Direct solid and gas changes

A liquid stage is not always part of the path

Ice can melt into water, then the liquid can evaporate. That path includes liquid. Under suitable conditions, a solid can instead change directly into gas. The actual path matters, not just the first and last states

Sublimation: Solid changes directly to gas, with no liquid stage. Some ice can become water vapour while it remains below its melting point. Water vapour is water in the gas state

Deposition: Gas changes directly to solid, with no liquid stage. Water vapour can become ice directly on a sufficiently cold surface under suitable conditions

Compare the liquid routes: Solid to liquid to gas involves melting then vaporisation. Gas to liquid to solid involves condensation then freezing. Neither two-stage path is a direct solid–gas change

A sample starts as solid and ends as gas. Does that information alone show that sublimation occurred?

Could the sample have been liquid between the two recorded states?

Follow particles, not shrinking dots

The next model follows twelve selected particles of one substance. Each small circle is a particle, not a visible grain or drop. A and B label two particles so you can follow them. Close circles near positions represent solid; separated circles represent gas

Same particles, same size: As solid becomes gas, the solid portion gets smaller because particles leave it. The individual particles do not shrink or vanish. Solid particles vibrate near positions; gas particles move through the space available

A selected region: The dashed rectangle is not a container. It marks the explanation's selected region, not all the particles in a real sample. Surrounding air is omitted

Snapshots, not measured motion: The three snapshots show different illustrative arrangements. They are not to scale, not seconds, and not measured particle paths. They do not measure temperature, pressure, energy, rate or density

Imagine some solid becoming gas directly. Predict where A will be, then compare snapshots 1 and 3. Check the particle sizes and the total count as well as the solid portion

What changes in the model?

Separating particles needs energy

Particles attract one another. Sublimation needs energy to separate particles from the close solid arrangement. Warmer surroundings can supply it; an electric heater is not required

Energy into a state change: Energy input can help particles separate rather than only raising temperature. Do not infer a particular temperature change when no readings are supplied

The reverse direction: When gas becomes solid by deposition, energy transfers out of the depositing material to the surface or surroundings as the close arrangement forms

Movement continues: Energy transfer out does not mean the particles become completely motionless. In the solid they vibrate near positions

A warmer surface supplies energy while some solid becomes gas directly. How can that energy help the change?

Think about particles leaving the close arrangement against attractions

Two paths can end with ice

Suppose a record establishes that water vapour becomes ice directly on a cold surface, without liquid forming. The initial state is gas, the final state is solid, and the path has no liquid stage. That is deposition

Explain the particles: The same water particles join a close solid arrangement and vibrate near positions. They do not change into a new substance or become larger particles

Explain the energy: The depositing water transfers energy to the surface and surroundings. The surface can receive energy even though it is cold

Do not identify the path from ice alone: If vapour first condensed into droplets and those droplets froze, the path would be condensation then freezing. An icy surface alone does not tell us which history occurred

Explain the recorded direct path

Water vapour reaches a cold surface and becomes ice there without first forming liquid droplets

Name this state change and explain which part of the record establishes a direct path

Water vapour is gas and ice is solid. No liquid forms along the stated path

Gas changing directly into solid is deposition. The presence of ice alone would not tell us this path; the record supplies the missing history

Predict what happens to A as gas becomes solid directly. Compare all three snapshots, then look at the particles that have joined the solid

Which explanation fits the last snapshot?

A gas becomes solid directly on a cold surface. Which direction of energy transfer is associated with forming that solid?

Consider the material joining the solid arrangement

Dry ice is not frozen water

Dry ice is solid carbon dioxide. At ordinary atmospheric pressure it can change directly into carbon dioxide gas without a liquid stage. The substance remains carbon dioxide

The gas is invisible: Carbon dioxide gas is not a visible white cloud. Where a nearby fog contains tiny liquid water droplets, those droplets are the visible material, not the carbon dioxide gas. Each droplet contains many water particles

Use the stated conditions: The direct change is not caused simply by heating quickly. The substance and conditions determine which state paths are possible

Conceptual example only: Dry ice can cause cold burns and dangerous carbon dioxide build-up. Do not handle it, put it in sealed containers or try a home demonstration. Practical work needs appropriate supervision and safety controls

A nearby white fog is examined and found to contain tiny liquid water droplets. What is the visible fog?

Use the observed droplet state rather than assuming that every cloud is gas

What does the evidence actually show?

An explanation should fit the whole recorded path. Seeing less solid later is an observation; saying that it sublimed is an inference about how the material changed. The inference needs evidence

Missing moments matter: Two photographs with no liquid visible do not prove that liquid never formed between them. Material might also have moved or changed in another way. Check observations over time and the substance and conditions

Heating speed is not a process name: Quick heating does not by itself remove the liquid stage. It does not prove sublimation, and it does not prove sublimation impossible either. Identify the actual state path

A model is not a measurement: The selected-particle snapshots explain an idea. They cannot establish what an unknown real sample did, how fast it changed or how many particles it contained

An observer sees a small solid in the morning and no visible solid that afternoon. No observations were made between those times. What is justified?

Separate the observed disappearance from an unrecorded state path

Someone claims that a sample must have sublimed because it was heated quickly. No state observations or material conditions are given. What is missing?

A rate of heating is not a record of which states occurred

Compare two complete state paths

Construct an explanation from two records

Written task, not automatically marked

Record A: water vapour forms liquid droplets on a cooled plate, then those droplets freeze. Record B: water vapour forms ice directly on another cooled plate, with no liquid stage. Both records track the same water substance without a chemical reaction

Write the ordered state path and process names for each record. Explain what happens to the water particles and the direction of energy transfer as the ice forms. Explain why a final photograph of ice cannot distinguish the two histories

Compare your explanation with the records

Record A follows gas to liquid to solid: condensation, then freezing. Record B follows gas directly to solid: deposition

In both records the water particles retain their identity and size. In the final solid they are close and vibrate near positions; forming that solid transfers energy out of the water to the plate and surroundings

The same final state can follow different paths. A photograph of ice does not reveal whether droplets formed first, so the time-ordered observations supply necessary evidence

Follow the path and explain the particles

Sublimation is solid directly to gas; deposition is gas directly to solid. Neither includes liquid. The same particles remain, while arrangement, movement and energy transfer explain the change

Energy direction: Sublimation needs energy for separation against attractions. Deposition transfers energy out as the solid arrangement forms

Evidence before a conclusion: A shrinking solid, an icy endpoint or quick heating alone does not establish the full state path. An illustrative model cannot replace the missing observations

Transfer to the model limits