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Melting Characteristics Guide

PhysicsBeginnerReading time: 3 min

Overview

Ice cream melts, steel melts, and candles melt too. The change of a substance from solid to liquid is called "Melting". However, have you noticed that when Hypo (sodium thiosulfate) melts, the temperature is stuck at 48∘C48^\circ\text{C}, while paraffin wax gets hotter and thinner as it burns? This experiment uses the classic "water bath" method to show you the essential differences between crystalline and amorphous substances during the melting process.

Background

The systematic study of melting phenomena began in the 18th century. In 1761, Scottish chemist Joseph Black first proposed the concept of "Latent Heat". He discovered that although ice continues to absorb heat during melting, its temperature remains constant—this heat is "hidden" to break the crystal structure. In the 19th century, scientists further revealed the regular arrangement of atoms inside crystals through X-ray diffraction, explaining why crystals have a fixed melting point, while the atomic arrangement of amorphous substances like glass and asphalt is disordered, so there is no clear phase change temperature. This discovery not only deepened our understanding of the microscopic structure of matter but also laid the foundation for modern materials science.

Key Concepts

Crystalline

Solids with regular internal particle arrangement and fixed melting points. Common examples include ice, table salt, hypo, and metals.

Amorphous

Solids with disordered internal particle arrangement and no fixed melting points. Common examples include paraffin, glass, rosin, and asphalt.

Melting Point

The specific temperature value that a crystal maintains during the melting process.

Formulas & Derivation

Heat Absorption Relation

Q=mcΔtQ = mc\Delta t
Before and after complete melting, absorbing heat causes the temperature to rise. However, during the melting process, the heat absorbed by the crystal is used to destroy the lattice structure rather than increasing the kinetic energy of molecules, so the temperature does not rise.

Experiment Steps

  1. 1

    Select Experiment Object

    First, select "Hypo". Note the initial state is solid granules around 20∘C20^\circ\text{C}. Using a water bath prevents local overheating.
  2. 2

    Start Heating and Observe Curve

    Click "Start Heating". Carefully observe the trend of the temperature-time curve and the changes in the appearance of the hypo. What happens to the state of the hypo when the temperature rises to a certain value?
  3. 3

    Analyze Curve Characteristics

    Continue to observe the temperature-time curve. Does the curve always rise? If a plateau appears, what state is the hypo in? What are the characteristics of the temperature?
  4. 4

    Compare with Amorphous

    Reset the experiment and switch to "Paraffin". Click heat, and compare how the temperature-time curve of paraffin differs from that of hypo? What is the state change of paraffin during the heating process?

Learning Outcomes

  • Summarize the conditions for crystal melting: reaching the melting point + continuing to absorb heat.
  • Understand the physical fact that crystals maintain a constant temperature while absorbing heat during melting.
  • Learn to describe the "temperature rise and softening" phenomenon during amorphous melting.
  • Master the purpose of the water bath heating method and the details of using thermometers.

Real-world Applications

  • Steelmaking: Utilizing the property that iron is a crystal to control furnace temperature for precise melting.
  • Casting: Utilizing the property that liquid metal cools and solidifies back into crystals.
  • Glass Processing: Utilizing the property that glass is amorphous and has no fixed melting point to stretch and blow it into art pieces while it softens.

Common Misconceptions

Misconception
When hypo reaches 48∘C48^\circ\text{C}, it will immediately melt completely.
Correct
Not necessarily. It also needs to continuously absorb heat from the surroundings. If the ambient temperature is also 48∘C48^\circ\text{C}, the hypo cannot melt.
Misconception
Crystals can only be in a liquid state at the melting point temperature.
Correct
False. At the melting point, a crystal can be solid (just reached melting point), liquid (just completely melted), or a solid-liquid coexistence state.

Further Reading

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