The fusion curve is the line on a phase diagram where a substance changes from solid to liquid. In Thermodynamics II, it shows how melting temperature changes with pressure for a pure substance.
The fusion curve is the solid-liquid boundary on a phase diagram in Thermodynamics II. Every point on the curve gives a pressure and temperature pair where the solid and liquid phases of a pure substance are in equilibrium, so the material can melt or freeze without a net change in phase.
If you picture a P-T diagram, the fusion curve separates the solid region from the liquid region. On one side, the substance stays solid at that pressure and temperature. On the other side, it stays liquid. Right on the curve, both phases can exist together, which is why the line is tied to melting point rather than to a single fixed melting temperature.
For most substances, the fusion curve slopes upward. That means higher pressure usually raises the melting point, because the solid phase is often denser than the liquid and pressure favors the phase with smaller volume. This is the pattern you expect in many engineering materials and in standard property diagrams.
Water is the famous exception. Ice takes up more volume than liquid water, so increasing pressure can lower the melting point instead of raising it. That unusual slope is why the fusion curve is not just a memorized line, but a clue about molecular structure and specific volume.
In Thermodynamics II, you usually read the fusion curve as part of a larger phase diagram rather than in isolation. It connects directly to phase equilibria, latent heat, and property tables. When a problem gives you pressure and temperature near the solid-liquid boundary, the curve tells you whether the substance is stable as a solid, a liquid, or a two-phase mixture at equilibrium.
The fusion curve shows up any time Thermodynamics II asks you to reason about phase behavior instead of just plugging values into a table. It tells you whether a substance can melt at a given pressure, which matters when you are working with pure substances, high-pressure systems, or materials exposed to unusual conditions.
This is also one of the first places where you see how pressure changes phase change behavior. A lot of students remember that a melting point is a single temperature, but in thermodynamics that is only true at one pressure. Once pressure changes, the solid-liquid equilibrium point moves, and the fusion curve shows that shift directly.
That matters in engineering situations like selecting materials for pressure vessels, refrigeration components, or systems where water can freeze under load. It also helps when you interpret a phase diagram during a problem set, because you need to know which region the state point falls in before you use the right property relation.
The curve also sets up later ideas in phase equilibria and energy transfer. If a substance crosses the fusion curve, you may need latent heat of fusion instead of sensible heat, and that changes the whole energy balance. In other words, the curve is a map for deciding what kind of thermodynamic calculation comes next.
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The fusion curve is one part of the full phase diagram. A phase diagram shows all the major boundaries between solid, liquid, and vapor regions, while the fusion curve focuses only on the solid-liquid boundary. When you read a diagram problem, you usually use the fusion curve together with the other lines to locate the stable phase at a given pressure and temperature.
melting point
The melting point is the temperature where solid and liquid are in equilibrium at a specified pressure, usually one atmosphere in basic chemistry. The fusion curve shows that this temperature is not fixed forever, it shifts as pressure changes. So if a Thermodynamics II problem changes pressure, the fusion curve tells you how the melting point moves.
Latent Heat of Fusion
The fusion curve tells you where melting happens, while Latent Heat of Fusion tells you how much energy it takes to make that phase change happen. If a state point crosses the solid-liquid boundary, you may need an energy balance that includes latent heat instead of just using specific heat. The curve and the latent heat go together in phase-change calculations.
triple point
The triple point is where the fusion curve meets the sublimation curve and the vaporization curve. At that single pressure and temperature, solid, liquid, and vapor can all coexist. In a phase diagram, the triple point helps anchor the fusion curve because it shows where the solid-liquid boundary begins and how it connects to the rest of the diagram.
A problem set or quiz will usually give you a phase diagram and ask you to identify where a state lies relative to the fusion curve. You might need to say whether the substance is solid, liquid, or on the melting boundary, then explain what happens if pressure increases. In a numerical problem, the curve helps you decide whether you are dealing with sensible heating or a phase change.
You may also see conceptual questions about why water is different from most substances. In that case, the move is to connect the negative slope of water’s fusion curve to the fact that ice has a larger specific volume than liquid water. If the prompt includes a pressure change, don’t assume the melting point stays the same, use the curve to reason it out.
The fusion curve is the solid-liquid boundary, while the sublimation curve is the solid-vapor boundary. They meet at the triple point, but they describe different phase changes. If a problem asks about melting, use the fusion curve. If it asks about a solid turning directly into vapor, that is the sublimation curve.
The fusion curve is the line on a P-T phase diagram where a pure substance melts or freezes at equilibrium.
For most substances, the curve slopes upward, so higher pressure means a higher melting point.
Water is the common exception because ice has a larger specific volume than liquid water, which can make the melting point drop as pressure rises.
In Thermodynamics II, the fusion curve helps you decide which phase is stable before you choose the right property table or energy balance.
When a state crosses the fusion curve, the calculation may need latent heat of fusion instead of only temperature change.
The fusion curve is the boundary on a phase diagram where a substance is exactly at the solid-liquid transition. It shows the pressure and temperature combinations where melting and freezing occur in equilibrium. In Thermodynamics II, you use it to tell whether a state point is solid, liquid, or right on the melting line.
No. Most substances have an upward-sloping fusion curve, which means pressure raises the melting point. Water is the classic exception, because ice is less dense than liquid water, so higher pressure can lower the melting point instead.
The fusion curve separates solid and liquid phases. The sublimation curve separates solid and vapor phases. They are both phase-boundary lines, but they describe different transitions and meet at the triple point.
You use it when a problem asks about phase behavior, phase diagrams, or how pressure changes melting. It also helps you decide whether to use latent heat of fusion in an energy balance. If a state point is near the solid-liquid boundary, the curve tells you which side of the boundary the substance is on.