(b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

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insert step 1> The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. In a vacuum, the external pressure is significantly reduced.

insert step 2> When the vacuum pump is turned on, it reduces the pressure inside the flask. This reduction in pressure lowers the boiling point of the water, allowing it to boil at a temperature lower than its normal boiling point of 100°C.

insert step 3> As the water boils, it undergoes a phase change from liquid to gas, which requires energy. This energy is taken from the water itself, causing the temperature of the remaining liquid water to decrease.

insert step 4> If the vacuum pump continues to operate, the pressure remains low, and the water continues to lose energy through evaporation. Eventually, the temperature of the water can drop to 0°C or below, causing it to freeze.

insert step 5> This phenomenon is an example of evaporative cooling, where the most energetic molecules escape as vapor, leaving behind molecules with lower kinetic energy, thus lowering the temperature of the liquid.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. When a vacuum pump is applied, the external pressure decreases, allowing the water to boil at a lower temperature than its normal boiling point of 100°C. This phenomenon occurs because the water molecules can escape into the vapor phase more easily under reduced pressure.

Freezing Point and Pressure

The freezing point of a substance is the temperature at which it transitions from a liquid to a solid. Under normal atmospheric pressure, water freezes at 0°C. However, when the pressure is significantly reduced, the freezing point can also decrease, allowing water to freeze even at temperatures above 0°C. This is due to the relationship between pressure and the phase changes of substances.

Phase Changes and Energy Transfer

Phase changes, such as boiling and freezing, involve the absorption or release of energy. Boiling requires energy input to overcome intermolecular forces, while freezing releases energy as molecules arrange into a solid structure. In a vacuum, the energy dynamics change, allowing water to transition between phases at lower temperatures due to the altered pressure conditions.

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