Vaporization

Boiling of water is a violent vaporization phenomenon. At this point, a large number of bubbles rise, become larger, to the surface of the water rupture, and the water vapor inside is emitted into the air. In the boiling process, although the water continues to be heated, but only so that the water continues to become water vapor, its temperature are kept constant.

The two conditions for liquid boiling

One is that the temperature of the liquid should reach the boiling point, and the other is that heat needs to be absorbed from outside. Boiling is a violent vaporization phenomenon that occurs both inside and on the surface of a liquid at a certain temperature. Different liquids have different boiling points, and even if the same liquid, its boiling point has to change with the change of the external atmospheric pressure.

Vaporization

The average distance of molecules in a liquid is much smaller than in a gas. Vaporization increases the average distance of the molecules and increases their volume dramatically, overcoming intermolecular gravity and working against atmospheric pressure. Therefore, vaporization to absorb heat. Unit mass of liquid into the same temperature vapor absorbed heat is called latent heat of vaporization, referred to as the heat of vaporization.

The heat of vaporization decreases with increasing temperature because at higher temperatures the liquid molecules have greater kinetic energy and the difference between the liquid and gas phases decreases. At the critical temperature, the material is in a critical state, the difference between the gas phase and the liquid phase disappears, the heat of vaporization is zero. Vaporization has two forms: evaporation and boiling.

Water boiling experiment

Experimental equipment

The iron stand, alcohol lamp, matches, asbestos mesh, beaker, cardboard with a hole in the center, thermometer, water, stopwatch

Experimental steps

①Install the experimental apparatus according to the diagram of the device;

②Heat the water with an alcohol lamp and observe;

③When the water temperature is close to 90 ℃ every 1 min to record the temperature, and observe the boiling phenomenon of water.

④Complete the curve of the relationship between temperature and time when water boils.

Experimental Phenomena

Water boiling phenomenon: the phenomenon of intense vaporization, a large number of bubbles rise, get bigger, to the surface of the water rupture, the inside of the water vapor emitted into the air. Although continue to heat, its temperature remains unchanged.

Experiment supplement

(1) Liquid boiling requires a certain temperature, different liquids have different boiling points at standard atmospheric pressure.

(2) The temperature of a liquid rises before it boils by absorbing heat, and stays the same after it boils by absorbing heat.

(3) the boiling point of the liquid is also related to the atmospheric pressure, the higher the pressure the higher the boiling point of the liquid, the pressure cooker is to take advantage of this principle.

(4) The similarities and differences between evaporation and boiling:

Directions for Examination

1. The conditions for water to boil: it reaches the boiling point and continues to absorb heat.

2, by the image tracing points, water boiling characteristics: the temperature remains the same, continue to absorb heat.

3, the beginning of heating to boiling time is too long, improve: ① with warm water; ② reduce the mass of water;

4, asbestos mesh role: so that the bottom of the beaker is heated evenly.

5, alcohol lamp removed, boiling continues, the reason: asbestos mesh has residual heat, water intermittent heat absorption;

6, water boiling point of 98 degrees, the reason: less than 1 standard atmospheric pressure.

7, boiling water cooled down to room temperature, temperature change characteristics: first fast and then slow.

8, if no matter how long it is heated, the water can not boil, reason: the fire is too small, the heat absorbed is less than the heat loss.

9, the order when installing the device: bottom-up.

10, before the water boils, the bubbles rising in the beaker are from large to small.