Physical Change of Solid to Liquid at Melting Point: What Happens?
When you heat an ice cube, it turns into water. The solid has changed into a liquid, but the substance is still H₂O. This is a physical change of solid to liquid at melting point: the material changes state without altering its chemical identity. The molecules stay the same; only their arrangement and energy level shift. For a pure substance, this happens at a single, fixed temperature. That temperature is 0 °C for water at standard pressure, and the temperature stays constant until the entire solid has melted.
What Defines a Physical Change
A physical change alters the form or appearance of a substance but not its chemical composition. Melting, freezing, boiling, and dissolving are all physical changes. The key distinction from a chemical change is that no new chemical bonds form or break. When ice melts, the water molecules remain H₂O. When salt dissolves in water, the sodium and chloride ions separate but they are still the same ions. You can recover the salt by letting the water evaporate Wikipedia.
Many physical changes are reversible by simple physical means. You can freeze liquid water back into ice. You can boil water into steam and then condense it back. This reversibility is a useful clue, though not a strict rule. Some physical changes, like hammering a piece of metal into a new shape, are not easily undone.
What Happens at the Melting Point
The melting point is the temperature at which a solid and its liquid form can coexist in equilibrium. At this temperature, the thermal energy of the molecules is high enough to overcome the forces holding them in the crystal lattice.
Consider a simple example: heating a piece of ice at -10 °C. As you add heat, the temperature of the ice rises until it reaches 0 °C. Then the temperature stops rising, even though you keep adding heat. All the added energy goes into breaking the hydrogen bonds that hold the water molecules in the ice crystal structure. This energy is called the latent heat of fusion. Only after every last bit of ice has melted does the temperature of the water start to rise again.
The same principle applies to any pure crystalline solid. For example, pure lead melts at 327.5 °C, and pure gold at 1064 °C National Institute of Standards and Technology. If you heat a block of lead, its temperature climbs until it hits 327.5 °C, then stays there until the entire block has melted.
Impurities Affect the Melting Point
Pure substances have sharp melting points. Impurities lower the melting point and cause it to occur over a range of temperatures. This is why salt is spread on icy roads. The salt dissolves in the thin layer of water on the ice, forming a mixture that freezes at a lower temperature than pure water. The same principle explains why a mixture of metals (an alloy) often melts at a lower temperature than any of its pure components, which is useful in applications like soldering.
The Role of Energy in Melting
The energy required to melt a solid is not small. For water, the latent heat of fusion is about 334 joules per gram. That means melting a single kilogram of ice at 0 °C into water at 0 °C takes 334,000 joules of energy. That is roughly the same amount needed to heat that same kilogram of water from 0 °C to 80 °C. This large energy requirement explains why ice stays cold for a long time in a drink. It absorbs heat from the liquid around it without warming up until it has fully melted.
Different substances have very different latent heats. For example, lead requires only about 23 joules per gram to melt, while aluminum needs about 400 joules per gram. These values reflect the strength of the forces holding the atoms or molecules together in the solid state.
How Pressure Affects the Melting Point
Pressure also influences the melting point, though the effect is usually small for most solids. For most substances, increasing pressure raises the melting point because it forces the molecules closer together, making it harder for the crystal lattice to break apart. Water is again the exception. Because ice is less dense than liquid water, applying pressure actually lowers its melting point. This is why ice skates glide so easily. The pressure of the blade on the ice creates a thin layer of liquid water that acts as a lubricant Royal Society of Chemistry.
For materials like rocks and minerals deep inside the Earth, pressure has a major effect. The melting point of mantle rock increases significantly with depth, which helps keep the interior of the planet solid despite extremely high temperatures.
Key Takeaways
- Melting is a physical change because the chemical composition of the substance remains unchanged. Only the physical state changes from solid to liquid.
- The melting point is the temperature at which a pure solid turns into a liquid. The temperature stays constant during the entire melting process because the added heat goes into breaking the crystal lattice (latent heat of fusion).
- Impurities lower and broaden the melting point, which is why salt is used to melt ice on roads.
- Physical changes like melting are generally reversible. You can freeze the liquid back into a solid.
