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# What is a Melting Point?

The matter is characterised by different states. There can be a transition from one state of matter to another. For instance, a solid can be transformed into a liquid and a liquid can be changed into a gas. For example, most of the solid metals, transform into liquid on heating and turn into vapour on further heating.

The change in matter state, for instance, from solid to liquid state occurs because of the following two characteristic factors :

• Temperature – There is a transition from solid to liquid state with the change, that is, an increase in temperature. On raising the temperature, ice changes into water. Also, liquid changes to the vapour state. Ice → Water → Vapour
• Pressure – Solids are characterised by closely packed particles, with no spaces between them. They don’t allow any compression, and therefore, there is no effect of pressure on solids. On constantly applying the pressure, the solid gets deformed and finally breaks.

### Conversion from solid to liquid

The process of a solid turning into a liquid state is called melting. It is also known as fusion. The reverse process of a liquid becoming a solid, is in turn called solidification

The temperature at which melting occurs for any pure substance is termed the melting point of that substance. It is considered to be a characteristic of that substance. Energy is required by the system to carry out the melting of a solid to a liquid. It is an endothermic reaction. Every pure substance requires a certain amount of energy which is required to change from a solid to a liquid state.

Energy is exclusively required to carry out the transition of the substance, from the solid to the liquid substance. The phase of a substance is changed, and the temperature remains unchanged. Therefore, melting is known to be an isothermal process, since the substance remains at the same unmodified temperature.

### Melting

Melting or fusion is the process by which a solid substance changes into a liquid on heating. On heating a substance, the particles vibrate with greater intensity and move more vigorously. Heat energy is produced. The particles gain sufficient kinetic energy to overcome the strong forces of attraction. They gain energy to break through to form small groups of particles. Solids, therefore, melt to form a liquid state.

Melting of a substance occurs at defined ambient pressure conditions. The energy supplied to melt a system is known as enthalpy of fusion or entropy of fusion. However, some of the substances pose an exception to this process, and heat has to be removed from these systems in order to melt them. e.g. Helium.

### What is Melting Point?

The temperature at which a solid substance melts and transforms into a liquid at atmospheric pressure is termed the melting point of a substance.

Different solids display varying melting points. For example, the melting point of wax is 63 °C. The greater the melting point of the substances, the more will be the intraparticle forces of attractions binding the particles. For instance, in the solid form of water, ice melts at a temperature of 0 °C to form liquid water. Therefore, the melting point of ice is 0 °C at atmospheric pressure.

At the melting point, the solid and liquid states of matter coexist together. The melting point of a substance is its characteristic property influenced by the applied pressure.

### Applications of Melting Point

The application can be observed during the process of snow skating. The weight of the skater gets concentrated on a thin line, there is considerable pressure applied on the ice slab. This results in the melting of ice beneath the shoe of the skater. This melting ice lubricates the undersides of his skating shoes. This makes it possible to skate on the ice.

### Heating Curve for the Conversion of Solid to Liquid

The heating curve denotes the temperature on the y-axis and the heat that has been supplied on the x-axis respectively. Let us assume a constant rate of heating, owing to which the x-axis can be shown as the amount of time which is shown as the substance is being heated. The curve majorly shows two main points:

• The portion of the graphs, where the temperature rises as and when heat is supplied to the system, and
• The portion of the graphs plateaus where the temperature remains at a constant level. Change in phase occurs at this plateaus.

### Sample Questions

Question 1: What causes phase change?

When the temperature or pressure change of a system occurs, the phase changes. Upon increasing the temperature or pressure of the system, the intramolecular forces of attraction increase. Upon decreasing the temperature, the molecules or atoms turn into a rigid structure.

Question 2: Does temperature change during the process of heating ice?

As we constantly supply heat energy to transform ice into water, the temperature of the system doesn’t change. The heat energy is absorbed by the ice changing slowing into the liquid state. Both the states of matter coexist together during this process.

Question 3: How does melting occur in the case of amorphous solids?

The non-crystalline solids, such as glass or pitch undergo melting by slowly decreasing viscosity with the simultaneous increase in the temperature. However, there is no sharp transformation from solid to liquid state.

Question 4: Define heat of fusion.

The amount of energy required by a pure substance to change from the solid to a liquid state is termed as the enthalpy of fusion (or heat of fusion) of the substance. It is denoted by ΔHfus.

Question 5: Differentiate between boiling or melting.

Boiling is the process of melting of liquid to gas state whereas melting is the transformation of solid to liquid state.

Question 6: How does energy change occur during melting?