2.6 Sublimation

Introduction

Most substances change from solid to liquid to gas when heated. But some substances skip the liquid stage completely. They go directly from solid to gas. This process is called sublimation.

Sublimation is not a rare or unusual process. It happens in everyday life — from the shrinking of dry ice to the smell of air fresheners. This section explains what sublimation is, how it works according to kinetic molecular theory, and how it is used in daily life.

Definition

Sublimation: Sublimation is the process in which molecules of a substance in the solid state go to the vapour state directly, without going through the liquid state.

Deposition (reverse of sublimation): Deposition is the process in which a gas changes directly into a solid without passing through the liquid state.

In simple words:

• Sublimation = solid → gas (skipping liquid)
• Deposition = gas → solid (skipping liquid)

Main Idea

• Sublimation happens when a solid turns directly into a gas.
• It occurs because some molecules in the solid have enough energy to break free from the solid structure and escape as gas.
• This can happen even below the melting point of the substance.
• Substances that undergo sublimation easily include carbon dioxide (dry ice), camphor, and naphthalene.
• The reverse process (gas → solid) is called deposition.

Core Concept

According to the kinetic molecular theory:

• Molecules are in constant random motion, even when the substance is below its melting point.
• The average speed of the molecules (kinetic energy) is directly proportional to the absolute temperature.
• If the molecules are below the melting point and are kept in the solid state by relatively weak intermolecular forces (compared to the strength of the covalent bonds within the molecule), then sometimes a molecule will have enough energy to break those intermolecular bonds and go into the vapour state.

Simple explanation: In a solid, particles are held together by forces. But some particles may have extra energy — enough to break free and escape as gas. If the forces holding the solid together are weak enough, this can happen without melting first.

Detailed Explanation

How Sublimation Works

Step-by-step:

• In a solid, particles are tightly packed in fixed positions.
• They vibrate around their fixed positions.
• Even below the melting point, some particles may have enough kinetic energy to overcome the intermolecular forces holding them in the solid.
• These particles escape directly into the gas phase.
• This is sublimation.

Why it happens:

• The intermolecular forces in the solid are relatively weak compared to the covalent bonds within the molecule.
• This means the solid can be held together by weak forces, making it easier for individual molecules to escape.
• Substances with weak intermolecular forces are more likely to sublime.

Key point: Sublimation occurs when the intermolecular forces are weak enough that some molecules can escape directly into the gas phase without the solid melting first.

Substances That Undergo Sublimation

Many substances undergo sublimation. The source specifically mentions:

• Carbon dioxide (dry ice) — used for cooling and fog effects
• Camphor — used in medicines and as a moth repellent
• Naphthalene — used in mothballs

These substances have relatively weak intermolecular forces, which allows them to sublime easily.

Application of Sublimation in Our Life

Sublimation has many practical applications in everyday life and industry.

1. Solid Air Fresheners

The solid air fresheners we often use in our cars and homes rely solely on the magic of sublimation. The solid fragrance within slowly transforms into a delightful vapour, filling the entire space with an enticing aroma that lingers in the air.

How it works:

• The solid air freshener contains a fragrance.
• The solid fragrance sublimes slowly into the air.
• The vapour spreads through the space, providing a continuous fragrance.

2. Textile Industry and 3D Printing

In the textile industry, 3D printing uses a sublimation process. Dye sublimation printers are now widely used for the printing process for a reduced cost.

Dye sublimation is used to print a variety of objects such as:

• T-shirts
• Pens
• Coffee mugs
• Bags

How it works:

• Special dyes are heated until they sublime.
• The vapour then bonds with the material (like fabric or plastic).
• This produces a permanent, high-quality print.

3. Food Industry

In the food industry, freeze-drying food items for preservation for a longer duration is due to the process of sublimation.

How it works:

• Food is frozen, and then the ice is sublimed directly into vapour under low pressure.
• This removes water without damaging the food structure.
• The food can be stored for a long time and rehydrated later.

Simple Examples of Deposition in Daily Life

Deposition is the reverse of sublimation — gas to solid directly.

Examples:

• Formation of snow from water vapour — water vapour in the atmosphere changes directly into ice crystals (snow) without becoming liquid water first.
• Black soot inside a vehicle exhaust — gases from the engine deposit as solid soot particles.
• Making of dry ice using carbon dioxide — carbon dioxide gas is cooled and deposited directly as solid dry ice.

Summary Table

Intermediate Level

Why sublimation happens:

• Molecules in a solid are always vibrating.
• Some molecules have enough energy to break free from the solid structure.
• If the intermolecular forces are weak, these molecules can escape directly into the gas phase.
• This happens even below the melting point.

Key conditions:

• Weak intermolecular forces
• Enough energy in some molecules to overcome those forces

Everyday applications:

• Solid air fresheners
• Dye sublimation printing
• Freeze-drying food

Deposition (reverse process):

• Gas → solid directly
• Example: snow formation, dry ice making

Important Points

• - Sublimation = solid → gas directly (no liquid stage).
• - Deposition = gas → solid directly (no liquid stage).
• - Sublimation happens because some molecules have enough energy to break free from the solid.
• - It occurs even below the melting point.
• - Substances that sublime easily: carbon dioxide (dry ice), camphor, naphthalene.
• - Applications: solid air fresheners, dye sublimation printing, freeze-drying food.
• - Examples of deposition: snow formation, soot in exhaust, dry ice making.
• - Sublimation is favoured by weak intermolecular forces and low pressure.

Remember

• - Sublimation = solid → gas.
• - Deposition = gas → solid.
• - No liquid stage in either process.
• - Dry ice, camphor, naphthalene sublime easily.
• - Solid air fresheners use sublimation.
• - Dye sublimation printing uses sublimation.
• - Freeze-drying uses sublimation.
• - Snow formation is an example of deposition.
• - Weak intermolecular forces favour sublimation.

Common Mistakes

Quick Revision

Sublimation is the process in which molecules of a substance in the solid state go to the vapour state directly, without going through the liquid state.

According to kinetic molecular theory:

• Molecules are in constant random motion, even below the melting point.
• The average speed of the molecules (kinetic energy) is directly proportional to the absolute temperature.
• If the molecules are below the melting point and are kept in the solid state by relatively weak intermolecular forces (compared to the strength of the covalent bonds within the molecule), then sometimes a molecule will have enough energy to break those intermolecular bonds and go into the vapour state.

Substances that undergo sublimation:

• Carbon dioxide (dry ice)
• Camphor
• Naphthalene

Applications of Sublimation:

• Solid air fresheners — solid fragrance sublimes into the air.
• Textile industry and 3D printing — dye sublimation printers for T-shirts, pens, mugs, bags.
• Food industry — freeze-drying food items for preservation.

Simple examples of deposition in daily life:

• Formation of snow from water vapour.
• Black soot inside a vehicle exhaust.
• Making of dry ice using carbon dioxide.

60-Second Revision

• Sublimation = solid → gas directly (no liquid).
• Deposition = gas → solid directly (no liquid).
• Examples of sublimation: dry ice, camphor, naphthalene.
• Applications: air fresheners, dye sublimation printing, freeze-drying.
• Examples of deposition: snow, soot, dry ice making.
• Weak intermolecular forces → easier sublimation.
• Sublimation can happen below the melting point.
• ⚠️ Warning: sublimation is not the same as evaporation — evaporation is liquid → gas, while sublimation is solid → gas.