There will be a lot of bubbles before the water is boiled, which means something

When the water is boiled, blisters emerge from the bottom, in fact, it is not necessarily the bottom, but on the heating object, such as boiling water with fire, the bottom of the kettle is directly heated and heats the water, so there are bubbles at the bottom of the kettle, if you use an electric kettle to boil water, you can see that there are bubbles coming out of the heating ring. There are bubbles because in the process of heating, the local temperature is high (the temperature of the heating parts is often much higher than 100°, and the boiling point of pure water is 100°), which produces water vapor, which is in a gaseous state, so of course it comes out in the form of a blister. The bubble grows larger and larger from the bottom to the top, because the pressure of the water at the bottom is high, the bubble is compressed, and the pressure on the upper layer is smaller, so the bubble gradually becomes larger until it bursts to the surface of the water.

When boiling, the water in the beaker is heated with a heat source, and when it does not reach boiling, the bubbles that appear from the bottom rise and shrink while disappearing into the water, and when the water boils, the bubbles appear from the bottom, and become larger while rising, until the surface breaks

Experiments show that before heating, a large number of gas molecules are attached to the wall and the surface of impurities in the water, which is the root cause of the bubble Before boiling, due to heating, bubbles are formed, at first, the bubbles are all gas molecules, there are no liquid molecules, so the surrounding liquid molecules have to evaporate into the bubble Due to the low temperature, the boiling point has not yet been reached, the saturated vapor pressure in the bubble is less than the external pressure, the bubble is compacted, and at the same time, it rises by the buoyancy of the water In the process of rising, the volume decreases, and the water vapor in the bubble becomes supersaturated. The liquid molecules have to be condensed again, so the bubble pressure is smaller, and then compressed, and then rises, until it disappears and boils, the temperature is high, after the initial formation of bubbles, the surrounding liquid evaporates into the bubble and becomes saturated steam, and the saturated air pressure is strong, and the external pressure can be balanced, the bubble rises due to buoyancy, so the external pressure becomes smaller, the pressure inside the bubble becomes larger, the volume expands, and the water vapor in the bubble becomes unsaturated steam, causing the surrounding liquid to evaporate into the bubble again until the liquid level bursts

When water boils, only the water at the bottom that is in contact with the heating part really reaches the boiling point. Other parts of the water are actually slightly below the boiling point.

The water in the heating area vaporizes and expands violently, causing the surrounding material to vibrate and produce a sound. And produce a large amount of water vapor, which is the bubble we see.

Can you ask the question a little bit clearer? I kind of don't know how to do it.

The water vapor bubble before boiling because the partial pressure of the water vapor cannot reach the saturated vapor pressure of the water at this air pressure, and the volume is gradually reduced during the floating process (basically cannot reach the water surface; not easy to observe). The bubbles increase in volume due to other gases dissolved in the water. This means that a certain amount of gas can be dissolved in the water, and the solubility of these gases decreases as the temperature increases.

Of course, when the water boils, the other gases are almost exhausted, and the bubbles that come out at this time are almost all water vapor.

The solubility of the gas decreases as the temperature increases.

The large number of bubbles produced before the water is boiled are the dissolved gases in the water.

The water turned into water vapor and bubbled up.

Why do bubbles go from large to small before water boils, and from small to large when boiling?

When the water boils, the reason why the bubble is small and large is that before boiling, the temperature of each water layer in the container is different, the temperature of the water layer near the heating surface is higher, the temperature near the water surface is lower, not only the pressure of the air in the bubble decreases with the decrease of water temperature in the rising process, but also the saturated vapor pressure is also decreasing after a part of the water vapor in the bubble condenses into water, and the external pressure is basically unchanged, at this time, the pressure outside the bubble is stronger than the pressure inside the bubble, so the volume of the bubble will decrease in the rising process.

A small amount of air is dissolved in the water, and air is also adsorbed in the small cavities on the surface of the container wall, and these small bubbles act as vaporized nuclei. The solubility of water to air and the adsorption capacity of the wall to the air decrease with the increase of temperature, when the water is heated, the bubbles are first generated on the wall of the heating surface After the bubbles are generated, because the water continues to be heated, a superheated water layer is formed near the heating surface, and it will continuously evaporate water vapor into the small bubbles. In addition to the original air, water vapor is added to the bubble, so that the sum of the pressure air pressure and vapor pressure in the bubble continues to increase, as a result, the volume of the bubble continues to expand, and the buoyancy of the bubble also increases.

When the buoyancy of the bubble is greater than the adhesion between the bubble and the wall, the bubble leaves the wall and begins to float.

Because the temperature of the water above is low, the water vapor is cooled and liquefied, the gas volume becomes smaller, and the bubbles become smaller. Boiling is a phenomenon in which a liquid is heated beyond its saturation temperature, and violent vaporization occurs both inside and on the surface of the liquid.

When all kinds of liquids boil, they have a definite temperature called the boiling point. Different liquids have different boiling points. Even if it is the same liquid, its boiling point varies with the air pressure outside

The higher the atmospheric pressure, the higher the boiling point of the liquid, and vice versa. Water has a boiling point of 100 at a standard atmospheric pressure, which is the most common.

The relationship between water pressure and bubbles

At a certain external pressure, boiling can only be carried out at a certain temperature (boiling point) and continuous heating. When the liquid is boiling, the temperature remains the same and it still absorbs heat. At this time, the saturated vapor pressure is equal to the external pressure p.

When the external pressure of the liquid increases, its boiling point increases; Otherwise, it decreases. The boiling point of different liquids is different at the same pressure. This is related to the saturation vapor pressure of the liquid.

If the saturated vapor pressure is the same as the outside world at the current temperature, the liquid boils, and the saturated vapor pressure of the liquid is positively correlated with the temperature of the liquid. For example, after the water in the circular flask boils, stop heating, stop boiling, pour a little cold water on the surface of the flask, so that the air pressure in the flask decreases, and the water boils again.

Many friends have experience in boiling water. When we boil the water in the kettle to a certain extent, we will see that there are a lot of small bubbles coming out. So what are these little bubbles? Why does it happen? Let me explain.

We know that water has three states: gaseous, liquid, and solid. There are a lot of small water vapor around us that we can't see, and we can't see it normally, but in winter, when we are hazing, we will see water mist, which is formed by the liquefaction of water vapor in the air, and this water vapor is gaseous water.

And solid water is our common ice (think of old popsicles). Liquid water is the drinking water we usually drink.

When we put liquid water in the kettle to heat, the water inside the kettle will absorb heat. Since the thermal conductivity of water is not good, the water in the pot will form a longitudinal temperature difference. The existence of this temperature difference makes the temperature of the bottom water of the pot and the temperature of the surface of the pot quite different.

Therefore, when the low water of the pot is heated to vaporization, the water on the surface of the pot still does not reach the temperature of vaporization, so the bubbles of the water are generated from the bottom of the pot and emerge from the surface of the pot. So what's in this bubble? Some people say that inside the bubble are hydrogen and oxygen.

Actually, no, the water is just heated, and there is no chemical reaction yet. So the composition has not changed. So what exactly is the gas inside a bubble?

In fact, it is formed by the heating and vaporization of water, and the actual composition is water vapor, that is, water changes from liquid to gaseous. This kind of water vapor is very powerful, and I think that Newton invented the steam engine after observing the water vapor pushing the lid of the pot.

This indicates that the water contains air, and the buoyancy of the air increases when it is heated and expands, causing the bubbles to float up and escape.

The bottom bubble before the water boils gradually shrinks as it rises.

Reason: During the heating process before boiling, because water is a poor conductor of heat, the water temperature of the lower part is higher, and the water temperature of the upper part is lower, and the gas is obviously affected by thermal expansion and contraction. All bubbles gradually shrink during ascent.

After the water boils, the bottom bubble gradually grows larger during the ascent.

Reason: After boiling, the temperature of all parts of the water in the container is the same, and the vapor after water vaporization forms bubbles, and the bubbles are affected by the pressure during the rising process.