Short Answer

Boiling point is the fixed temperature at which a liquid changes into a gas. At this temperature, bubbles of vapour form inside the liquid and rise to the surface. The boiling point of a substance does not change during the process, even though heat is continuously supplied.

Different liquids have different boiling points. For example, water boils at 100°C at normal atmospheric pressure. The boiling point helps us identify substances and understand their behaviour under heat.

Detailed Explanation :

Boiling point

The boiling point is the specific temperature at which a liquid changes into a gas throughout its entire volume. When a liquid is heated, its particles gain energy and move faster. As the temperature keeps rising, the vapour pressure inside the liquid increases. When this vapour pressure becomes equal to the surrounding atmospheric pressure, the liquid begins to boil. At this moment, bubbles of vapour form inside the liquid and rise to the surface, producing rapid bubbling.

The boiling point is a characteristic property of each liquid and remains constant during the entire boiling process. Even though heat continues to be supplied, the temperature does not increase until all the liquid has completely changed into vapour.

Meaning of boiling point

Boiling point can be defined as:

“The fixed temperature at which a liquid changes into vapour and boiling starts throughout the liquid.”

At this temperature, liquid and vapour exist together.

How boiling takes place

To understand boiling point clearly, it is important to study what happens inside the liquid:

1. Particles gain heat energy
   As the liquid absorbs heat, particles move faster and spread farther apart.
2. Vapour pressure increases
   This internal pressure rises as particles try to escape.
3. Equality of pressures
   When the vapour pressure becomes equal to the atmospheric pressure, boiling begins.
4. Formation of bubbles
   Bubbles form not just at the surface but throughout the liquid.
5. Temperature stays constant
   Even though heat continues to be supplied, the thermometer reading does not increase.
6. Latent heat of vaporization works
   All the heat goes into changing the state, and not into increasing the temperature.

This explains why water stays at 100°C while it boils, even if more heat is applied.

Boiling point of common substances

Different liquids boil at different temperatures due to differences in bonding strength and molecular structure.

Examples:

• Water → 100°C
• Ethanol → 78°C
• Mercury → 357°C
• Liquid nitrogen → –196°C
• Acetone → 56°C

These differences show the variety in molecular behaviour.

Factors affecting boiling point

Although the boiling point is constant for a pure liquid, several factors can influence it:

1. Atmospheric pressure

Boiling point depends on outside pressure.

• Low pressure → lower boiling point
  (Water boils at less than 100°C in mountains.)
• High pressure → higher boiling point
  (In a pressure cooker, water boils above 100°C.)

2. Impurities

Adding impurities increases the boiling point.

Example: Saltwater boils at a slightly higher temperature than pure water.

3. Nature of the liquid

Strong molecular forces → high boiling point
Weak molecular forces → low boiling point

4. Heating rate

Faster heating can make boiling appear sooner, but the true boiling point remains unchanged.

Importance of boiling point

The boiling point is an important physical property. It helps in:

1. Identifying liquids

Each pure liquid has a unique boiling point, useful in laboratory identification.

2. Checking purity

Pure substances have fixed boiling points.
Impure substances boil over a range of temperatures.

3. Cooking

Boiling helps cook food. At higher altitudes, lower boiling points affect cooking time.

4. Industrial use

Distillation, purification, and chemical processes rely on accurate boiling points.

5. Weather and climate

Boiling and evaporation play roles in cloud formation and the water cycle.

Boiling point and evaporation

Boiling and evaporation are both processes that convert a liquid into vapour, but they differ:

• Boiling happens at the boiling point.
• Evaporation happens at any temperature.
• Boiling occurs throughout the liquid.
• Evaporation occurs only at the surface.

These differences show why boiling is fast and evaporation is slow.

Conclusion

Boiling point is the fixed temperature at which a liquid changes into a gas with the formation of bubbles throughout the liquid. It is a constant property of pure liquids and is influenced by atmospheric pressure and impurities. Understanding boiling point helps in science, cooking, industry, and natural processes. It is an essential concept in studying how matter behaves under heat.