Particulate Nature of Matter · Lesson 2 of 6
Solids and Melting
“Use the particle model to explain why solids keep their shape and how heating can melt them.”
• Relate strong interparticle attraction and close packing to a solid’s fixed shape and volume. • Describe particle vibration in a solid without claiming that particles are completely still. • Explain how heating can allow a solid to become a liquid. • Define melting point and compare the melting points in the chapter table.
What holds a solid together?
A key, wooden block, iron nail, piece of rock salt, stone, and piece of aluminium differ in how easily they can be bent, broken, or dented. Yet each is a solid: it has a definite shape and a definite volume. The shared properties invite a particle-level explanation.
Particles attract one another. The strength of this interparticle attraction depends on the material and on how far apart its particles are. Even a small increase in distance can greatly weaken the attraction. The balance between particle energy and attraction helps determine a substance’s physical state.
The attractive force between the constituent particles of matter. Its strength depends on the substance and on particle separation.
Close packing and small vibrations
In a solid, the particles are packed closely and attract one another strongly. Each remains near a particular position instead of passing freely around its neighbours. A solid therefore resists flowing and normally retains its shape and volume. A particle is not frozen motionless: it vibrates to and fro around its position.
Try comparing the shape and size of the six objects and then attempting to hammer them, as in the solid-object investigation. One may deform more readily than another. That difference does not make it a liquid. The useful shared observation is that these objects retain a solid form rather than flowing to fill a container.
Problem
A metal piece becomes dented when struck. Is it still a solid?
- 1.A hammer can change an object’s shape by applying a large force.
- 2.A dent is different from the material flowing freely to take the shape of any vessel.
- 3.The metal remains a solid even if its previous shape is altered.
“Fixed positions” describes where solid particles stay on average. They can still vibrate, and heating makes their vibrations more vigorous.
How a solid melts
Heating supplies energy to the particles. Their vibrations grow more vigorous. At a sufficiently high temperature, the attraction is no longer able to keep them in their earlier fixed positions. They can move past one another, and the material changes from solid to liquid. The substance can remain the same while its physical state changes.
The minimum temperature at which a solid melts into a liquid at atmospheric pressure.
The melting point is a property of the particular material. A substance whose particles are held together more strongly generally needs a higher temperature to melt. Compare the values below; they are temperatures, not amounts of heat.
| Solid | Melting point at atmospheric pressure |
|---|---|
| Ice | 0 °C |
| Urea | 133 °C |
| Iron | 1538 °C |
Problem
Ice, urea, and iron begin well below their melting points. Which reaches its melting point first as the temperature rises steadily?
- 1.Compare the listed temperatures: 0 °C, 133 °C, and 1538 °C.
- 2.Ice reaches its melting point at the lowest temperature.
- 3.That does not mean every sample melts instantly or at the same rate; melting also takes time and energy.
In most materials, particles in the liquid state are somewhat farther apart than in the solid state. Ice is a useful exception: its particles are farther apart than the particles in liquid water. We will still use the general solid model to compare how strongly particles are held and how freely they move.
Problem
A piece of ice melts and becomes water. Has melting produced a different substance?
- 1.Ice and liquid water are two states of the same substance.
- 2.Heating changes the arrangement and movement of its particles.
- 3.This is a change of state, not evidence that the material has become a new substance.
Quiz
Which combination describes an ordinary solid?
What can particles in a solid normally do?
What does heating a solid first do at the particle level?
At atmospheric pressure, which listed material has the highest melting point?
Which statement about ice and liquid water is correct?
Practice Problems
- Explain why a key retains its shape when placed in differently shaped boxes.
- Draw solid particles and show small vibration arrows without showing them travelling through the whole object.
- Describe, in order, how heating can change a solid into a liquid.
- Use the melting-point table to order ice, urea, and iron from lowest to highest melting point. State what the numbers mean.
- A dented aluminium sheet is still solid. Explain why “fixed shape” does not mean “impossible to reshape.”
- Explain the difference between melting a piece of ice and making a new substance.
Key Takeaways
• Attraction between particles and their separation help determine physical state. • Solid particles stay close and vibrate near positions, giving solids a definite shape and volume. • Heating increases vibration until particles can leave their fixed positions and the solid melts. • Melting point is a temperature measured at atmospheric pressure; it differs among substances. • Ice has a different spacing pattern from most solids: its particles are farther apart than in liquid water.