FULL REVIEW
Full Review — Phase Changes and Latent Heat — Algebra-Based
Review the essential ideas, relationships, and problem-solving tools for Phase Changes and Latent Heat.
TIME
45–60 minutes
BEST FOR
A complete unit review
FINISH WITH
A readiness check
After this full review, you'll be able to...
recall the essential ideas, apply them to representative problems, and determine what to study next.
Choose how you want to review
Course Alignment
This Physics Sensei Unit Review is an independent learning resource. Use it to reinforce key concepts, prepare for homework, or review before a quiz or exam.
RESOURCE: Physics Sensei Unit Review | UNIT ID: THM-U04 | TOPIC: Phase Changes and Latent Heat | COURSE LEVEL: Algebra-based introductory physics
BEST USED ✓ After learning the unit ✓ Before starting homework ✓ Before a quiz or exam
Your Review Plan
Complete these six stages in order. Each stage builds on the previous one and prepares you for the final readiness check.
6 Stages • Approximately 45–60 minutes.
Warm-Up Check
Before you begin, take a moment to see what you already remember. Do not worry about getting everything right. This is only a starting point.
ACTIVITY 1
Recall Activity 1
Recall the phase-change vocabulary and energy sign.
Classify melting, freezing, vaporization, condensation, sublimation, and deposition as energy absorbed or released by the sample.
Reveal Answers
Absorbed: melting, vaporization, sublimation. Released: freezing, condensation, deposition.
Why it works: The direction toward a less bound phase requires energy input.
ACTIVITY 2
Recall Activity 2
Select the correct thermal relationship.
Choose Q = mcΔT or Q = mL for each: warming liquid water; melting ice at 0 °C; boiling water at 100 °C.
Reveal Answers
Warming liquid: Q = mcΔT. Melting: Q = mLf. Boiling: Q = mLv.
Why it works: Match the equation to the physical stage, not just to the word “heat.”
ACTIVITY 3
Recall Activity 3
Interpret a heating curve quantitatively.
A heating curve has a flat segment at the melting point. What physical quantity is changing even though temperature is not?
Reveal Answers
The phase fraction and intermolecular potential energy change while temperature remains approximately constant.
Why it works: Latent heat changes phase without increasing average kinetic energy during the plateau.
Ready to strengthen your understanding?
You've refreshed what you already know. Next, you'll reinforce the essential concepts that will help you solve problems with confidence. Need to see the learning path again?
Core Concepts
Let's rebuild the key ideas one step at a time. Focus on understanding the relationships before worrying about solving problems.
KEY CONCEPT 1
Latent Heat and Phase-Change Energy
At a phase boundary, the energy transferred to a mass m is proportional to the mass and the appropriate latent heat. For melting/freezing use Lf; for vaporization/condensation use Lv. The sign of Q depends on whether the sample gains or loses energy.
Q = mL
Example: Melting 0.250 kg of ice: Q = (0.250 kg)(334 kJ/kg) = 83.5 kJ.
Sensei note: Separate the magnitude mL from the sign convention used in the problem.
KEY CONCEPT 2
Heating Curves and Piecewise Energy
A heating curve is naturally piecewise. Sloped segments use Q = mcΔT; phase-change plateaus use Q = mL. A process that crosses several regions requires one energy calculation per region.
Qtotal = ΣQstage; use Q = mcΔT within a phase and Q = mL during a phase change.
Example: Ice below 0 °C → ice at 0 °C → liquid at 0 °C requires warming the solid, then melting it.
Sensei note: Do not combine ΔT across a phase-change plateau as though one heat capacity applied everywhere.
KEY CONCEPT 3
Phase Diagrams and Transition Conditions
A phase diagram maps the stable phase as a function of temperature and pressure. Crossing a phase boundary changes the stable phase; the transition temperature depends on pressure. Triple and critical points identify special boundaries of phase behavior.
A phase diagram identifies the stable phase from temperature and pressure.
Example: Lowering pressure can shift the boiling temperature because the liquid–vapor boundary is pressure dependent.
Sensei note: Use a heating curve for energy versus temperature at a fixed path; use a phase diagram to determine which phase is stable at T and P.
Ready to apply these ideas?
You've reinforced the essential concepts. Now it's time to put them into practice by working through guided examples and building your problem-solving confidence. Need a quick reminder?
Guided Practice
Now it's time to apply what you've reviewed.
Work through each activity in order. The examples become gradually more challenging, and each one prepares you for the final readiness check.
PRACTICE 1
Worked Example
Compute one latent-heat stage and label the energy direction.
How much energy is needed to vaporize 0.030 kg of water at its boiling point if Lv = 2260 kJ/kg?
Reveal Answers
Q = mLv = (0.030)(2260) = 67.8 kJ absorbed.
Why it works: Vaporization is a liquid→gas phase change, so use Lv.
PRACTICE 2
Guided Problem
Build a two-stage energy calculation.
A 0.150 kg ice sample at 0 °C melts and then the resulting water warms by 20 °C. Use Lf = 334 kJ/kg and cwater = 4.18 kJ/(kg·°C). Find each stage and the total.
Reveal Answers
Melting: Q1 = (0.150)(334) = 50.1 kJ. Warming: Q2 = (0.150)(4.18)(20) = 12.54 kJ. Total = 62.64 kJ absorbed.
Why it works: The path crosses a phase change and then a temperature-change region, so it must be split.
PRACTICE 3
Independent Problem
Solve independently and report the total energy.
A 0.040 kg water-vapor sample condenses at the boiling point. Using Lv = 2260 kJ/kg, find Q for the sample if energy leaving the sample is negative.
Reveal Answers
Q = −mLv = −(0.040)(2260) = −90.4 kJ.
Why it works: Condensation releases energy from the sample, so Q is negative under the stated convention.
Ready to check your understanding?
You've practiced the essential skills with guidance. Now it's time to solve a few short problems on your own and confirm you're ready to move forward. Need a quick reminder?
Confidence Check
You've rebuilt the key ideas and practiced them with guidance. Now try these short questions on your own to check your understanding before moving on.
QUICK CHECK 1
Model Selection
Name the equation and latent heat.
A solid melts at its melting point. Which equation is used for the transition?
Reveal Answers
Q = mLf.
Why it works: Solid↔liquid uses the latent heat of fusion.
QUICK CHECK 2
Two-Stage Calculation
Calculate the total energy.
A 0.100 kg sample melts with Lf = 200 kJ/kg, then the liquid warms by 10 °C with c = 2.0 kJ/(kg·°C). Find Qtotal.
Reveal Answers
Melting: 20 kJ. Warming: 2 kJ. Total: 22 kJ absorbed.
Why it works: Add the phase-change and temperature-change stage energies.
QUICK CHECK 3
Heating-Curve Interpretation
Explain the flat segment.
Why can a heating curve be horizontal while energy is still entering the sample?
Reveal Answers
The incoming energy is latent heat used to change phase and intermolecular potential energy, so temperature remains approximately constant.
Why it works: A horizontal segment represents coexistence of phases during the transition.
How did it go?
You've checked your understanding. Take one final look at the essential ideas before deciding what to do next. Need a quick reminder?
Summary
Before moving on, take one final look at the most important ideas from this review.
KEY TAKEAWAY 1
Choose the equation from the physical stage
Use Q = mcΔT inside one phase and Q = mL during a phase change.
KEY TAKEAWAY 2
Latent heat is mass proportional
For a given transition, doubling the mass doubles the required or released phase-change energy.
KEY TAKEAWAY 3
Treat heating curves piecewise
Calculate each slope or plateau separately and sum the energy contributions.
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