In love with mathematics and physics

Author: Zhang Guijun
Publisher:
Publish Date: 2006-10-01
Features: Why Boiling Water Makes Noise, But Boiled Water Doesn't
In daily life, experienced people can tell if water is boiling by listening to the sound in the kettle. When the noise is loud, the water hasn't boiled yet, but after a while, the noise decreases, and the water finally boils. What's the reason behind this?
When the water is heated to about 50°C, a "hiss" sound is produced. This is because the bottom part of the kettle heats up first, causing the air dissolved in the water to expand and form small bubbles that attach to the bottom and sides. As heating continues, these bubbles grow larger, and the surrounding water evaporates into steam, entering the bubbles. With increasing temperature, some bubbles rise due to buoyancy. At this point, the upper part of the container is cooler than the lower part, so the bubbles gradually shrink in size as they rise, and the buoyant force acting on them decreases. During the ascent, most bubbles burst under water pressure and produce noise. These sounds travel through the water to the kettle, causing resonance and creating a loud noise, but the water hasn't actually boiled. This is the principle behind "boiling water makes noise."
When the water boils, the temperature is uniform throughout the container, so the bubbles formed by water vapor at the bottom detach and rise without being submerged. Instead, they are replenished by surrounding water vapor, growing larger until they burst at the surface. At this point, the noise is smaller, which explains "boiled water doesn't make noise."
How Is Rain Induced Artificially?
Artificial rainfall involves dispersing catalysts in clouds or around them when natural rainfall conditions are insufficient, creating conditions to promote rainfall. Currently, artificial rainfall is divided into cold-cloud and warm-cloud rainfall.
Cold-cloud rainfall relies on the ice-crystal effect, primarily depending on the presence and quantity of ice crystals. The goal of catalyzing cold-cloud rainfall is to supplement ice crystals. Common methods include:
1. Introducing cooling agents, such as dry ice, into the cloud to rapidly cool the water vapor and form ice crystals.
2. Introducing artificial ice crystals, such as silver iodide, into the cloud.
When there are enough ice crystals in the cloud, water droplets easily collide with and adhere to them, causing the ice crystals to grow larger. This is the ice-crystal effect. Once the ice crystals grow to a certain size, they begin to fall. During the descent, they collide and merge with each other, and moisture in the air continuously condenses on their surfaces, causing the ice crystals to grow larger until they are no longer supported by updrafts, at which point they fall. At a certain height above the ground, the ice crystals melt due to heat, forming rain.
Warm-cloud rainfall primarily depends on the quantity of large water droplets in the cloud. The goal of catalyzing warm-cloud rainfall is to create conditions for forming many large water droplets. Common methods include:
1. Introducing hygroscopic substances, such as salt or urea, which continuously absorb water vapor in the cloud to form water droplets.
2. Introducing large water droplets directly, allowing them to collide and merge with small water droplets in the cloud, forming numerous large water droplets to achieve the goal of rainfall.
China commonly uses catalysts such as dry ice, salt, urea, silver iodide, ethylene acetal, and tetraethylene acetal. Among these, ethylene acetal is inexpensive, has a readily available raw material source, and exhibits a significant catalytic effect, making it a relatively good catalyst.