Cooking invariably involves the boiling of water at regular intervals of time. It’s an absolutely integral part of cooking. You put water in a container, keep it on the stove, and cover it (for maximum utilization of heat energy), and when the water starts to make noise and the cover begins to tremble, you know that the water has successfully boiled. Then, you take the boiled water and put… WAIT! Has it ever crossed your mind why water is the loudest just before it boils?
You may already know that (under standard conditions) water boils at 100 Degrees Centigrade, but what actually happens at 100 Degrees Centigrade that makes the water boil? At that specific temperature, water molecules gain enough energy from the heat to break free from the bonds that hold them together (intermolecular forces) and assume a gaseous form.
But just as water boils, it’s particularly loud. Why does that happen?
When water is heated, air dissolved in water forms bubbles at the bottom of the container. After attaining a specific size, these bubbles are released from the bottom to reach the top layer of water in the container. A little contributor to the sound that you hear just before water begins to boil comes from these bubbles, namely when they are created at the bottom. Although this sound is very low (a sound of frequency of ~ 100 Hz), it is still a contributor to the overall sound.
Upon being released from the bottom of the container where they are created, they start to move up through layers of water that are much cooler than the layers at the bottom. However, in the process, these bubbles lose sufficient energy, become cooler and therefore start to collapse. When a bubble collapses (or disappears, leaving a cavity), the water around it sort of ‘slaps into itself’. This process is known as cavitation, and is quite noisy and destructive in nature.
However, as the container continues to be heated, these bubbles become bigger and collapse harder until they get hot enough to reach the surface of the water. At this point, they pop gently and the noise is quelled. Eventually, when all the steam has made it to the top, the noise stops altogether, which is why once the water has boiled, it becomes quiet.
This is one of those everyday phenomena that we observe all the time, but barely pay it any attention. So, next time you are busy doing something entirely different than cooking in the room next to the kitchen, you can make use of this audible cue of nature to determine if the water is about to boil and save yourself from that last-minute dash to the kitchen to shut off the stove and avoid an overflowed pot!
- Boiling For Making Water Potable – Wikipedia
- What Causes Bubbles To Form When Boiling Water? Where Are The Bubbles Coming From? – Science Line (University of California, Santa Barbara)