Melting
Leave an ice cube on a table on a warm day, and within minutes it is a puddle of water. Leave a bar of iron in a furnace hot enough, and it too turns to liquid — just at a temperature more than a thousand degrees higher. Both changes are the same process, melting, and the huge gap between 0 degrees Celsius and 1538 degrees Celsius is not an accident. It is a direct clue about how tightly the particles of ice and the particles of iron are held together. This article builds melting from the particle picture of matter you already know: particles in a solid are locked in fixed positions by strong interparticle attraction and can only vibrate. We define melting and melting point precisely, walk through why heating a solid produces a flat temperature plateau at the melting point rather than a smooth rise, and use two worked examples — the ice-on-a-stove graph and the iron ingot — to fix the reasoning so it is exam-ready, not just familiar.
Melting is the solid-to-liquid change that begins at a substance's melting point, when particles gain just enough thermal energy to break free of their fixed vibrating positions and start sliding past one another, and the strength of interparticle attraction is exactly why that temperature differs so wildly from substance to substance.
The core idea
Melting is the change of a solid into a liquid. The melting point of a substance is the minimum temperature at which that solid turns into a liquid, at atmospheric pressure. It is a fixed, measured property of each substance — not something calculated from a formula.
Recall the particle picture of a solid. Particles in a solid are packed close together with very little interparticle space, and they are held in fixed positions by strong interparticle attraction. Because the attraction is so strong, each particle cannot wander off — it can only vibrate about the spot it is locked into. This is why a solid keeps a fixed shape and a fixed volume: its particles are not free to move around.
What heating does to those particles. Supplying heat to a solid gives its particles thermal energy, and thermal energy shows up as motion — the particles vibrate faster and through a wider swing as the temperature rises. Nothing about their fixed positions changes yet; they are just shaking more violently in place.
What happens at the melting point. At the melting point, particles have absorbed enough thermal energy to partly overcome the strong interparticle attraction that was locking them in place. Once that attraction is partly overcome, particles can slip out of their fixed positions and slide past their neighbours, while still staying relatively close together. That looser, sliding arrangement — free to move within a limited space, no longer vibrating around one fixed spot — is precisely the particle behaviour that defines the liquid state. Melting point is therefore the temperature marking a change in particle behaviour, not merely a number on a thermometer. The textbook gives three specific melting-point values as reference facts, and it is worth noticing how far apart they sit:
| Substance | Melting point |
|---|---|
| Ice | 0 degrees C |
| Urea | 133 degrees C |
| Iron | 1538 degrees C |
Why it works
The observed jump in melting point from ice at 0 degrees C to iron at 1538 degrees C is a direct, explainable consequence of interparticle attraction — not a coincidence to memorise — and the reasoning below shows exactly which particle-level property is responsible and why more of that property demands more heat before melting can begin.
Step 1. Every solid's particles are held in their fixed vibrating positions by interparticle attraction, but the strength of that attraction is not the same for every substance — it depends on what the particles are and how they are bonded.
Step 2. Melting requires enough thermal energy to partly overcome interparticle attraction. Weak interparticle attraction, as in ice, is overcome with only a small amount of thermal energy, so a low temperature (0 degrees C) is enough to free the particles into the liquid's sliding arrangement.
Step 3. Strong interparticle attraction, as in iron, resists being overcome. Far more thermal energy has to be pumped into iron's particles before that strong attraction is partly broken down, which is why the furnace must reach a temperature as high as 1538 degrees C before iron starts turning liquid.
Step 4. Because melting point tracks the strength of interparticle attraction rather than any single external condition, substances with very different bonding show very different melting points — this is why the same physical event (melting) can happen at 0 degrees C for one substance and over 1500 degrees C for another, and why melting point is treated as a distinguishing property of a substance.
Worked example
Question: A small ingot of pure iron is placed in a furnace and heated. At what approximate temperature would you expect the iron to start turning into a liquid, and why is this temperature so much higher than the melting point of ice? Also explain, using the same reasoning, why a beaker of crushed ice being heated on a stove shows its thermometer reading climb steadily from minus 5 degrees C to 0 degrees C and then stay fixed at 0 degrees C for several minutes even though the flame keeps burning.
Step 1. From the melting-point table, iron's melting point is 1538 degrees C, so the iron ingot is expected to start melting only once the furnace reaches around this temperature.
Step 2. Melting point depends on the nature of the substance, specifically on how strong the interparticle attraction between its particles is. Iron's particles are held together by much stronger interparticle attraction than water's particles, whose melting point (as ice) is only 0 degrees C.
Step 3. Because iron's particles need far more thermal energy to overcome this much stronger attraction and break free of their fixed vibrating positions, iron requires a far higher temperature to melt than ice does.
Step 4. The same idea explains the stove graph. Below 0 degrees C, all the heat from the flame raises the temperature of solid ice by making its particles vibrate more vigorously about their fixed positions. At 0 degrees C, the ice's particles have gained just enough thermal energy to start overcoming ice's (comparatively weak) interparticle attraction. While the reading stays flat at 0 degrees C, the heat supplied is being used entirely to break particles free from their fixed positions rather than to raise temperature further — that is exactly why the thermometer does not move during melting. Once all the ice has become water, further heating raises the liquid's temperature again.
Answer: iron begins melting at about 1538 degrees C, roughly 1538 degrees higher than ice's 0 degrees C, because iron's interparticle attraction is far stronger; the flat 0 degrees C plateau on the ice graph is the same physics — heat consumed in overcoming attraction, not in raising temperature.
Common traps
- Confusing melting with evaporation or boiling — melting is specifically the solid-to-liquid change, not a liquid-to-gas change.
- Believing temperature keeps rising throughout the heating of a solid — at the melting point, temperature stays constant while the solid turns to liquid, because the heat supplied goes into overcoming interparticle attraction, not into raising temperature.
- Thinking melting point is calculated from a formula — at this level it is a measured, given property of each substance, like the values for ice, urea, and iron, not something derived mathematically.
- Assuming all solids melt at low temperatures like ice — melting point varies enormously between substances, from 0 degrees C for ice to 1538 degrees C for iron, because interparticle attraction strength varies enormously too.
Ask the tutor: curious why the thermometer refuses to budge while ice is melting on the stove even with the flame still on — ask your AI tutor to walk through the heating-and-melting graph step by step.
If heat is still being supplied while ice melts at 0 degrees C, where does that heat go?
It goes into overcoming the interparticle attraction holding the ice particles in their fixed vibrating positions, not into raising the temperature. Before 0 degrees C, heat makes particles vibrate faster, which is what a rising temperature means. At 0 degrees C, particles have enough energy to start breaking free of those fixed positions, and every bit of extra heat during the plateau is spent finishing that break-free process across the whole sample. Only once every particle has escaped its fixed position — the ice is fully water — does further heat go back to raising temperature, now of the liquid.
Why do different substances have such different melting points?
Melting point tracks the strength of interparticle attraction inside that substance. Weakly attracted particles, like those in ice, need only a small amount of thermal energy to be pulled free of their fixed positions, so ice melts at a low temperature, 0 degrees C. Strongly attracted particles, like those in iron, resist being pulled free and need a huge amount of thermal energy first, so iron only melts at 1538 degrees C. The melting point is essentially a report of how hard interparticle attraction fights back against particles trying to move.
Is melting point something I calculate, like speed or area?
No — at this level melting point is a measured property, looked up or found experimentally for each substance, not calculated from a formula. The chapter gives you specific values (0 degrees C for ice, 133 degrees C for urea, 1538 degrees C for iron) as facts to know, along with the particle-level reasoning for why they differ. You are expected to explain melting using interparticle attraction and thermal energy, not to plug numbers into an equation.