Melting and freezing are physical changes that describe how matter moves between the solid and liquid states. These changes happen when thermal energy is added or removed from a substance. Water is the most familiar example, since ice melts into liquid water when warmed and liquid water freezes into ice when cooled.
Understanding these changes helps explain weather, cooking, refrigeration, and many natural cycles on Earth.
At the particle level, melting happens when particles gain enough energy to move past one another instead of staying locked in place. Freezing happens when particles lose energy and slow down until they form a more fixed arrangement. For pure water at normal atmospheric pressure, both changes occur at 0 degrees C.
During melting or freezing, the temperature stays constant until the phase change is complete because energy is used to change state rather than raise or lower temperature.
Understanding Melting and Freezing
A solid is held together by attractions between its particles. In many solids, the particles vibrate around fixed positions. Heating makes those vibrations stronger.
At the melting point, added energy is mainly used to weaken enough attractions for particles to rearrange and flow. This energy increases the particles' potential energy, not mainly their speed. Temperature measures average particle motion, so it does not show this hidden energy change well.
The energy needed for a complete change depends on the mass of the sample. It equals the mass multiplied by the latent heat of fusion. Different substances have different latent heats because their particle attractions have different strengths.
A heating or cooling graph makes this process easier to see. Before melting, a graph of temperature against energy has an upward slope. The substance is still one phase, so added energy raises particle motion.
During the change, the graph has a flat section called a plateau. Its length shows how much energy is required to change the whole sample. After all the solid has become liquid, the slope rises again.
Cooling graphs show the reverse pattern. Learning to identify the sloped sections and flat sections is important in chemistry questions.
A flat graph does not mean that no energy is moving. It means energy is changing the arrangement of particles.
Real materials do not always behave like perfectly pure laboratory samples. A pure substance usually changes state at one well-defined temperature under fixed pressure. A mixture often changes over a temperature range instead.
Dissolved salt lowers the freezing temperature of water. Road salt works because salty water can remain liquid below the temperature where pure water would form ice. Impurities can make a melting point less sharp, which helps scientists check whether a sample is pure.
Pressure can matter too. Most substances need higher temperatures to melt under greater pressure. Water is unusual because solid ice has a more open structure than liquid water, so pressure can slightly favor the liquid state.
Freezing can begin only when particles form a small stable cluster in the solid arrangement. This starting process is called nucleation. Dust, scratches, and the walls of a container can provide places where crystals begin.
Very clean water can sometimes cool below its usual freezing temperature without forming ice. It is then supercooled and may freeze quickly after a bump or a seed crystal is added. When water freezes, it releases energy to its surroundings.
This matters in weather, because freezing droplets can affect the temperature of clouds. It matters at home too. Water expands as it forms ice, which can crack pipes and containers.
When studying phase changes, separate the ideas of temperature, energy transfer, particle motion, and particle attractions. They are connected, but they are not the same thing.
Key Facts
- Melting is the change from solid to liquid.
- Freezing is the change from liquid to solid.
- For pure water at atm, melting point = freezing point = degrees C.
- for melting or freezing, where is heat, is mass, and is latent heat of fusion.
- If heat is added, particles gain kinetic energy and melting becomes more likely.
- During a phase change, temperature remains constant until all of the substance has changed state.
Vocabulary
- phase change
- A phase change is a change of state, such as from solid to liquid or liquid to solid.
- melting point
- The melting point is the temperature at which a solid changes into a liquid.
- freezing point
- The freezing point is the temperature at which a liquid changes into a solid.
- thermal energy
- Thermal energy is the energy associated with the motion of particles in a substance.
- latent heat of fusion
- Latent heat of fusion is the energy needed per unit mass to melt a solid or released per unit mass when a liquid freezes.
Common Mistakes to Avoid
- Thinking melting and dissolving are the same thing, which is wrong because melting is a change of state while dissolving involves mixing one substance into another.
- Assuming temperature always changes when heat is added or removed, which is wrong because during melting or freezing the energy goes into changing the state instead.
- Believing water freezes at any cold temperature, which is wrong because pure water at normal pressure freezes specifically at 0 degrees C.
- Confusing particle size with particle motion, which is wrong because particles do not get bigger or smaller during melting or freezing, they only change how they move and arrange themselves.
Practice Questions
- 1 A block of ice melts completely. If the latent heat of fusion of water is , how much heat is required?
- 2 How much heat is released when of liquid water at freezes? Use .
- 3 A cup of ice water stays at 0 degrees C while some of the ice is still present. Explain why the temperature does not rise even though heat is entering the cup.