Short Answer

The boiling point of a liquid is affected by several factors, mainly atmospheric pressure, impurities, and the nature of the liquid. When atmospheric pressure decreases, the boiling point becomes lower, and when pressure increases, the boiling point becomes higher.

Impurities such as salt increase the boiling point, while pure liquids boil at fixed temperatures. The strength of attraction between particles also affects boiling: liquids with strong forces of attraction have high boiling points, and those with weak forces have low boiling points.

Detailed Explanation :

Factors affecting boiling point

The boiling point of a liquid is the temperature at which it changes into a gas throughout the entire liquid. Although every pure liquid has a fixed boiling point at a given pressure, several conditions can change this temperature. Understanding these factors helps explain why water boils at different temperatures in different places, why pressure cookers work efficiently, and why impurities change the behaviour of liquids.

Boiling occurs when the vapour pressure of a liquid becomes equal to the atmospheric pressure acting on its surface. This means the boiling point depends not only on the liquid itself but also on the external conditions. The major factors affecting the boiling point include atmospheric pressure, impurities, the nature of the liquid, and heating conditions.

1. Atmospheric pressure

Atmospheric pressure has the greatest effect on the boiling point.

• Lower pressure → lower boiling point
  At high altitudes (like mountains), air pressure is low.
  So, water boils at a temperature lower than 100°C.
  This is why cooking takes longer in hill areas.
• Higher pressure → higher boiling point
  In a pressure cooker, the pressure inside is higher.
  So, water boils above 100°C, making food cook faster.

Reason:
Boiling happens when vapour pressure equals atmospheric pressure. When external pressure is low, vapour pressure matches it quickly, so boiling starts earlier.

2. Impurities in the liquid

Impurities generally increase the boiling point of a liquid.

• Example: Adding salt to water increases its boiling point.
• Sugar, salt, and other dissolved substances make the liquid require more heat to boil.

Reason:
Impurities bond with liquid particles and make it harder for the particles to escape into vapour. More heat is needed, so the boiling point rises. This effect is known as boiling point elevation.

3. Nature of the liquid

Different liquids boil at different temperatures due to differences in their molecular forces.

• Liquids with strong intermolecular forces have high boiling points.
  Example: Water has hydrogen bonding, so it has a relatively high boiling point (100°C).
• Liquids with weak intermolecular forces have low boiling points.
  Example: Petrol and alcohol evaporate easily and boil at lower temperatures.

Reason:
Stronger forces require more heat energy to break the particles apart.

4. Vapour pressure

Liquids with higher vapour pressure boil at lower temperatures.

• Example: Alcohol has high vapour pressure, so it boils at only 78°C.
• Liquids with low vapour pressure require more heat to reach boiling.

Reason:
When a liquid naturally creates more vapour, its vapour pressure reaches atmospheric pressure faster.

5. Heating conditions

Rapid heating can cause uneven boiling or boiling before the liquid reaches its true boiling point. Slow, steady heating shows the correct boiling behaviour.

• Slow heating → accurate boiling point
• Fast heating → may cause superheating (temperature rises slightly above boiling point before bubbles form)

6. Altitude

Although altitude is related to pressure, it has a significant practical impact.

• At higher altitudes → low pressure → low boiling point
• At sea level → normal pressure → normal boiling point
• Below sea level → higher pressure → higher boiling point

Example: Water boils at 90–95°C in many hill stations.

7. Presence of dissolved gases

Dissolved gases may slightly reduce the boiling point by lowering cohesive forces inside the liquid. When water is heated for a long time, these gases escape, which can change the boiling characteristics.

Examples demonstrating factors affecting boiling point

1. Pressure cooker cooks food faster because increased pressure raises the boiling point.
2. Saltwater boils at a higher temperature than pure water, explaining why seawater behaves differently when heated.
3. High-altitude cooking takes longer because water boils at a lower temperature.
4. Petrol boils easily due to weak forces, which is why it evaporates quickly from open containers.

These examples show how different factors change the boiling temperature in real life.

Conclusion

The boiling point of a liquid is influenced by atmospheric pressure, impurities, the nature of the liquid, vapour pressure, and heating conditions. Lower pressure lowers the boiling point, while impurities raise it. Liquids with strong intermolecular forces have higher boiling points, while those with weak forces boil easily. Understanding these factors helps explain everyday phenomena like cooking differences at high altitudes, the working of pressure cookers, and the behaviour of different liquids when heated.