Melting and Boiling Points

What is the Melting Point of a Substance?

The temperature at which water transforms from solid ice to liquid water is known as the melting point. At this temperature, water’s solid and liquid phases are in balance. 

Because the melting point of water varies significantly depending on pressure, one cannot designate a particular temperature as its melting point. However, the melting point of pure water ice at 1 atmosphere of pressure is quite close to 0 °C, which is 32 °F or 273.15 K for practical purposes. 

What is the Definition of the Term “Boiling Point”?

The temperature at which a liquid’s vapour pressure equals the atmospheric pressure of the liquid’s surroundings is known as the boiling point. The liquid turns into a vapour at this temperature.

The boiling point of any substance is the temperature at which it transitions from the liquid to the gaseous state. For water, this occurs at a temperature of 100 oC. The ice/water melting point and the liquid water/vapour boiling point constructed the Celsius scale. The boiling point of each material is different.

Example of Boiling Water 

You can boil water by two methods:

• Increase the temperature
• Lower the air pressure

At sea level, the air pressure causes water to boil at 100 oC. In the absence of air, water may boil at a significantly lower temperature. 

Therefore, if it weren’t for the skin, which keeps the blood compressed, body temperature would be high enough to cause the blood to boil. When air pressure is low, water boils at temperatures much below 100 °C.

Understanding Melting Point Briefly 

The melting point of any pure substance is the temperature at which the solid and liquid phases may exist in equilibrium. Beyond this threshold, the substance undergoes solid-to-liquid conversion.

When you apply heat to it, any solid object’s temperature will rise to its melting point. The substance’s temperature remains constant at this stage as it continues to melt into a liquid state. Any remaining heat from the melting process will cause the temperature of the resultant liquid to rise even more.

What Causes Melting?

• Solids are nothing but crystals bound by strong intermolecular attraction forces. Heat weakens these forces binding the crystal lattice.
• As per the well-known melting hypothesis, when increased pressure or heat raises a material’s temperature, it induces rapid movements in its molecules. When the distance covered by this back and forth movement (amplitude of vibration) becomes more than the substance’s interatomic distance, it causes vibrational instability and leads to melting.
• In simple words, the diminishing of the intermolecular forces weakens the lattice structure leading the molecules to break away from each other in opposite directions. The instability of the crystal lattice causes the melting of a solid material.

The Effect of Pressure on Melting Point

In most circumstances, increasing pressure raises a material’s melting point. This fact indicates that you would have to apply more heat to melt a material under high-pressure circumstances.  The only exception to this rule is ice.

The melting point temperature drops when you put more pressure on the ice because it shrinks as it melts into water. We can also use LeChatelier’s principle to describe the same feature.

Boiling Point Elevation and Impurities

We know that pure water boils at 100 °C at 1 atmosphere of pressure. But when we add a little quantity of salt to that water, something fascinating occurs to the boiling point. If you add non-volatile contaminants to boiling water, such as sugar, the boiling point will rise.

Experiments prove this fact. The amount of solute added directly relates to the change in boiling point. Compared to pure solvents, this aspect shows that solutions can have a greater boiling point. 

Interesting Facts About Boiling Point

• Did you know that the boiling point would decrease if you ascend a mountain? It is because the greater the temperature, the more vapour you will discover!
• As the term implies, a solvent is a liquid you may use to dilute or dissolve a solution. However, did you know that water is an excellent and potent solvent? Water can dissolve a wide range of substances. People, for example, frequently use it as a solvent in drinks such as coffee and tea.
• Although adding various solutes to boiling water tends to raise its boiling point. Alcohol is a form of volatile solute that, when added to it, lowers the water’s boiling point.
• You can use boiling water as a disinfectant. Yes! It is because the scorching heat of boiling water damages microbes and bacteria, making it an excellent way to disinfect and clean your home. The microbes are killed in roughly a minute of boiling water.

Some Elements’ Melting and Boiling Points

The melting and boiling points of various essential elements are listed in a tabular column below. A few important melting and boiling points are: 

• Boiling point of mercury = 63 K  
• Silver melting point = 1234.15 K

Conclusion

The melting point of a solid is commonly assumed to be the same as the freezing point of the equivalent liquid. However, because liquids freeze in various crystal structures and impurities reduce the freezing point, the actual freezing point may differ from the melting point. 

As a result, the melting point is preferable when describing a material. Finally, the melting and boiling points are the temperatures reached when converting a solid to a liquid form, and air pressure equals the vapour pressure, respectively.