Pressure is the force exerted per unit area on a surface. It is a scalar quantity measured in pascals in the SI system. Pressure can arise from liquids gases or solids and plays a key role in various phenomena like fluid flow atmospheric behavior and mechanical systems.
Chapter 10 of Class 11 Physics focuses on thermal properties of matter. It explores concepts like heat temperature specific heat thermal expansion and heat transfer. Topics include conduction convection and radiation. The chapter emphasizes understanding physical behaviors due to temperature changes and their practical applications in various natural and engineered systems.
Effect of Pressure on the Boiling Point of a Liquid:
The boiling point of a liquid is that temperature at which its vapor pressure becomes equal to the external pressure acting on it. When the pressure is increased, the boiling point of the liquid also increases, and when the pressure is decreased, the boiling point decreases. This is because a high pressure requires much energy (heat) to enable the liquid molecules to escape into the vapor phase, while a low pressure will make it easier to vaporize.
Explanation Using a Simple Experiment
To understand how pressure affects the boiling point of a liquid, you can perform a simple experiment using a saucepan, a thermometer, and a vacuum pump or a simple setup to create a change in pressure.
Experiment:
1. Materials Needed:
– A saucepan or beaker
– A thermometer
– A vacuum pump (for lowering pressure) or a pressure cooker (for increasing pressure)
– Water or another liquid (such as alcohol)
2. Procedures:
Experiment 1 (Effect of Decreasing Pressure):
1. Put water in the saucepan and place it on a heat source.
2. Dip the thermometer into the water and measure the temperature.
3. Boil the water and note the temperature at which it starts boiling. The boiling point of water at atmospheric pressure is usually about 100°C.
4. Reduce the pressure using a vacuum pump. Alternatively, you can do this in a simple vacuum chamber, if available.
5. Notice that the water starts boiling at a temperature less than 100°C as the pressure is reduced.
Experiment 2: Effect of Increasing Pressure
1. Repeat the same setup but use a pressure cooker instead of the open saucepan.
2. When the pressure cooker is sealed, it increases the pressure inside.
3. You will notice that as the pressure is increased, it takes longer for the water to boil, and the boiling point is above 100°C.
3. Observations:
– In Experiment 1, as the pressure inside the chamber decreases, the water boils at a temperature lower than its normal boiling point (100°C at 1 atmosphere).
In Experiment 2, the higher temperature at which the water boils results from the increased pressure inside the pressure cooker.
Explanation in Science Terms:
At Low Pressure: Fewer air molecules exert force on the liquid’s surface when pressure is low, so it’s easier for liquid molecules to go into the vapor phase. In this case, the liquid will boil at a lower temperature. That is why water boils at a lower temperature at high altitude where the pressure of the atmosphere is low.
– At Higher Pressure: When the pressure is increased (like in a pressure cooker), the liquid’s vapor pressure needs to overcome a higher external pressure to escape into the gas phase. Therefore, the liquid boils at a higher temperature to provide the necessary energy for the molecules to break free from the liquid phase.
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