Interpret evaporation, boiling and condensation

Two ways from liquid to gas

Liquid particles remain close but can move past neighbours. Gas particles are more separated and move through the space available. Vaporisation means liquid becoming gas. Evaporation and boiling are two ways it happens

1. 1Evaporation Particles escape from a liquid's surface. This can happen below the boiling point, such as a thin water film drying in a room
2. 2Boiling Vapour of the substance forms bubbles within the liquid at its boiling point under the stated pressure. Surface evaporation can also continue
3. 3The reverse state direction Condensation means gas becoming liquid. Vapour is the substance in its gas state, not newly made air

Read the particle explanation

The next pictures follow fourteen selected particles of one substance. Equal small circles represent particles, not drops. A and B track two particles changing state; C stays liquid and changes neighbours

1. 1Regions and boundaries Pale blue marks liquid. A large orange ring, when present, marks a vapour bubble containing smaller particle circles. The dashed rectangle is the explanatory region, not a wall
2. 2Two different scales The lower sample sketch represents what is happening to the sample; the upper view explains it with particles at a different scale. A real drop or bubble contains many more particles than these selected few. Air and other particles are omitted
3. 3Snapshots, not a timed simulation Snapshot numbers label separate arrangements, not seconds. They do not measure paths, speed, rate, temperature, pressure or volume. Each particle keeps the same drawn size

Predict where A and B will be after leaving the surface. Compare snapshots 1 and 3, then follow C within the liquid

What changes between the first and last snapshots?

A vapour bubble is a region, not a particle

During boiling, gas made from the liquid forms bubbles within it. Each bubble contains many particles of the same substance. A larger bubble is not one enlarged particle, and it is not an empty hole

1. 1Do not identify boiling from one bubble alone A bubble might instead contain gas released from solution or introduced from elsewhere. The gas source, repeated observations, temperature and pressure conditions matter
2. 2Pressure is part of the condition Pressure describes a push per unit area on the sample. A substance's boiling point can change when pressure changes. Use the boiling point supplied for the stated pressure, not one universal value

Compare snapshots 1 and 3. Follow A and B into the large ring, then compare the ring with the small circles inside it

How should the growing ring be interpreted?

A liquid loses material from its surface at a temperature below its boiling point. No vapour bubbles form inside it. Which conclusion fits?

Use both the location of the change and the temperature condition

Where do droplets on a cold object come from?

Imagine a dry solid metal disc with no water inside it. After it is cooled in air containing water vapour, liquid droplets form on its outside. The air already contains a water source: invisible water vapour. Some of it changes into liquid on the cool surface

1. 1Name the direction Water goes from gas to liquid, so this is condensation. It does not require liquid to leak through the metal or the metal to turn into water
2. 2Water vapour is invisible A visible white cloud above hot water contains tiny liquid droplets, each made of many particles. The gas itself is invisible
3. 3Not only at the normal boiling point Water vapour mixed with air can condense on a sufficiently cool surface far below water's normal boiling temperature. Do not use the boiling point alone to judge this air-and-surface example

The lower sketch shows droplets attached to a cool surface. The upper view explains a small part of a droplet boundary. Predict what happens to A and B, then compare snapshots 1 and 3

Which particle change explains the growing liquid portion?

A visible cloud above warm water is made of tiny liquid droplets. What does this tell us about the cloud?

A droplet's small size does not make it a gas particle

Separating particles needs energy

Particles attract one another. To become gas, particles must separate against those attractions. During steady boiling, a heater can supply the energy for this change. The particles do not need to grow or become another substance

1. 1Condensation transfers energy out As gas joins the closer liquid arrangement, energy is transferred from the condensing material to the surface or surroundings. The receiving surface can warm
2. 2Recall melting and freezing Energy transfer does not always change temperature. A pure substance can boil at steady temperature while energy continues to enter and more liquid becomes gas

A heater keeps supplying energy to a pure liquid boiling at fixed pressure. More vapour forms, but temperature stays steady. What is the energy doing?

Compare the changing state with the unchanging thermometer reading

Why evaporation can cool a liquid

Kinetic energy means energy of movement. Liquid particles do not all have the same kinetic energy; those that escape tend to have more. Average kinetic energy means the total movement energy divided by the number of particles. No calculation is needed here

1. 1Follow the remaining liquid If particles with more kinetic energy leave, the average kinetic energy of those left behind can fall. Temperature is linked to this movement energy, not the total energy stored in the sample. The remaining liquid can cool without its particles shrinking
2. 2Check energy coming in as well Warmer surroundings can transfer energy back into the liquid. If that replaces what evaporation removes, evaporation can continue without a further temperature fall
3. 3Not all vaporisation uses an external heater Surface evaporation can draw energy from the remaining liquid. Do not assume it needs the whole sample to reach boiling or an external heater to be switched on

A wet surface is drying while its temperature stays steady. Does that prove evaporation has stopped?

Consider whether energy entering from the surroundings can offset cooling

A boiling point is not a progress meter

A hypothetical pure substance boils at 76 °C at a stated pressure. While liquid and vapour coexist at that fixed pressure, more of the liquid can become vapour without changing the temperature. Coexist means the two states are present together

1. 1Exactly at the point A reading of 76 °C alone cannot tell whether little or much liquid remains. It does not mean half liquid, and it does not prove that vaporisation has finished
2. 2Compare conditions The same pure substance can boil at different temperatures under different pressures. Compare measurements at matching pressures before using them to question identity or purity
3. 3Do not transfer this rule to every air mixture This is a pure-substance, fixed-pressure transition. Condensation of water vapour from air onto a cool object is not restricted to water's normal boiling temperature

A pure substance's boiling point is 76 °C at the stated pressure. The only recorded observation is a sample temperature of 76 °C. How much is gas?

A temperature at the transition point does not show how far the change has progressed

Record the change, not just one clue

A bubble or a steady thermometer reading alone cannot establish boiling. Record what forms, where it forms, the sample temperature and the pressure conditions. Link repeated observations to the starting and finishing states

1. 1Use independent differences To compare evaporation and boiling, give a location difference and a temperature-condition difference. Saying evaporation happens at the surface and only the surface repeats one idea; it is not two differences
2. 2Real measurements Uneven temperature, heat loss and measurement uncertainty can affect readings. A model's selected pictures are not experimental data
3. 3Practical work Heating samples, observing safely and measuring reliably require supervised practical work. Selecting a good observation record does not certify those skills or an independent written explanation

A photograph shows one bubble in a liquid, without a temperature or gas-content record. Which claim is supported?

An unexplained bubble can have more than one source

Follow the substance through the change

Evaporation and boiling both change liquid into gas; condensation changes gas into liquid. The particles remain the same substance and size. Their arrangement and separation change

1. 1Use observations Evaporation occurs at the surface and can happen below boiling. Boiling forms vapour within the liquid under the stated conditions. Droplets on a cool object can come from vapour in air
2. 2Use energy and conditions Separating particles needs energy; condensation transfers energy out. Evaporation can cool the remaining liquid, but incoming energy can offset that effect. A boiling-point reading alone does not give phase proportions