

How Pressure Affects the Melting Point of Substances
Matter generally occurs in three states around us – solid, liquid and gas. The change of state from solid to liquid is known as melting. Similarly, when a liquid turns into a solid state, it is freezing.
What is Melting?
Melting refers to the change of state from solid to liquid. The most common example of melting is ice. Keep ice cubes in a small bowl at room temperature. You will notice that this ice slowly turns into water. After a certain amount of time, the ice crystals completely disappear, and only water remains in this bowl.
The change of state from a solid to a liquid occurs due to the presence of hyperactive atoms. These atoms in the ice molecule gain energy and successfully escape from this cube. Such a change results in the frozen water to transform its state and turn into the liquid form.
Quick Question – 1
Q. Where Do Hyperactive Ice Atoms Derive Energy from to Escape the Ice Cube?
Ans. The ice atoms acquire energy in the form of heat when the ice cube is kept at room temperature. The atoms use this energy to escape from ice, thereby changing to its liquid form.
Temperature Increase During Change from Solid to Liquid
Now that you understand what is melting, you would think that the temperature of ice would increase as it absorbs heat and melts. However, such is not the case. In fact, research indicates that temperature does not increase or decrease when such a change of state is underway.
In fact, the temperature will only increase after the entire ice cube melts into water. Therefore, you can say that equilibrium exists between the liquid and solid states during such a change of state.
True or False
Q. Water’s Temperature Decreases While Transforming Into Ice.
Ans. False. Water’s temperature remains the same while this process of change is underway. It can only increase after all water molecules have changed into ice molecules.
Understanding Melting Point
Melting point is the temperature at which solid and liquid phases of any pure material can exist in equilibrium. Beyond this point, the material experiences the conversion of solid to liquid.
When heat is applied on any solid object, its temperature will increase to its melting point. At this point, the temperature of this material remains constant as long as it is still melting into its liquid form. After this melting is complete, any excess, residual heat will lead to a further increase in temperature for the resulting liquid.
Now that you understand what is melting point, here is a look at how pressure affects melting points.
Pressure and its Effect on Melting Point
In most cases, increased pressure increases the melting point for a material. This means that under high-pressure conditions, you would need to apply more heat to cause a material to melt. Ice is a rare exception to this rule.
When you increase pressure on ice, it lowers the melting point temperature. This happens primarily because ice shrinks when it turns into water. The same trait can also be explained using LeChatelier’s principle.
Example 1
One of the examples of change of state of matter in everyday life is that of an ice skater skating. The pressure from the skates causes the ice to melt. However, it refreezes as soon as the pressure is removed, that is, the skater moves off.
If you want to know more about how to change solid to liquid, attend our live online classes, conducted by expert teachers. Apart from these classes, you can also take advantage of a wide range of study materials. You can even download our Vedantu app for convenient access to online classes.
FAQs on Melting Point Explained: What, Why & How in Physics
1. What is the melting point of a substance as explained in Physics?
In Physics, the melting point is defined as the specific, constant temperature at which a solid substance changes its state to become a liquid at standard atmospheric pressure. At this temperature, the solid and liquid phases of the substance exist in equilibrium. It is a characteristic physical property used to identify pure crystalline substances.
2. On a molecular level, why does a solid begin to melt when it is heated?
When a solid is heated, the supplied thermal energy increases the kinetic energy of its constituent particles (atoms or molecules). This causes them to vibrate more vigorously about their fixed positions in the crystal lattice. At the melting point, the vibrations become so intense that the particles overcome the intermolecular forces of attraction holding them in a rigid structure. They break free from the lattice and begin to move past one another, transitioning the substance into a liquid state.
3. How does a change in pressure impact the melting point of a substance?
The effect of pressure on the melting point depends on how the substance's volume changes upon melting. For most substances that expand on melting (e.g., wax), an increase in pressure raises the melting point. Conversely, for substances that contract on melting, like ice, an increase in pressure lowers the melting point. This is because higher pressure favours the state with the smaller volume.
4. What is the difference between the melting point and the freezing point?
For a pure crystalline substance, the melting point and freezing point occur at the same temperature. The key difference is the direction of the phase transition:
- Melting Point: The temperature at which a solid turns into a liquid.
- Freezing Point: The temperature at which a liquid turns into a solid.
5. Do all solids have a sharp and definite melting point? Explain with examples.
No, not all solids have a sharp melting point. This property distinguishes between two types of solids:
- Crystalline Solids: These have a regular, ordered arrangement of particles and thus a sharp, definite melting point. Examples include ice (0°C), table salt (801°C), and iron (1538°C).
- Amorphous Solids: These lack a regular structure and instead soften gradually over a range of temperatures. Examples include glass, rubber, and plastic.
6. What is the role of latent heat of fusion during the melting process?
During melting, the temperature of a substance remains constant at its melting point even though heat is being continuously supplied. This hidden heat energy, known as the latent heat of fusion, is used to break the bonds of the solid's crystal structure and facilitate the phase change to liquid. It increases the potential energy of the molecules, not their kinetic energy, which is why the temperature does not rise until all the solid has melted.
7. How does adding an impurity, like salt to ice, affect its melting point?
Adding an impurity to a pure crystalline solid typically lowers its melting point. This phenomenon is called freezing point depression. The impurity particles disrupt the orderly formation of the crystal lattice, making it more difficult for the substance to solidify. Consequently, more energy must be removed (i.e., the temperature must drop lower) for the substance to freeze or melt. This is the principle behind using salt to melt ice on roads in winter.
8. Can you give some examples of melting points for common substances?
Yes, here are the approximate melting points for several common substances at standard pressure:
- Water (Ice): 0°C (32°F)
- Table Salt (NaCl): 801°C (1474°F)
- Iron (Fe): 1538°C (2800°F)
- Tungsten (W): 3422°C (6192°F)
- Mercury (Hg): -38.83°C (-37.89°F)
- Gallium (Ga): 29.76°C (85.57°F), which allows it to melt in the human hand.

















