NDA Chemistry · Matter and Its States
States of Matter, Phase Changes and Diffusion
Matter exists as solid, liquid or gas depending on how tightly its particles are held; adding or removing heat moves it between these states through six named phase changes, and particles spread on their own by diffusion.
Why this matters
The foundation subtopic and a reliable scorer — 7 PYQs, almost every year. The bank tests three things: the properties of the three states (and which elements are liquid at room temperature), the names of the six phase changes (deposition and sublimation are the favourites), and the meaning of diffusion. Dry ice as a sublimation example shows up almost every other year. Learn the phase-change hexagon and the dry-ice fact and most of this subtopic is automatic.
Concept 1 of 4
The three states of matter
Intuition
Everything around you is one of three states. The difference is only how tightly the particles are packed and how freely they move: locked in place (solid), touching but sliding (liquid), or far apart and flying free (gas). Heating loosens the packing; cooling tightens it.
Definition
The three states and their particle picture:
- Solid — particles packed close in a fixed pattern, vibrating in place. Fixed shape and fixed volume; nearly incompressible.
- Liquid — particles close but free to move past one another. Fixed volume but no fixed shape (takes the shape of its container); nearly incompressible.
- Gas — particles far apart, moving fast and randomly. No fixed shape and no fixed volume (fills the container); highly compressible.
- At room temperature (about 25 °C) and normal pressure, only two elements are liquid: mercury (Hg) and bromine (Br₂). Gallium (melts at 30 °C) and caesium (melts at 28 °C) are solids at 25 °C but melt in a warm hand.
| State | Shape | Volume | Compressibility |
|---|---|---|---|
| Solid | Fixed | Fixed | Almost none |
| Liquid | Takes container's shape | Fixed | Almost none |
| Gas | Takes container's shape | Fills container | High |
| Liquid elements at 25 °C | — | — | — Only mercury (Hg) and bromine (Br₂) are liquid at room temperature and normal pressure. Gallium and caesium are solids at 25 °C (they melt only just above it). |
Solid = fixed shape + fixed volume; Liquid = fixed volume only; Gas = neither.
Practice this conceptself-check · 4 quick reps
Try it yourself
Which two elements are liquid at room temperature (about 25 °C) and normal pressure?
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which state of matter has a fixed volume but no fixed shape?
- 2.Which state of matter is highly compressible?
- 3.Name the only two elements that are liquid at room temperature.
- 4.Does a gas have a fixed volume?
From the bank · past-year question
Which one of the following pairs of elements is liquid at room temperature and at normal pressure?
[Q105 · Sep · 2021]
Gallium is a solid at room temperature
Gallium feels like it should be liquid (it melts in your hand at 30 °C), but at room temperature (25 °C) it is a solid. The only elements liquid at 25 °C are mercury and bromine.
Concept 2 of 4
The six phase changes
Intuition
Heating and cooling move matter between the three states. There are six named transitions — one each way between every pair of states. The two that students forget are the direct solid↔gas pair: sublimation (solid → gas) and deposition (gas → solid).
Definition
The six interconversions of state:
- Melting (fusion) — solid → liquid (add heat).
- Freezing (solidification) — liquid → solid (remove heat).
- Vaporisation (boiling/evaporation) — liquid → gas (add heat).
- Condensation — gas → liquid (remove heat).
- Sublimation — solid → gas directly, skipping liquid (e.g. dry ice, camphor, naphthalene, iodine, anthracene).
- Deposition — gas → solid directly, skipping liquid (e.g. sulphur vapour forming a crust on rocks; frost forming on a cold surface).
Two sub-points the bank loves:
- Boiling is a bulk phenomenon (happens throughout the liquid at the boiling point), but evaporation is a surface phenomenon (happens only at the surface, at any temperature).
- All phase changes between states of the same substance are physical changes — no new substance forms.
| Phase change | Direction | Heat | Example |
|---|---|---|---|
| Melting | Solid → liquid | Absorbed | Ice → water |
| Freezing | Liquid → solid | Released | Water → ice |
| Vaporisation | Liquid → gas | Absorbed | Water → steam |
| Condensation | Gas → liquid | Released | Steam → water droplets |
| Sublimation | Solid → gas | Absorbed | Dry ice → CO₂ gas; camphor Sublimation skips the liquid state entirely — solid goes straight to gas. |
| Deposition | Gas → solid | Released | Sulphur vapour → solid crust; frost Deposition is the reverse of sublimation: gas straight to solid, no liquid. |
Sublimation and deposition are the direct solid↔gas pair, skipping liquid.
Practice this conceptself-check · 5 quick reps
Try it yourself
Vapours of sulphur escaping from a volcano form a solid crust on the rocks. Name the phase change, and state how it differs from condensation.
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Name the phase change from solid directly to gas.
- 2.Name the phase change from gas directly to solid.
- 3.Is boiling a surface or a bulk phenomenon?
- 4.Liquid → solid is called?
- 5.Frost forming on a cold window is which phase change?
From the bank · past-year question
Vapours of sulphur escaping from a volcano often form a crust on the rocks. The process involved is an example of
[Q108 · Sep · 2022]
Boiling is bulk, evaporation is surface
The correct statement is: boiling is a bulk phenomenon (throughout the liquid, only at the boiling point) but evaporation is a surface phenomenon (only at the surface, at any temperature). Any statement that swaps these two, or calls both 'surface' or both 'bulk', is wrong.
Deposition ≠ condensation
A gas turning into a solid directly is deposition, not condensation. Condensation is gas → liquid. The sulphur-crust and frost examples are deposition.
Concept 3 of 4
Dry ice — solid carbon dioxide
Intuition
'Dry ice' is the bank's single most-repeated fact in this chapter. It is solid carbon dioxide, not frozen water — and it is 'dry' because it sublimes (solid → gas) with no wet liquid stage in between.
Definition
Everything the bank asks about dry ice:
- Dry ice is solid carbon dioxide (CO₂) — NOT frozen water.
- It sublimes: solid CO₂ → CO₂ gas directly, leaving no liquid (hence 'dry').
- Used as a refrigerant and to make stage mist: the cold CO₂ gas chills the air, condensing atmospheric water vapour into a visible fog. (The dry ice itself sublimes; the visible mist is water vapour condensing.)
| Question asked | Answer |
|---|---|
| What is dry ice? | Solid carbon dioxide (CO₂) |
| Is dry ice frozen water? | No — it is solid CO₂ |
| Phase change when dry ice 'disappears' | Sublimation (solid → gas) Dry ice never melts to a liquid at normal pressure — it sublimes straight to gas. |
| Why mist forms on a stage | Cold CO₂ gas condenses atmospheric water vapour |
Practice this concept3 quick reps
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Dry ice is the solid form of which compound?
- 2.What phase change does dry ice undergo at normal pressure?
- 3.Is dry ice frozen water?
From the bank · past-year question
Dry ice is used on a performing stage to produce mist in air. The process involved is an example of
[Q109 · Sep · 2022]
Dry ice is CO₂, not ice
Dry ice is solid carbon dioxide, not frozen water. It sublimes (solid → gas) with no liquid stage — that is why it is called 'dry'.
Concept 4 of 4
Diffusion
Intuition
Particles are always moving, so two substances placed together mix on their own without any stirring — a smell spreading across a room, a drop of ink colouring a glass of water. That spontaneous intermixing is diffusion.
Definition
Key facts about diffusion:
- Diffusion is the spontaneous intermixing of the particles of two different types of matter on their own (no stirring needed).
- It happens because particles are in constant random motion.
- Rate of diffusion: gas > liquid > solid — faster where particles move more freely. (Solids barely diffuse at all.)
- Diffusion is faster at higher temperature (particles move faster).
| State | Diffusion rate | Everyday example |
|---|---|---|
| Gas | Fastest | Perfume smell spreading across a room |
| Liquid | Moderate | Ink drop colouring water |
| Solid | Slowest (negligible) | Two metals welded together over years Solids diffuse extremely slowly because their particles are locked in place. |
Diffusion is fastest in gases, slowest in solids, and faster when hotter.
Practice this concept3 quick reps
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.What is the spontaneous intermixing of particles of two substances called?
- 2.In which state is diffusion fastest?
- 3.Does diffusion get faster or slower at higher temperature?
From the bank · past-year question
The intermixing of particles of two different types of matter on their own is called
[Q56 · Sep · 2024]
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
The three states of matter4 rows
| State | Shape | Volume | Compressibility |
|---|---|---|---|
| Solid | Fixed | Fixed | Almost none |
| Liquid | Takes container's shape | Fixed | Almost none |
| Gas | Takes container's shape | Fills container | High |
| Liquid elements at 25 °C | — | — | — Only mercury (Hg) and bromine (Br₂) are liquid at room temperature and normal pressure. Gallium and caesium are solids at 25 °C (they melt only just above it). |
Solid = fixed shape + fixed volume; Liquid = fixed volume only; Gas = neither.
The six phase changes6 rows
| Phase change | Direction | Heat | Example |
|---|---|---|---|
| Melting | Solid → liquid | Absorbed | Ice → water |
| Freezing | Liquid → solid | Released | Water → ice |
| Vaporisation | Liquid → gas | Absorbed | Water → steam |
| Condensation | Gas → liquid | Released | Steam → water droplets |
| Sublimation | Solid → gas | Absorbed | Dry ice → CO₂ gas; camphor Sublimation skips the liquid state entirely — solid goes straight to gas. |
| Deposition | Gas → solid | Released | Sulphur vapour → solid crust; frost Deposition is the reverse of sublimation: gas straight to solid, no liquid. |
Sublimation and deposition are the direct solid↔gas pair, skipping liquid.
Dry ice — solid carbon dioxide4 rows
| Question asked | Answer |
|---|---|
| What is dry ice? | Solid carbon dioxide (CO₂) |
| Is dry ice frozen water? | No — it is solid CO₂ |
| Phase change when dry ice 'disappears' | Sublimation (solid → gas) Dry ice never melts to a liquid at normal pressure — it sublimes straight to gas. |
| Why mist forms on a stage | Cold CO₂ gas condenses atmospheric water vapour |
Diffusion3 rows
| State | Diffusion rate | Everyday example |
|---|---|---|
| Gas | Fastest | Perfume smell spreading across a room |
| Liquid | Moderate | Ink drop colouring water |
| Solid | Slowest (negligible) | Two metals welded together over years Solids diffuse extremely slowly because their particles are locked in place. |
Diffusion is fastest in gases, slowest in solids, and faster when hotter.
Watch out for (4)
- Gallium is a solid at room temperature→ The three states of matter
- Boiling is bulk, evaporation is surface→ The six phase changes
- Deposition ≠ condensation→ The six phase changes
- Dry ice is CO₂, not ice→ Dry ice — solid carbon dioxide
Mastery check — 3 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
Which one of the following statements is correct?
[Q58 · Sep · 2019]
Which one of the following is termed as 'Dry ice'?
[Q76 · Sep · 2019]
Which one of the following is called dry ice?
[Q82 · Sep · 2018]
Drill every past-year question on this subtopic
7 questions from the bank — paginated, with cart and Word-export support.