Water at room temperature is placed in a flask connected by rubber tubing to a vacuum pump, and the pump is turned on. After several minutes, the volume of the water has decreased, and what remains has turned to ice. Explain

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The water is at room temperature, which means it is in liquid form. The flask is connected to a vacuum pump, which will reduce the pressure inside the flask when turned on.

When the vacuum pump is turned on, it decreases the pressure inside the flask. According to the phase diagram of water, reducing the pressure can lower the boiling point of water.

As the pressure decreases, the water begins to boil at a lower temperature. Boiling is an endothermic process, meaning it absorbs heat from the surroundings, which in this case is the remaining water.

The heat absorbed during boiling causes the temperature of the remaining water to drop. If the temperature drops below 0°C, the water will freeze, turning into ice.

The vacuum pump reduces the pressure, causing the water to boil and absorb heat, which cools the remaining water enough to freeze it into ice.>

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Key Concepts

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

Phase Changes

Phase changes refer to the transitions between solid, liquid, and gas states of matter. In this scenario, water initially in the liquid phase undergoes a phase change to ice, which occurs when the temperature drops below the freezing point. Understanding phase changes is crucial for explaining how water can freeze under reduced pressure.

Vapor Pressure and Boiling Point

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form. When the vacuum pump is activated, it lowers the pressure above the water, which can cause the boiling point of water to decrease. This reduction in boiling point can lead to rapid evaporation, which explains the decrease in water volume.

Cooling Effect of Evaporation

The cooling effect of evaporation occurs because the molecules with the highest kinetic energy escape first, leaving behind cooler molecules. As water evaporates in the vacuum, it absorbs heat from the remaining liquid, leading to a drop in temperature that can cause the remaining water to freeze. This principle is essential for understanding the observed phenomenon in the flask.

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