Why doesn't the liquid in Torricelli's experiment boil?

The relationship between the boiling point and pressure of mercury is the same as that of water. The lower the external pressure, the lower the boiling point (but the specific degree of reduction is different).

To make water boil at room temperature, the external pressure must be less than the saturated vapor pressure of water at room temperature (equivalent to the pressure of 17.5 mm Hg at 20℃). Put a glass of water into a vacuum container, the external pressure is less than the saturated vapor pressure, and the water will boil. However, with the evaporation of water, the vacuum container will soon be filled with steam, and the steam pressure will gradually increase. When it reaches the saturated vapor pressure of water, boiling will stop. In order to keep boiling, the generated steam must be continuously pumped out of the container to ensure that the internal pressure is less than the saturated vapor pressure of water.

Now, assuming that the vacuum container is relatively small, it is conceivable that after water is put in, the internal pressure will reach the saturated vapor pressure in a short time, and the boiling will end soon. When the container becomes smaller and smaller, even the boiling phenomenon is invisible to the naked eye, the water vapor in the container is saturated. This is because the surface of water will evaporate at any temperature. As long as this evaporated water vapor can fill the vacuum area and reach the saturated vapor pressure, boiling phenomenon will not be seen.

This is the case with Torricelli's experiment. Of course, there is a tendency to form a vacuum above the water column in the process of water column falling (note that there is a trend, but it cannot form a vacuum), but at the same time, a small amount of water (large surface area per unit mass) left on the column wall will evaporate quickly (instead of boiling), making the vacuum zone full and saturated, so boiling phenomenon cannot be observed. The evaporation of water on the surface of the water column also contributes to the steam saturation above, but it can be ignored compared with the small surface area of the column wall.

Note that although the water molecules inside tend to become gaseous, the pressure it produces is the saturated vapor pressure at this temperature. When the external pressure is greater than or equal to the saturated vapor pressure, this part of the gas that may be generated cannot escape from the liquid surface, and the bubbles are "killed in the cradle" by the external pressure as soon as they are in the "budding state", so the boiling phenomenon of bubbles escaping from the liquid surface cannot be observed by naked eyes.

There are many specific details involved in the actual process (my answer above is not completely true, but it is basically close to the truth. With the improvement of your knowledge reserve and cognitive ability, you will find that the truth becomes more and more complicated), so it is impossible to explain all these details clearly in middle school education. The landlord raised this question, which shows that the landlord has thought it over, but it is not legal yet. This requires a step-by-step process, and gradually cultivate your own insight.

If you have any questions, please ask.