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Maharashtra State BoardHSC Science (Computer Science) 11th Standard
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Syllabus

Boiling Point and Pressure

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Estimated time: 14 minutes
  • Activity: Boiling Point & Pressure Experiment
  • Boiling of Water at High Altitudes
  • Pressure Cooker Speeds Up Cooking
  • Key Points: Boiling Point and Pressure
Maharashtra State Board: Class 11

Activity: Boiling Point & Pressure Experiment

This activity demonstrates how the boiling point depends on external pressure using a round-bottom flask. The experiment has two parts — one proving that increased pressure raises the boiling point, and another proving that decreased pressure lowers it.

Apparatus Required:
Round-bottom flask (500 mL)
Bunsen burner/spirit lamp
Thermometer (0–110°C)
Cork with two holes
Retort stand with clamp
Ice-cold water (in beaker)
Airtight rubber cork
Steam outlet tube

Labelled diagram of the boiling point experiment setup showing a round-bottom flask on a burner with thermometer and steam outlet

Fig.: Experimental setup: Round-bottom flask with thermometer and steam outlet, heated over a Bunsen burner

Part A: Increased Pressure Raises Boiling Point

Procedure & Observations

  1. Set up: Fill a round-bottom flask more than half with water. Insert a thermometer and a steam outlet tube through the cork, then place the flask on a burner.
  2. Initial Heating: Heat the water and observe small bubbles forming.
    Observation: These bubbles are dissolved air escaping, not steam.
  3. Steam Formation: As the temperature increases, steam bubbles form at the bottom.
    Observation: They collapse before reaching the surface because the upper water is still cooler.
  4. Boiling Stage: When the water reaches 100°C, steam bubbles reach the surface.
    Observation: Steam inside the flask is invisible but forms a fog when it comes out and cools.
  5. Key Test: Close the steam outlet for a few seconds.
    Observation: Boiling stops even though the temperature is still 100°C because the pressure inside increases.

Conclusion:
The boiling point increases with increasing external pressure.

Part B: Decreased Pressure Lowers Boiling Point

Procedure & Observations

  • Cool down: Remove the burner. Allow the water to cool to approximately 80°C.
  • Seal the flask: Remove the thermometer and steam outlet tube. Seal the flask with an airtight cork.
  • Invert: Turn the flask upside down and mount it on a retort stand.
  • Pour ice-cold water over the outer surface of the inverted flask.

Observation: Water inside the flask begins to boil vigorously — even though it is only at approximately 80°C!

Diagram of inverted flask experiment showing reduced pressure boiling at approximately 80 degrees Celsius

Fig.: Inverted flask experiment: Ice-cold water condenses vapour, reducing pressure, causing water to boil at ~80°C

Maharashtra State Board: Class 11

Boiling of Water at High Altitudes

At high altitudes (e.g., hill stations like Shimla, Darjeeling, or Leh), atmospheric pressure is significantly lower than at sea level because there is less air above to exert pressure.

Since boiling point depends on external pressure, water boils at a temperature well below 100°C at these elevations — for example, at approximately 92°C at 2,400 m (8,000 ft).

Since food requires a certain minimum temperature to cook properly (proteins denature, starches gelatinise), the lower boiling point means the water is simply not hot enough to cook food efficiently. The cooking process becomes much slower, and food may remain undercooked.

Maharashtra State Board: Class 11

Pressure Cooker Speeds Up Cooking

A pressure cooker is a sealed vessel that prevents steam from escaping. As water boils and generates steam, the trapped steam increases the internal pressure to about 1.5–2 atmospheres.

At this elevated pressure, water boils at approximately 120°C instead of 100°C. The higher temperature means food receives more thermal energy per unit time, leading to faster and more thorough cooking.

Additionally, the trapped high-temperature steam surrounds the food, cooking it evenly from all sides and retaining moisture — further speeding up the process.

Maharashtra State Board: Class 11

Key Points: Boiling Point and Pressure

  • Boiling occurs when a liquid's vapour pressure equals the external atmospheric pressure — it's a bulk phenomenon happening throughout the liquid.
  • Increasing pressure raises the boiling point — e.g., in a pressure cooker, water boils at ~120°C because the trapped steam increases internal pressure to ~1.5–2 atm.
  • Decreasing pressure lowers the boiling point — e.g., at 2,400 m on a mountain, water boils at ~92°C because atmospheric pressure is lower.
  • The flask experiment demonstrates both effects: closing the outlet (↑ pressure → boiling stops) and pouring cold water on the inverted flask (↓ pressure → boiling resumes at ~80°C).
  • Cooking at high altitudes is slower because the reduced boiling point means water cannot reach the temperature needed to cook food efficiently.
  • Pressure cookers cook faster by raising the boiling point above 100°C through increased internal pressure, providing more thermal energy to food.

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