 5.06 × 10⁶ |
Heat is energy transferred because two systems have different temperatures. Spontaneous transfer runs from the higher-temperature system to the lower-temperature system until they reach thermal equilibrium. After the transfer, the energy belongs to the systems' internal energy. The symbol Q records the amount transferred, in joules (J).
Within one phase, added energy usually raises average particle kinetic energy and temperature. During melting or boiling, added energy instead changes intermolecular arrangement and potential energy. That is why temperature can remain constant even while energy continues to enter. Removing energy reverses the changes: gas condenses and liquid freezes at the same transition temperatures at a given pressure.
Solid → liquid is melting; liquid → solid is freezing. Liquid → gas is vaporization or boiling; gas → liquid is condensation. Melting and vaporization absorb energy. Freezing and condensation release energy to the surroundings. A melting point equals the freezing point for the same substance and pressure; a boiling point equals the condensation point under the same conditions.
Read an ideal heating curve from left to right as energy enters: solid warms on a rising segment, melting occurs on a level segment, liquid warms on the next rise, boiling occurs on the next level segment, and gas warms on the final rise. A rising segment means temperature and average kinetic energy increase within one phase. A level segment means phase and particle arrangement change while energy still enters.
Read the shape before the labels.
Temperature changes within one phase.
Phase changes. Energy still transfers, but temperature does not.
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Q in J, m in kg, c in J/(kg·K), and ΔT = Tf − Ti in K or Celsius degrees.
Lf is latent heat of fusion in J/kg. Use its magnitude, then assign the sign from energy direction.
Lv is latent heat of vaporization in J/kg.
For an isolated exchange, energy gained plus energy lost equals zero.
If both are the same phase and temperature changes, start with Q = mcΔT.
Melt, freeze, vaporize, boil, or condense points to Q = mL while the transition occurs.
Heating ice below 0 °C until it becomes warm water needs separate warming, melting, and warming calculations. Add the Q values.
With ΔT = Tf − Ti, warming gives positive Q and cooling gives negative Q. A negative result means energy left the chosen system.
Convert grams to kilograms before using constants written per kilogram.
Circle the action word in the problem.
Use mcΔT unless the path crosses a phase boundary.
Use mL for that transition.
Energy into the named system is positive; energy out is negative.
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How much energy enters the water?
Q = 1.47 × 10⁵ J.
How much energy transfers relative to the copper?
Q = −8.92 × 10³ J. The negative sign means energy left the copper.
How much energy is required?
Q = 1.3 × 10⁵ J to two significant figures.
Energy still enters or leaves. It changes intermolecular potential energy rather than average kinetic energy.
The temperature is constant during the transition, so latent heat supplies the missing energy calculation.
A constant in J/kg requires kilograms.
Q is positive for energy entering that system and negative for energy leaving it.
Choose the physical process first. The arithmetic comes second.
