Change of state diagrams

The triangle has three corners and three sides. Place solid, liquid, and gas at the three corners of a triangle. Every side connects two states directly, and every side carries exactly two arrows, pointing in opposite directions — a change and its reverse. That gives $3$ sides $\times$ $2$ arrows $=$ $6$ changes of state in total, matching the six names you need to know.

Reading each side.

• Solid ↔ Liquid: melting (fusion) goes solid → liquid; freezing (solidification) goes liquid → solid.
• Liquid ↔ Gas: vaporisation (covering both quiet evaporation and vigorous boiling) goes liquid → gas; condensation (liquefaction) goes gas → liquid.
• Solid ↔ Gas: sublimation goes solid → gas directly; deposition (desublimation) goes gas → solid directly — both skip the liquid state entirely.

Direction tells you the heat flow. Any arrow moving solid → liquid → gas (melting, vaporisation, sublimation) means heat is being supplied: particles gain kinetic energy and break free of the stronger, more ordered attractions of the earlier state. Any arrow moving the opposite way, gas → liquid → solid (condensation, freezing, deposition), means heat is being removed: particles lose energy and settle into a more ordered arrangement.

Why the triangle needs a third side. Most substances change state one step at a time, through the liquid stage. But a small set of substances — camphor, naphthalene (mothballs), dry ice — jump directly between solid and gas under ordinary conditions. Without the solid–gas side, the diagram would be a straight line and could never show sublimation or deposition.

Explaining every entry in the triangle from a single physical cause makes the diagram easy to reconstruct even if you forget the picture — the goal is to show that all six arrow names follow from just two facts: particles in a solid are the most tightly bound and least energetic, particles in a gas are the least bound and most energetic, and adding or removing heat moves a substance between these states by changing how much kinetic energy its particles carry relative to the attraction holding them together.

Step 1. Picture the three states by how their particles behave. In a solid, particles vibrate about fixed positions, held by strong inter-particle attraction — the most ordered, least energetic arrangement of the three. In a liquid, particles have enough energy to slide past one another while still staying close together. In a gas, particles have enough energy to escape the attraction almost entirely and move freely, far apart — the least ordered, most energetic arrangement.

Step 2. Heat added always pushes a substance toward the less-ordered, more-energetic state on the triangle, because heat is energy being transferred in, and that extra energy lets particles overcome more of the attraction binding them. This single rule generates three of the six arrow names at once: solid → liquid is melting, liquid → gas is vaporisation, and solid → gas is sublimation — every "heat in" arrow on the triangle, regardless of which side it sits on.

Step 3. Heat removed always pushes a substance the opposite way, toward the more-ordered, less-energetic state, because energy is leaving and the particles that remain have less kinetic energy to resist the attraction pulling them together. This generates the other three arrows as a mirror set: liquid → solid is freezing, gas → liquid is condensation, and gas → solid is deposition — every "heat out" arrow, and each one is the exact reverse of one of the Step 2 arrows.

Step 4. The reason the solid–gas side exists at all, rather than forcing every substance through liquid first, is that "more energetic" and "less energetic" are about the total energy balance, not a fixed route. For most substances, the jump from the tightly-bound solid state to the freely-moving gas state is too large to make directly at ordinary pressure, so it happens in two smaller steps via the liquid stage. A few substances — camphor, naphthalene, dry ice — have weak enough inter-particle attraction in the solid state that the whole jump can happen in one step at ordinary conditions, giving direct sublimation and deposition.

Question: An open pot of ice is heated continuously on a stove until nothing is left in the pot. Using the change-of-state triangle, name, in order, every change of state the water undergoes, and state whether heat is being added or removed at each step.

Step 1. Start at the solid corner (ice). As heat is supplied, follow the arrow from solid to liquid: this is melting (fusion), and heat is being added — ice turns to liquid water at its melting point, $0\ ^\circ\text{C}$.

Step 2. Continued heating raises the water's temperature until it reaches its boiling point; the applicable arrow is liquid → gas: vaporisation (boiling), heat again being added — bubbles form throughout the liquid and steam escapes rapidly.

Step 3. Since the pot is open, escaping steam is not trapped and cooled back within the system, so no condensation arrow applies here — the process simply ends once all the liquid has vaporised, with no third change of state to name.

Step 4. Reading the triangle in order, the water undergoes melting, then vaporisation — both heat-added arrows, tracing the solid → liquid → gas path along two sides of the triangle. (Ordinary evaporation is also happening quietly at the surface throughout the heating, but the dominant, fixed-temperature change once boiling begins is vaporisation.)

Competitive stretch

A block of dry ice (solid carbon dioxide) left in an open dish at room temperature shrinks steadily and disappears, producing a visible white mist, without ever forming a liquid puddle. Step 1. The starting state is solid and the ending state is gas — carbon dioxide vapour; the visible mist is actually tiny water droplets condensed from air moisture cooled by the very cold gas, not the carbon dioxide itself. Step 2. On the triangle, the arrow going directly from solid to gas, without passing through liquid, is labelled sublimation, and heat is being added throughout, drawn from the warmer surrounding air. Step 3. The liquid corner is skipped because dry ice, like camphor and naphthalene, has weak enough solid-state attraction to sublime at ordinary atmospheric conditions — confirming that the third side of the triangle (solid ↔ gas) is needed, not just the solid–liquid and liquid–gas sides.

• Drawing the diagram as a straight line, solid — liquid — gas, and forgetting the third side connecting solid directly to gas — this misses sublimation and deposition entirely.
• Mixing up which arrow means which change: check direction first. Heat added moves solid → liquid → gas; heat removed moves the opposite way, and every side has exactly one arrow of each kind.
• Treating vaporisation as a name only for boiling — it is the general term for the whole liquid → gas arrow, covering both quiet evaporation and vigorous boiling.
• Trying to draw a temperature-time heating or cooling curve alongside this diagram — that graphical treatment, and latent heat, are not part of the Class 8 syllabus; the triangle alone is enough here.

Ask the tutor: Want to try drawing the full triangle from memory, or trace what happens to a specific substance as it is steadily heated or cooled? Ask the AI tutor to check your version arrow by arrow.