Which of the Following Describes the Process of Melting? A Clear Guide
If you are faced with a multiple-choice question that asks “which of the following describes the process of melting?”, the direct answer is that melting is the physical change of a solid into a liquid when it absorbs enough heat energy. The solid’s internal structure breaks down as its particles gain energy and move faster, allowing them to slide past one another.
What Happens During Melting
Melting is a phase transition where a solid becomes a liquid. For this to happen, the solid must absorb thermal energy. In a solid, atoms or molecules are locked into a fixed, ordered arrangement (a crystal lattice). They vibrate in place but cannot move freely.
When heat is added, those vibrations become more intense. At a specific temperature, the vibrations overcome the forces holding the solid together. The lattice breaks apart, and the particles become free to move past each other. The substance is now a liquid.
A key detail is that the temperature of the substance stays constant during the entire melting process, even though heat is still being added. That heat energy goes entirely into breaking the bonds between particles, not into raising the temperature. This constant temperature is called the melting point.
The Melting Point
Every pure substance has a characteristic melting point at a given pressure. For example, the melting point of ice (solid water) is 32°F (0°C) at standard atmospheric pressure Britannica. Pure iron melts at about 2,800°F (1,538°C). The melting point of gold is around 1,948°F (1,064°C).
If the pressure changes, the melting point can shift slightly. For most substances, increasing pressure raises the melting point. Water is a notable exception: increasing pressure lowers its melting point, which is why ice skates can glide on a thin layer of liquid water.
How Melting Differs from Other Phase Changes
Phase changes can be confusing because many of them involve heat but produce different results. Here is a quick comparison.
| Process | Starting state | Ending state | Energy change |
|---|---|---|---|
| Melting | Solid | Liquid | Absorbs heat (endothermic) |
| Freezing | Liquid | Solid | Releases heat (exothermic) |
| Boiling | Liquid | Gas | Absorbs heat (endothermic) |
| Condensation | Gas | Liquid | Releases heat (exothermic) |
| Sublimation | Solid | Gas | Absorbs heat (endothermic) |
| Deposition | Gas | Solid | Releases heat (exothermic) |
Melting is endothermic — it requires an input of energy. Freezing is the reverse: it is exothermic and releases heat. When water freezes into ice, it gives off the same amount of heat that was needed to melt it.
A Simple Worked Example
Imagine you have an ice cube at 20°F (-7°C). You place it in a room at 70°F (21°C). The ice cube absorbs heat from the air. Its temperature rises until it reaches 32°F (0°C). At that point, the temperature stops rising. The ice cube begins to melt, and the added heat goes into breaking the crystal structure. Once all the ice has become liquid water, the temperature of the water will start to rise again.
This example shows that melting is not instant. It requires a certain amount of energy, known as the latent heat of fusion. For water, that value is about 334 joules per gram. To melt one gram of ice at 0°C into water at 0°C, you must supply 334 joules of energy.
Common Misconceptions About Melting
Several ideas about melting are incorrect but widely repeated.
Misconception: Melting and dissolving are the same. Melting is a phase change of a pure substance. Dissolving is a physical change where a substance (the solute) mixes with another substance (the solvent) to form a solution. For example, sugar dissolving in water is not melting. The sugar remains solid on a molecular level, but its crystals break apart into individual molecules that spread through the water.
Misconception: Melting always happens at a specific temperature. This is true for pure crystalline solids like ice or metals. But many materials, such as glass, wax, or plastic, do not have a sharp melting point. They soften over a range of temperatures. These are called amorphous solids. They gradually become less rigid until they flow like a liquid.
Misconception: Heat is the only way to cause melting. Pressure can also cause melting. As mentioned earlier, increasing pressure on ice lowers its melting point. This is why a heavy object like a glacier can melt the ice beneath it, allowing the glacier to slide.
Key Takeaways
- Melting is the phase change from solid to liquid caused by absorbing heat energy.
- The temperature remains constant at the melting point until all the solid has melted.
- Melting is endothermic (absorbs heat), while freezing is exothermic (releases heat).
- Pure crystalline substances have a sharp melting point; amorphous solids soften over a range.
- Melting is not the same as dissolving, and pressure can also trigger melting.
