---
title: "Latent Heat of Sublimation | Thermodynamics II"
description: "Latent heat of sublimation is the energy a solid absorbs to turn directly into vapor, a core pure-substance property in Thermodynamics II."
canonical: "https://fiveable.me/thermodynamics-ii/key-terms/latent-heat-of-sublimation"
type: "key-term"
subject: "Thermodynamics II"
unit: "Unit 1"
---

# Latent Heat of Sublimation | Thermodynamics II

## Definition

Latent heat of sublimation is the energy needed to change a solid directly into a gas without becoming a liquid first. In Thermodynamics II, it shows up in phase behavior and pure-substance property analysis.

## What It Is

Latent heat of sublimation is the energy a substance absorbs when it goes from solid straight to vapor in Thermodynamics II. The temperature stays at the sublimation point while the phase change is happening, so the heat input does not raise the temperature yet. It goes into breaking the intermolecular attractions that hold the solid together and letting molecules escape into the gas phase.

For a pure substance, this is a property of the material, not of the amount of heat transfer path you choose. If you know the mass and the latent heat of sublimation, you can estimate the energy needed with a relation like Q = mL, where L is the specific latent heat. The exact value depends on the substance and on the pressure, since sublimation happens where the solid and vapor are in equilibrium on the phase diagram.

This term sits in the same family as latent heat of fusion and latent heat of vaporization, but sublimation is the direct solid-to-gas route. For most substances, it takes more energy than melting or boiling because you are not just loosening particles into a liquid. You are going all the way to a gas, which means the solid lattice has to be disrupted enough for particles to move freely.

Dry ice is the classic example. At atmospheric pressure, solid carbon dioxide does not melt into a liquid under normal conditions, so it sublimates directly into gas. Iodine does something similar, which is why you may see purple vapor when it is heated in a lab.

In phase diagrams, sublimation shows up along the sublimation curve, the boundary between solid and vapor regions. When a Thermodynamics II problem gives you pressure and temperature near that boundary, you are usually checking whether the substance can sublime, or whether you need to use another phase-change relation instead.

## Why It Matters

Latent heat of sublimation shows up any time Thermodynamics II asks you to track energy through a phase change of a pure substance. That means it connects directly to phase diagrams, property tables, and energy balances. If you are doing a problem set on heating dry ice, freeze-drying, or a solid-vapor equilibrium, this is the number that tells you how much heat is absorbed before the material fully becomes gas.

It also sharpens your reading of phase behavior. A lot of students assume every solid becomes liquid first, but that is not always true. Knowing when sublimation happens helps you pick the right phase path on a diagram and avoid using the wrong latent heat value.

In engineering contexts, this matters for refrigeration, material handling, and processes where moisture or refrigerants change phase. If you can identify sublimation from the conditions given, you can set up the energy balance correctly and explain why temperature stays constant during the phase change.

## Connections

### Sublimation

Sublimation is the actual phase change from solid to gas, while latent heat of sublimation is the energy needed to make that change happen. In Thermodynamics II problems, you usually identify the process first, then use the latent heat value to calculate heat transfer or explain the phase path on a diagram.

### [Latent Heat of Fusion](/thermodynamics-ii/key-terms/latent-heat-of-fusion)

Latent heat of fusion applies when a solid melts into a liquid. It is smaller than the latent heat of sublimation for most substances because the material only has to leave the solid lattice, not become a vapor. Comparing the two helps you see why direct solid-to-gas transition takes more energy.

### [Latent Heat of Vaporization](/thermodynamics-ii/key-terms/latent-heat-of-vaporization)

Latent heat of vaporization is the energy required to turn a liquid into a gas. Sublimation skips the liquid stage, so the latent heat of sublimation is often treated as the sum of the fusion and vaporization steps for a substance at the same pressure, with phase-diagram details in mind.

### Phase Diagram

A phase diagram shows where solid, liquid, and gas are stable at different temperatures and pressures. The sublimation curve on that diagram marks the boundary where solid and vapor coexist. If you can read that curve, you can tell when sublimation is possible and when another phase change will happen instead.

## On the AP Exam

A quiz problem may give you the mass of dry ice, the latent heat of sublimation, and ask for the heat absorbed as it turns to CO2 gas. The move is to identify the phase change first, then use Q = mL with the sublimation value, not the fusion or vaporization value. If the question includes a phase diagram, you may also need to point to the sublimation curve and explain why the solid goes straight to vapor at that pressure. In short-answer work, a good response says that the temperature stays constant during the phase change because the added energy changes phase, not temperature.

## latent heat of sublimation vs Latent Heat of Vaporization

These are easy to mix up because both involve liquid or solid turning into gas, but they are not the same step. Latent heat of vaporization is liquid to gas, while latent heat of sublimation is solid to gas with no liquid stage. If a problem starts with a solid, use sublimation.

## Key Takeaways

- Latent heat of sublimation is the energy a solid absorbs to become a gas directly.
- During sublimation, temperature stays constant until the phase change is complete.
- For most substances, latent heat of sublimation is larger than latent heat of fusion or vaporization.
- In Thermodynamics II, you use it with pure substances, phase diagrams, and energy balance calculations.
- Dry ice is the standard example, since it sublimates at atmospheric pressure instead of melting first.

## FAQs

### What is latent heat of sublimation in Thermodynamics II?

It is the heat required to change a solid directly into vapor without passing through the liquid phase. In Thermodynamics II, you treat it as a pure-substance property tied to phase change, not a temperature increase. The temperature stays fixed during the change.

### How is latent heat of sublimation different from latent heat of vaporization?

Vaporization is liquid to gas, while sublimation is solid to gas. That means sublimation skips one whole phase, so it usually requires more energy. If the starting state is a solid, you should not use the vaporization value.

### Why is latent heat of sublimation larger than latent heat of fusion?

Fusion only breaks the structure enough for a solid to become a liquid. Sublimation has to free particles all the way into the gas phase, which takes more energy. That is why the value is usually much higher.

### What is an example of sublimation in Thermodynamics II?

Dry ice is the cleanest example. At atmospheric pressure, solid carbon dioxide turns straight into gas, so it is a common lab and homework example for phase change energy calculations. Iodine is another substance that can show sublimation.

## Related Study Guides

- [1.2 Properties of Pure Substances](/thermodynamics-ii/unit-1/properties-pure-substances/study-guide/WR0waFNGu4myoZTA)

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