Melting Point

Introduction

Ice always melts at 0 °C, whether it is a single cube or an iceberg. Pure gold always melts at 1064 °C. Every pure solid has its own melting point , a fixed temperature at which it changes from solid to liquid. Because it is so reliable, the melting point is one of the most useful properties chemists use to identify substances and check their purity.

Core explanation

Definition. The melting point of a substance is the temperature at which the pure solid turns into a liquid. At this temperature, solid and liquid can exist together.

Melting point equals freezing point. A pure liquid freezes at exactly the same temperature at which the solid melts. Water freezes at 0 °C and ice melts at 0 °C. The two names describe the same temperature approached from opposite directions.

Some melting points (at normal pressure):

Substance Melting point (°C) --- --- Oxygen −218 Ethanol −114 Mercury −39 Water (ice) 0 Sodium chloride 801 Copper 1085 Iron 1538 Tungsten 3422

Why the temperature is fixed. In a pure solid, every particle is the same and every particle is held in the same kind of position by the same forces. So all the particles need the same amount of energy to break free. They start to break free at one particular temperature. While melting continues, the energy supplied goes into freeing particles, not into making them move faster, so the temperature stays at the melting point until all the solid has melted.

Melting point and forces of attraction. The stronger the forces holding particles together, the more energy is needed to free them, and the higher the melting point. Oxygen molecules attract each other very weakly, so oxygen melts at −218 °C. The particles in sodium chloride and in metals attract each other very strongly, so these substances melt at hundreds or thousands of degrees.

Melting point as a characteristic property. Since every sample of a pure substance melts at the same temperature, measuring a melting point can help identify an unknown solid. Impurities change the result: an impure solid usually melts at a lower temperature and over a range of temperatures rather than sharply.

Step-by-step reasoning

To use melting point data:

1. Find the melting point of the substance in a data table. 2. Compare it with the temperature you are interested in. 3. Below the melting point, the substance is solid; above it (but below the boiling point), it is liquid. 4. A higher melting point suggests stronger forces of attraction between particles.

Visual explanation

Draw a thermometer scale from −250 °C to 3500 °C. Mark each substance from the table at its melting point. Weak-attraction substances such as oxygen cluster at the cold end; strong-attraction substances such as metals and salts sit high up. The scale shows how widely melting points vary.

Real-world analogy

A melting point is like the pass mark for an exam that every particle must reach. In a pure substance, every particle faces the same test, so they all pass at the same mark. In a mixture, some particles face an easier test, so the "passing" happens over a spread of marks.

Real-world example

Tungsten was chosen for the filaments in old-style light bulbs because its melting point, about 3422 °C, is the highest of any metal. The filament could glow white hot at around 2500 °C without melting. A copper or iron filament would have melted almost at once.

Why?

Why do pure substances melt sharply while mixtures melt over a range? In a pure substance, identical particles are held by identical forces, so they all break free at the same temperature. In a mixture, different particles disrupt the regular pattern, so some are freed more easily than others and melting is spread out.

Common misconception

"The melting point is the temperature at which the solid starts to get soft." For a pure crystalline solid, the melting point is the fixed temperature at which solid and liquid exist together and the solid turns to liquid. Gradual softening is typical of impure substances and non-crystalline materials such as wax.

Worked example

Question: A white solid melts sharply at 801 °C. Use the table to identify it, and explain what a sharp melting point tells you.

Reasoning: In the table, sodium chloride melts at 801 °C. A sharp melting point at one temperature indicates the sample is pure, because an impure sample would melt lower and over a range.

Answer: Sodium chloride; the sharp melting point shows it is pure.

Quick check

1. At what temperature does pure water freeze? Answer: 0 °C, the same as the melting point of ice.

Exam focus

Learn the definition and remember that melting point and freezing point are the same temperature. Be ready to read data tables, compare melting points and link a high melting point to strong forces of attraction. Know that impurities lower the melting point and cause melting over a range.

Advanced insight

Melting points depend slightly on pressure. For most substances, higher pressure raises the melting point, but ice is unusual: because ice is less dense than liquid water, pressure lowers its melting point very slightly. The effect is small — enormous pressures are needed to shift it by a few degrees.

Summary

The melting point is the fixed temperature at which a pure solid turns into a liquid; it is the same as the freezing point. The temperature stays constant during melting because energy goes into freeing particles. Stronger forces between particles mean higher melting points. Pure substances melt sharply; impure ones melt lower and over a range.

Practice questions

1. Define melting point. Answer: The temperature at which a pure solid turns into a liquid. 2. Mercury melts at −39 °C. What state is mercury in at room temperature (20 °C)? Explain. Answer: Liquid, because 20 °C is above its melting point (and below its boiling point). 3. Explain why copper has a much higher melting point than oxygen. Answer: The forces of attraction between copper particles are much stronger than those between oxygen molecules, so much more energy is needed to free them. 4. A sample melts between 118 °C and 124 °C. What does this suggest? Answer: The sample is impure, because it melts over a range rather than at one sharp temperature.