Why Does Water Make Noise Before Boiling but Quiet Down Near the Boiling Point?

Why does water make a lot of noise before it boils, but become quiet as it nears the boiling point?

Why does water make a lot of noise before it boils, but stops making a lot of noise when it gets close to boiling point?

Become a special member of Arjan Ambassador in order to benefit from and access special facilities and different departments in Arjan Ambassador.

The sound of the kettle is so loud that you think the water has boiled. You get up from the table and head to the kitchen, but just as you reach for the kettle, the sound goes down. The machine light is still on and the water is still heating. A few seconds later, large bubbles rise to the surface and the water begins to bubble. But why does the kettle make so much noise before it boils and then calm down just as the water reaches the boiling point?

You haven't taken the kettle off the stand yet. You look at the water through the glass. The surface of the water is almost still, but near the bottom of the container, small bubbles have formed on the metal plate. Some detach from the surface and rise, while others disappear before reaching the surface of the water.

When you turn on the kettle, the electric element at the bottom of the device starts to generate heat. This heat is first transferred to the water near the hot plate. Contrary to what you might think, all the water in the kettle does not heat up at the same time and at the same rate. In the first few minutes, the water near the bottom can be much hotter than the water higher up.

This temperature difference causes the water in the kettle to move. Warm water usually expands and its density decreases as the temperature increases. As a result, it moves upwards and cooler and denser water takes its place. This continuous movement in physics is called convection.

If you could see the movement of water inside the kettle with the help of a suitable laboratory dye, you would notice that the hot water moves from the bottom to the top and the cooler water from the other parts returns to the bottom. These currents spread the heat inside the container and gradually reduce the temperature difference of different parts of the water. However, this movement alone is not the reason for that loud and continuous sound you hear from the kettle.

The interesting thing is that for the formation of steam bubbles, it is not necessary that all the water in the kettle has reached the boiling temperature. At normal pressure near sea level, pure water boils at about 100°C, but a very thin layer of water in direct contact with the hot surface can reach the conditions necessary to form steam sooner.

At this point, even though most of the water has not yet reached its boiling point, the temperature around the element has risen enough that steam bubbles form on the hot surface. These bubbles gradually get bigger and separate from the bottom of the kettle. Their rapid size changes also play a role in producing the sound you hear.

But the interesting thing is that not all bubbles reach the surface of the water. If you look closely, you'll see some of them suddenly disappear as you go up. Where do these bubbles go and what is the connection between their disappearance and the sound of the kettle?

If you look closely at the wall of the kettle, you will see tiny bubbles clinging to its surface. Some of the bubbles slowly detach from the wall and rise up, but some others behave strangely; They appear suddenly and disappear before they reach the surface of the water. What is the reason for this difference? For the answer, you need to know that not all the bubbles in the kettle are of the same type.

The water you pour into the kettle from the tap has some dissolved gas in it. Gases such as oxygen and nitrogen can enter water from the air and remain in it. As the temperature rises, the solubility of many of these gases decreases and some of them come out of the water in the form of tiny bubbles. For this reason, you may see a lot of bubbles on the kettle wall even before the water reaches the boiling point. These bubbles are not necessarily a sign of boiling, some of them are formed from gases that were previously dissolved in the water.

But there is another group of bubbles that are directly related to the conversion of water into steam and play an important role in producing the sound of the kettle. If you could look at the metal bottom of a kettle under a microscope, you would notice that its surface, contrary to its smooth appearance, is made up of very small bumps. Microscopic scratches and holes can be the perfect place for vapor bubbles to start forming. These places are called germination centers in physics.

When the temperature of the water around these points rises enough, the first bubbles of steam are formed, which are very small at first, but as more steam enters, their volume increases. Finally, when the buoyancy force overcomes the sum of the forces holding the bubble at the surface, the bubble separates from the bottom and moves upward. This is where the difference in temperature of different parts of the water becomes important.

Suppose that the water near the element has reached the temperature necessary to form steam, but a few centimeters above it is still cooler. After the steam bubble separates from the floor, it enters this cooler area and part of the steam inside turns into liquid water again. This process is called condensation; The same thing that happens when water vapor hits cold glass and drops form on it. With the difference that inside the kettle, condensation can occur in a very short time.

Since steam occupies a much larger volume than the same amount of liquid water, the conversion of steam to liquid will cause the bubble to shrink. If this change happens too quickly, the bubble may contract severely or collapse completely.

Sometimes all these events happen in a few thousandths of seconds or even less; So fast that the human eye cannot follow their details. However, high-speed cameras can record the moment of formation, growth and collapse of these small bubbles.

Therefore, what you see behind the glass of the kettle is only a small part of what is happening inside the water. Near the element, vapor bubbles form successively, grow larger, and some disappear on the way up after separating from the hot surface.

These changes do not just change the appearance of the water. The rapid expansion and contraction of the bubbles also changes the pressure of the water around them, creating waves that can help produce sound. If these small bubbles play a role in producing the sound of the kettle, why does the sound sometimes become less as the water approaches the boiling point?

The hotter the water in the kettle gets, the louder it sounds. The soft hissing sound that you hear at the beginning gradually turns into a loud and continuous roaring sound. However, the water level is still relatively calm and not many bubbles reach it. Why does the kettle sound so loud at this stage?

As we said, the rapid growth and collapse of steam bubbles changes the pressure of the water around them. When a large number of bubbles are successively formed and destroyed, the pressure waves resulting from this process combine with each other and part of the vibrations are transferred to the body of the kettle. The result is a sound that sounds continuous to our ears.

At this stage, the water temperature difference near the element and the higher parts is still large. Because of this, many of the steam bubbles, after separating from the foam, enter the cooler water and burst quickly. The higher the number of these occurrences, the more intense the sound produced can be.

But as the water continues to warm, the situation changes. Convection currents gradually spread the heat in different parts of the kettle, and the temperature of the water in the higher parts approaches the boiling point. As a result, the steam bubbles encounter warmer water as they rise, and the possibility of their rapid collapse decreases.

In such conditions, more bubbles can reach the surface of the water without being destroyed. By reducing fast decays, one of the main sources of kettle noise is weakened. Therefore, the continuous sound of the device may be noticeably reduced before the water starts to boil.

Of course, not all kettle noise is caused by the collapse of bubbles. Their rapid growth, detachment from the hot surface, coalescence of bubbles and changes in water flow can also produce sound. It has even been observed in some experiments that the growth of the bubble, independent of its collapse, can create a significant pressure wave. Therefore, not all of the kettle's noise can be attributed to bubbles bursting.

When water reaches its boiling point, a large number of steam bubbles rise to the surface of the water. The opening of the bubbles on the surface and the turbulence caused by the movement of the water creates a familiar gurgling sound. This sound is different from the continuous roar before boiling. In other words, when water boils, in addition to the intensity of the sound, its type also changes.

Of course, this pattern is not the same in all kettles. Factors such as the amount of water, the power of the element, the shape of the container and the material of the body can affect the sound produced. As a result, in some kettles, the pre-boiling sound may be louder and in others, the sound of water gurgling can be heard more.

Each vapor bubble doesn't make a very loud noise on its own, but when a large number of them grow and burst in a short period of time, the resulting sounds can be quite noticeable. However, how does this sound reach our ears from inside the water?

Sound is a type of mechanical wave that is transmitted through the vibration of particles in the environment. The pressure waves caused by the change in the volume of the bubbles spread in the water and upon reaching the bottom and walls of the kettle, they transfer part of their energy to them. This energy causes the body of the kettle to vibrate, and finally, the vibrations reach our ears through the surrounding air.

Of course, the body of the kettle does not only transmit the sound, but can also affect its intensity and characteristics. Depending on its material, shape and dimensions, any object vibrates more easily at certain frequencies. For this reason, two kettles with the same element and amount of water may produce different sounds.

To better understand this difference, we need to familiarize ourselves with two characteristics of sound: intensity and frequency. Sound intensity is related to the amount of sound energy, while frequency indicates how many times an oscillation is repeated per second. The higher the frequency, the lower the sound is usually heard and as it decreases, the bass is heard.

The size of the bubbles, the speed of their volume change and the movement of the water around them affect the frequency and intensity of the waves produced. On the other hand, all these waves are not audible to humans. Some oscillations caused by boiling may have a frequency higher than the range of human hearing, which requires appropriate measuring equipment to be recorded.

Meanwhile, the collapse of vapor bubbles is sometimes confused with a phenomenon called cavitation. Cavitation usually occurs when the local pressure of a liquid decreases and vapor voids form. These holes may collapse quickly upon entering the high-pressure areas; What happens, for example, in industrial pumps and propellers.

The main difference is that the steam bubbles in the kettle are formed mainly by the heating of the water, not local pressure drop. Although the collapse of bubbles in both cases can create pressure waves, the mechanism of their formation is not the same. Therefore, the sound we hear from the kettle is the result of a series of oscillations created in the water and their transmission to the body and the surrounding air. The number and behavior of the bubbles, along with the characteristics of the kettle itself, determine how loud and how loud the sound is.

The behavior of the bubbles and the sound they produce is not only important in the kettle. Boiling is one of the important heat transfer processes in industrial equipment such as power plants, boilers and cooling systems. In this equipment, it is very important to check the condition of boiling, because changing its conditions can affect the performance and safety of the device.

In many of these systems, the liquid is inside tubes or closed chambers, and direct observation of the bubbles is not possible. On the other hand, temperature and pressure measurements are not always sufficient to detect all changes in the boiling process. For this reason, researchers also use other methods such as sound recording and analysis.

As we have seen, changing the behavior of the bubbles can change the characteristics of the sound produced. By installing sound or vibration sensors on the equipment, these changes can be recorded and checked. For example, researchers can measure the sound of a device under different boiling conditions and determine what patterns are associated with changing heat transfer conditions.

Today, machine learning methods are also used to analyze these data. By examining the sound patterns and comparing them with the recorded data, these methods can help identify different boiling conditions and detect some changes in equipment performance.

Of course, sound alone is not enough to accurately assess the condition of a device. The type of equipment, the location of the sensor, the type of liquid and the noise of the environment also affect the measurement results. Therefore, to achieve reliable results, acoustic data are usually analyzed alongside information such as temperature and pressure.

This issue can even be investigated on a smaller scale with a simple experiment. It is enough to record the sound of the kettle from the time it is turned on until the water boils completely. If you simultaneously measure the temperature of the water with a suitable instrument, you can compare the changes in sound with the increase in temperature.