Short Answer

Water boils at a lower temperature at high altitudes because the external atmospheric pressure is lower. Boiling occurs when the vapor pressure of water becomes equal to the outside pressure. At high altitudes, this outside pressure is reduced, so water reaches the required vapor pressure at a lower temperature.

As a result, water starts boiling even though it has not reached 100°C. This is why cooking takes longer in hill stations or mountainous regions, where the boiling temperature of water is lower than at sea level.

Detailed Explanation

Water Boiling at Lower Temperature at High Altitudes

Water boils when its vapor pressure becomes equal to the external pressure acting on it. At sea level, the atmospheric pressure is high, so water must reach 100°C for its vapor pressure to match the external pressure. However, at high altitudes, atmospheric pressure is much lower. Because the pressure outside the liquid is reduced, water does not need to reach 100°C to match this pressure. Instead, it reaches equilibrium at a lower temperature, causing boiling to occur earlier.

This phenomenon explains why boiling point varies with altitude and why boiling temperature is not a fixed value but depends on external conditions.

Atmospheric Pressure and Boiling

Atmospheric pressure is the weight of the air pressing down on everything on the Earth’s surface. At sea level:

• Air density is high
• Atmospheric pressure is high
• Water needs to reach 100°C to make its vapor pressure equal to this pressure

At high altitudes:

• Air is thinner
• Atmospheric pressure is lower
• Water requires less vapor pressure to match the outside pressure
• This happens at a lower temperature

Thus, the boiling point decreases as altitude increases.

Role of Vapor Pressure

Vapor pressure increases with temperature. Water boils when vapor pressure equals external pressure:

• At sea level: vapor pressure must reach 1 atm
• At high altitude: vapor pressure may only need to reach 0.8 atm or even less

Since reaching lower vapor pressure requires lower temperature, water boils earlier.

For example:
At about 2,000 meters above sea level, water boils around 93°C instead of 100°C.

Effect of Lower Boiling Temperature on Cooking

Because water boils at a lower temperature, it never gets hot enough for normal cooking processes. Even though water is boiling, its temperature is lower than usual:

• Food takes longer to cook
• Tough foods like rice, pulses, and potatoes remain partly uncooked
• Steaming and boiling processes slow down

This is why pressure cookers are commonly used in mountainous regions. They increase the internal pressure, raising the boiling point back toward 100°C or more.

Relation Between Altitude and Boiling Point

As altitude increases:

• External air pressure decreases
• Boiling point of water decreases

This change can be summarized as:

• Higher altitude → lower pressure → lower boiling point
• Lower altitude → higher pressure → higher boiling point

This relationship is important in chemistry, weather science, cooking, and industrial processes.

Scientific Explanation Using Molecular Behavior

At high altitudes:

• Fewer air molecules press down on the water
• Molecules of water require less energy to escape into vapor
• They escape at lower temperature
• Formation of bubbles becomes easier at lower temperature

Thus, boiling occurs earlier than expected.

Real-Life Examples

1. Hill Stations

At places like Shimla, Manali, or Darjeeling, water boils below 100°C. Tea, rice, or pulses take longer to cook.

2. Mountaineering

Climbers at high altitudes boil water at very low temperatures, sometimes below 90°C.

3. Pressure Cooker Use

Pressure cookers increase internal pressure and allow water to boil above 100°C, helping food cook normally even at high altitudes.

4. Scientific Experiments

Laboratory experiments that rely on boiling must adjust for altitude to ensure accurate measurements.

Importance of This Concept

Understanding why water boils at lower temperatures at high altitudes is useful because it helps:

• Adjust cooking techniques
• Understand atmospheric pressure changes
• Predict boiling behaviour
• Design pressure-based equipment
• Perform chemistry experiments accurately

This idea also helps explain natural processes like evaporation, cloud formation, and weather conditions.

Conclusion

Water boils at a lower temperature at high altitudes because the external atmospheric pressure is reduced. When pressure is low, water’s vapor pressure becomes equal to the outside pressure at a lower temperature, causing boiling to occur early. This affects cooking, scientific experiments, and daily activities in mountainous regions. The relationship between altitude, pressure, and boiling point highlights how physical conditions influence phase changes.