---
title: "Latent Heat | General Chemistry II"
description: "Latent heat is energy absorbed or released during a phase change without a temperature change, a core thermodynamics idea in General Chemistry II."
canonical: "https://fiveable.me/general-chemistry-ii/key-terms/latent-heat"
type: "key-term"
subject: "General Chemistry II"
unit: "Unit 6"
---

# Latent Heat | General Chemistry II

## Definition

Latent heat is the energy a substance absorbs or releases during a phase change without changing temperature. In General Chemistry II, you see it when melting, freezing, boiling, or condensing is analyzed with thermodynamics.

## What It Is

Latent heat is the energy involved when a substance changes phase in General Chemistry II, while its temperature stays constant. That means the energy is not going into speeding up particles the way sensible heat does. Instead, it is being used to overcome or form intermolecular forces so the substance can move from solid to liquid, liquid to gas, or the reverse.

A heating curve makes this really clear. When ice warms from below 0°C to 0°C, the temperature rises because the added heat increases particle motion. But once the sample reaches the melting point, extra heat goes into melting the solid lattice, not into raising temperature. The flat part of the heating curve is where latent heat shows up.

Chemists usually talk about two main kinds: latent heat of fusion and latent heat of vaporization. Fusion is the energy needed to melt a solid or the energy released when a liquid freezes. Vaporization is the energy needed to turn a liquid into a gas or the energy released when a gas condenses. For water, the latent heat of vaporization is especially large because hydrogen bonding makes it take a lot of energy to separate water molecules into the gas phase.

This is also why phase changes are such good thermodynamics examples. During the change, the system is still exchanging energy with the surroundings, but the average kinetic energy of the particles is not increasing. The added or removed energy changes the arrangement of particles instead. That is why latent heat connects directly to enthalpy and to the idea that state changes can involve large energy transfers without a temperature change.

In problems, you often see latent heat in equations like q = mL, where m is mass and L is the latent heat per gram or per mole. The sign depends on direction. Melting and vaporization absorb heat, while freezing and condensation release it.

## Why It Matters

Latent heat shows up any time General Chemistry II connects thermodynamics to real matter instead of abstract symbols. It gives you a concrete way to see how energy conservation works during a phase transition, which is a big step up from just tracking temperature changes with specific heat.

It also explains why some substances are so resistant to phase change. Water is the classic example. Its high latent heat of vaporization is tied to hydrogen bonding, so a lot of energy is needed before liquid water becomes steam. That is why evaporation cools surfaces, why sweating works, and why boiling water can absorb so much heat without a temperature increase.

In thermodynamics, latent heat ties into enthalpy because phase changes at constant pressure are usually discussed with enthalpy changes, not just raw heat flow. If you can tell when energy is being used to change particle arrangement instead of particle speed, you will have an easier time with heating curves, calorimetry, and any problem that mixes temperature change with a phase change.

It also helps you avoid a common mistake: assuming more heat always means a higher temperature. During melting or boiling, that is not true. The temperature can stay flat while energy still enters or leaves the system.

## Connections

### enthalpy

Latent heat is often discussed through enthalpy changes, especially for phase transitions at constant pressure. When you see values like 94Hfus or 94Hvap, those are enthalpy changes that describe the same energy transfer as latent heat, just written in the thermodynamics language used in chemistry.

### [phase transition](/general-chemistry-ii/key-terms/phase-transition)

A phase transition is the actual change from solid to liquid, liquid to gas, or the reverse. Latent heat is the energy exchange that makes that transition happen. The phase transition is the process, while latent heat is the heat absorbed or released during that process.

### specific heat

Specific heat describes how much energy changes temperature within a single phase. Latent heat is different because temperature stays constant during the phase change. On problem sets, you often have to decide whether to use q = mc94T or q = mL based on whether the substance is warming up or changing phase.

### [Specific heat capacity](/general-chemistry-ii/key-terms/specific-heat-capacity)

Specific heat capacity is the per-mass heat required to raise temperature by 1 degree, and it applies only when the substance stays in the same phase. Latent heat takes over when the sample reaches a melting or boiling point and the temperature stops changing even though heat is still flowing.

## On the AP Exam

A quiz question or problem set item will usually give you a heating curve, a mass, or a phase change scenario and ask what happens to temperature, heat flow, or energy. Your job is to spot whether the sample is crossing a phase boundary. If it is, you use latent heat, not specific heat, because the temperature is flat while the substance melts, boils, freezes, or condenses.

You may also be asked to calculate the heat required to melt ice, boil water, or condense steam. In those problems, you identify the direction of heat flow, choose the correct latent heat value, and use q = mL or q = n94H if the course uses moles. If the question mixes heating and phase change, you handle each segment separately.

## latent heat vs specific heat

Specific heat changes temperature within one phase, while latent heat is energy used during a phase change at constant temperature. If the graph shows a slope, think specific heat. If it shows a flat plateau at melting or boiling, think latent heat.

## Key Takeaways

- Latent heat is the energy absorbed or released during a phase change without a temperature change.
- Melting, freezing, boiling, and condensing are the main phase changes where latent heat shows up.
- In General Chemistry II, latent heat connects directly to thermodynamics, enthalpy, and heating curves.
- Use latent heat when the sample is changing phase, and use specific heat when the temperature is changing within one phase.
- Water has a large latent heat of vaporization because hydrogen bonding takes a lot of energy to overcome.

## FAQs

### What is latent heat in General Chemistry II?

Latent heat is the heat absorbed or released when a substance changes phase without changing temperature. In General Chemistry II, it shows up in melting, freezing, boiling, and condensing problems, often alongside heating curves and enthalpy changes.

### What is the difference between latent heat and specific heat?

Specific heat describes how much energy changes the temperature of a substance in one phase. Latent heat describes energy used during a phase change, when the temperature stays constant. That is why you use different formulas for sloped versus flat parts of a heating curve.

### Why does temperature stay constant during a phase change?

The added or removed energy is going into changing intermolecular attractions and particle arrangement, not into changing average kinetic energy. Since temperature measures average kinetic energy, it stays flat until the phase change is finished.

### How do you calculate heat using latent heat?

Use q = mL when the problem gives mass and a latent heat value per gram, or q = n94H if it uses moles and molar enthalpy values. Make sure you match the process, such as fusion for melting or vaporization for boiling, and keep track of whether the process absorbs or releases heat.

## Related Study Guides

- [6.1 Laws of thermodynamics and state functions](/general-chemistry-ii/unit-6/laws-thermodynamics-state-functions/study-guide/2FzjyiOeYmqJyrfn)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

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