Melting Point

Introduction

Ice melts at 0 °C, chocolate melts in your hand, and iron needs a blast furnace at over 1500 °C. The temperature at which a solid turns into a liquid is its melting point . It is a quantitative, characteristic physical property that tells chemists a great deal about a substance, including how strongly its particles are held together.

Core explanation

Definition. The melting point is the temperature at which a solid changes into a liquid. For a pure substance, it is the same temperature as the freezing point — the temperature at which the liquid turns back into a solid. Water melts at 0 °C and freezes at 0 °C.

Some melting points (°C, at normal pressure):

Substance Melting point --- --- Oxygen −218 Mercury −39 Water (ice) 0 Gallium 30 Sulfur 115 Lead 327 Aluminium 660 Sodium chloride 801 Copper 1085 Iron 1538 Tungsten 3422

Predicting state. If room temperature (about 20–25 °C) is below a substance's melting point, it is a solid at room temperature. Gallium melts at about 30 °C, so a piece held in the hand slowly melts. Mercury melts at −39 °C, so it is a liquid at room temperature.

Temperature stays constant while melting. If you heat ice steadily and record the temperature, it rises to 0 °C and then stays at 0 °C until all the ice has melted, even though heating continues. The energy supplied is being used to overcome the attractions between particles, letting them break out of their fixed positions, rather than to make them move faster. This flat section on a heating curve is called a plateau .

What melting point tells us. A high melting point means strong forces hold the particles together — a lot of energy is needed to free them. Metals like tungsten and ionic compounds like sodium chloride have high melting points; small molecules like oxygen have very low melting points because the forces between their molecules are weak.

Step-by-step reasoning

To decide the state of lead at 400 °C:

1. Find the melting point of lead: 327 °C. 2. Compare: 400 °C is above the melting point. 3. Check the boiling point (1749 °C): 400 °C is below it. 4. So lead is a liquid at 400 °C.

Visual explanation

A heating curve for ice: temperature on the y-axis, time on the x-axis. The line rises from −20 °C to 0 °C (solid warming), runs flat at 0 °C (melting — solid and liquid present together), rises again from 0 °C (liquid warming). The simulation shows particles in a lattice vibrating harder until they break free and begin sliding past one another during the flat section.

Real-world analogy

Imagine a class of students holding hands in rows. As the music gets louder (heating), they dance more energetically but keep holding hands. At a certain point (the melting point) they start letting go; while they are letting go, the music's extra energy goes into breaking handholds, not into dancing faster. Only when everyone has let go does the dancing speed up again.

Real-world example

Electrical fuses and some fire sprinkler systems use metals or alloys with carefully chosen low melting points. In a sprinkler, a small plug or glass bulb gives way when the room temperature rises to a set value (often around 68 °C), releasing the water automatically when a fire starts.

Why?

Why does the temperature stay constant during melting? Temperature measures the average kinetic energy of the particles. During melting, the supplied energy is used to overcome the forces holding particles in the solid structure, not to speed them up. Once every particle is free, the added energy again raises their speed, and the temperature climbs.

Common misconception

"Ice at 0 °C and water at 0 °C contain the same energy." They are at the same temperature, but the water contains more energy — the energy that was used to break the particles out of the ice structure. This is why ice at 0 °C cools a drink much more effectively than water at 0 °C.

Worked example

Question: Using the table, state whether each is solid, liquid or gas at 25 °C: (a) gallium (bp 2400 °C), (b) mercury (bp 357 °C), (c) oxygen (bp −183 °C).

Reasoning: (a) 25 °C is below gallium's melting point (30 °C), so solid. (b) 25 °C is between −39 °C and 357 °C, so liquid. (c) 25 °C is above oxygen's boiling point, so gas.

Answer: (a) solid; (b) liquid; (c) gas.

Quick check

1. What happens to the temperature of a pure solid while it is melting? Answer: It stays constant at the melting point until all of the solid has melted.

Exam focus

State that melting point is the temperature at which a solid turns into a liquid, and that for pure substances it equals the freezing point. Explain plateaus on heating curves by saying the energy is used to overcome forces between particles rather than to increase their kinetic energy. Use melting and boiling points to predict states.

Advanced insight

The energy needed to melt a substance without changing its temperature is its enthalpy of fusion (for ice, about 6.0 kJ per mole or 334 J per gram). Pressure also affects melting points, though only slightly for most substances; for ice, very high pressure lowers the melting point a little.

Summary

The melting point is the temperature at which a solid becomes a liquid; for a pure substance it equals the freezing point. During melting the temperature stays constant because energy is used to overcome attractions between particles. High melting points indicate strong forces between particles. Comparing a temperature with melting and boiling points lets you predict a substance's state.

Practice questions

1. Tin melts at 232 °C. Is it solid or liquid at 200 °C? Answer: Solid. 2. Why does a heating curve for a pure solid have a flat section? Answer: During melting, the energy supplied is used to overcome the forces between particles, so the temperature does not rise. 3. What is the freezing point of pure water? Answer: 0 °C (the same as its melting point). 4. Which is held together by stronger forces between particles: iron or oxygen? Use melting points to explain. Answer: Iron, because its melting point (1538 °C) is far higher than oxygen's (−218 °C), so much more energy is needed to separate its particles.