When water is brought to a boil, air bubbles form due to the decrease in the solubility of oxygen and other gases dissolved in the water as the temperature increases.
When you heat water in a saucepan, initially, the heat transfers from the bottom to the top of the container. This thermal energy increases the agitation of the molecules, which then begin to move faster and faster. Once the water reaches 100 °C (at normal atmospheric pressure), it becomes so intense that the liquid water transforms into steam. This steam appears in the form of small bubbles that generally form on the imperfections or scratches at the bottom of the container. Why in these specific spots? Simply because these small defects are easy points where the steam can start to cling and form bubbles visible to the naked eye. When the bubbles become large enough, they quickly rise to the top and escape into the air. That's why you see these clusters of bubbles forming at the bottom and rising like a little army to the surface when the water is boiling!
When you heat water, its molecules start moving faster and faster. The more they move, the more they accumulate energy. Eventually, this agitation allows the molecules to overcome the attractive force that holds them together in a liquid state. When this tipping point is reached, water transitions from a liquid state to a gaseous state: this is what we call steam. The bubbles you see forming in your pot of hot water are simply pockets filled with this water vapor. The higher the water temperature, the more these little steam bubbles appear, grow, rush to the surface, and burst, releasing the vapor into the air.
Atmospheric pressure acts like a weight pushing down on the surface of the water. When it's high, the water needs more energy (thus a higher temperature) to form bubbles and boil. Conversely, when you go up in altitude, the pressure decreases. As a result, water boils at a lower temperature, so bubbles form more quickly even though the water is less hot. That's why, at the top of a mountain, cooking pasta takes longer: the water boils faster, indeed, but it's less hot, so cooking is slower.
The water we commonly use often contains small amounts of air or other dissolved gases, naturally captured during its time in the atmosphere or underground. When we start to heat it, these gases feel cramped: the heat decreases their ability to remain dissolved, and they prefer to escape and form tiny bubbles, often well before a full boil. These first bubbles that appear when heating the water are often just simple gases being released, not yet pure water vapor. That's why you sometimes see a few bubbles at the bottom of your pot, long before a good rolling boil.
Impurities in water give a serious boost to bubble formation. Why? Because these tiny particles provide perfect spots for bubbles to form easily. These spots are called nucleation sites. Without these little "bubble anchors," water could even become slightly superheated, meaning it exceeds 100°C without boiling right away (and that’s quite risky). For example, a slightly scratched pot or a bit of limescale at the bottom? Boom, immediate bubbles. Conversely, an ultra-clean pot and ultra-pure water make bubble formation more difficult, to the point of delaying boiling a bit.
The bubbles you see at the beginning of the water heating process are not always water vapor. Initially, they often consist of dissolved gases such as oxygen, nitrogen, or carbon dioxide that escape from the heated water.
Adding salt to water does not make it boil faster, as many people think, but it slightly increases its boiling point. Therefore, your salted water takes a bit longer to boil, but it will cook your pasta more quickly once it reaches a boil!
The phenomenon of bubbles during boiling can also depend on the container used. A damaged container with micro-cracks or imperfections often facilitates the formation of bubbles, unlike a perfectly smooth container which will delay their formation.
A superheated liquid can slightly exceed its boiling point without forming bubbles. However, as soon as the first bubble forms, it triggers a violent and potentially dangerous boiling phenomenon known as 'explosive boiling' or 'violent superheating'.
Yes, the boiling point of water depends on atmospheric pressure. At high altitudes, where the pressure is lower, water boils at temperatures below 100°C. This is why it is often necessary to adjust cooking times in mountainous regions.
Salt raises the boiling point of water through a phenomenon called boiling point elevation, which is due to the presence of additional ions that make it more difficult for water molecules to escape as gas. However, this effect is generally limited in practical terms and typically increases the temperature by only a few tenths of a degree.
Increasing the intensity of the fire under the pot does indeed speed up the heating of the water and allows it to reach boiling more quickly. However, this does not change the boiling point, which mainly depends on atmospheric pressure; thus, water cannot exceed 100°C at normal atmospheric pressure.
As the temperature increases, the amount of water vapor produced grows rapidly, leading to the formation of large water vapor bubbles. Due to the heat, these enlarged bubbles rise quickly and burst at the surface.
The characteristic sound heard accompanies the rapid movement of steam bubbles forming and bursting in the boiling water. When these bubbles burst near the surface or move turbulently through the hot water, they create audible vibrations.
These small bubbles primarily come from dissolved gases, such as oxygen and nitrogen, which are gradually released as the temperature of the water increases. Before actual boiling occurs, these bubbles rise slowly due to their small size and the small amount of water vapor they still contain.
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