Melting

Introduction

An ice lolly drips on a hot day, butter softens in a warm pan and a chocolate bar turns sticky in a pocket. All three are examples of melting : a solid turning into a liquid. Melting happens when particles in a solid gain enough energy to break out of their fixed positions. This page describes what happens to particles as a solid melts and why energy must be supplied.

Core explanation

Particles in a solid. In a solid, particles are packed closely in a regular arrangement. Strong forces of attraction hold each particle in a fixed position. The particles are not still — they vibrate about these positions — but they cannot move from place to place. That is why solids have a fixed shape.

Heating makes the vibrations bigger. When a solid is heated, energy is transferred to its particles. They vibrate faster and with larger movements. The temperature of the solid rises. The solid also expands slightly, because the more vigorous vibrations push the particles a little further apart.

Breaking free. Eventually the particles vibrate so strongly that they can partly overcome the forces holding them in place. The regular arrangement breaks down. Particles are still close together and still attract each other, but they can now slide past one another. The solid has become a liquid.

What changes and what does not.

- Arrangement changes from regular to random. - Movement changes from vibrating on the spot to sliding past each other. - Spacing changes only slightly; particles remain close together. - The particles themselves do not change.

Melting takes in energy. Energy is needed to overcome the forces of attraction that held particles in fixed positions. Melting is therefore an endothermic change: it takes in energy from the surroundings. This is why melting ice cools a drink — the ice takes energy from the liquid around it.

Melting happens at a particular temperature. A pure solid melts at a fixed temperature called its melting point. For ice this is 0 °C; for iron it is about 1538 °C. While a pure solid is melting, its temperature stays constant until all of it has melted, because the energy supplied is used to free particles rather than to speed them up.

Stronger forces, more energy. Substances whose particles attract each other strongly need more energy to melt, so they melt at higher temperatures.

Step-by-step reasoning

To describe melting using the particle model:

1. Start with particles vibrating in fixed positions in a regular pattern. 2. Heat is supplied, so the particles gain energy and vibrate more. 3. At the melting point, particles overcome enough of the attractions to leave fixed positions. 4. The particles can now slide past each other: the substance is a liquid.

Visual explanation

Draw a grid of circles in neat rows with short wiggle marks around each to show vibration. Next to it, draw the same number of circles still touching but jumbled, with curved arrows showing sliding. An arrow labelled "heat in — melting" connects the two pictures. SIM-STATE-001 shows the lattice shaking harder until it collapses into a liquid.

Real-world analogy

Picture a school assembly where pupils stand in straight rows, shuffling on the spot. As they get more restless, they fidget more and more until the rows break up and pupils start weaving between each other while still staying close. The orderly rows have "melted" into a moving crowd.

Real-world example

Road engineers spread salt on icy roads, and ice rinks use chillers to stop ice from melting. In metal casting, metals such as aluminium (melting point about 660 °C) are melted in furnaces so they can be poured into moulds. The shape changes, but the metal is the same substance throughout.

Why?

Why must energy be supplied to melt a solid? The particles in a solid are held by forces of attraction. To let them move past one another, these attractions must be partly overcome, and that requires energy. Without a supply of energy, the particles stay locked in position.

Common misconception

"Melting and dissolving are the same thing." Melting involves only one substance changing from solid to liquid because of heating. Dissolving involves a solid spreading out among the particles of a solvent, such as sugar in water, and can happen without heating.

Worked example

Question: A block of ice at −10 °C is heated steadily. Describe what happens to the particles from −10 °C until the ice has completely melted.

Reasoning: From −10 °C to 0 °C the water molecules gain energy and vibrate faster, so the temperature rises. At 0 °C the energy supplied is used to overcome the attractions holding molecules in fixed positions, so the temperature stays at 0 °C while the regular pattern breaks down.

Answer: The molecules vibrate more and more strongly until 0 °C; then, at a constant 0 °C, they break free from fixed positions and begin to slide past each other, forming liquid water.

Quick check

1. Is melting an endothermic or exothermic change? Answer: Endothermic, because it takes in energy from the surroundings.

Exam focus

Use precise language: particles "gain energy", "vibrate more", "overcome forces of attraction" and "move from fixed positions". Do not say particles "melt" or "get bigger". Remember that the particles in the liquid are still close together.

Advanced insight

Some solids, such as glass, wax and chocolate, do not melt at one sharp temperature. They soften gradually over a range because their particles are not arranged in a perfectly regular crystal. Chocolate makers carefully control the crystal form of cocoa butter so that it melts at about body temperature — in the mouth, but not in the hand.

Summary

Melting is the change from solid to liquid. Heating makes the particles in a solid vibrate more strongly until they overcome enough of the forces of attraction to leave their fixed positions and slide past each other. Melting takes in energy and, for a pure substance, happens at a fixed temperature called the melting point.

Practice questions

1. Describe the arrangement and movement of particles before and after melting. Answer: Before: regular arrangement, particles vibrating about fixed positions. After: random arrangement, particles close together but sliding past each other. 2. Explain why ice cubes cool a drink. Answer: Melting takes in energy; the ice takes this energy from the drink, so the drink's temperature falls. 3. Why does a solid expand slightly as it is heated before melting? Answer: The particles vibrate more strongly, taking up a little more space, so they move slightly further apart. 4. Suggest why iron melts at a much higher temperature than ice. Answer: The forces of attraction between iron particles are much stronger, so more energy is needed to overcome them.