8000万人进城之后－时事观察－时政频道－中工网

Boiling water

百度 “我会一辈子陪在他的身边。

Boiling is a process where a liquid changes into a gas or vapor when it gets hot enough. This happens at a special temperature called the boiling point. Unlike evaporation, which only happens at the surface of the liquid, boiling happens throughout the whole liquid. This happens because bubbles of vapor form inside the liquid and rise up to the surface. When the bubbles reach the surface, the gas escapes into the air or the space around it. The temperature at which a liquid boils can change depending on the pressure around it. For example, at normal air pressure (like at sea level), each liquid has its own boiling point. But if you go higher up in the mountains where the air pressure is lower, the boiling point gets lower too.

Boiling is something we see every day and is very important in many jobs and industries. For example, we use boiling when we cook food, clean things by killing germs, separate liquids in distillation, and make chemicals. Boiling starts when the pressure of the vapor inside the liquid becomes the same or higher than the pressure around the liquid. This lets bubbles form inside the liquid. These bubbles get bigger and rise to the surface, which makes the liquid bubble and boil. When a liquid boils, it uses a special kind of heat called the latent heat of vaporization. This heat helps change the liquid into gas, but it does not make the temperature go up while boiling is happening.

Boiling is a bit more complicated than just bubbles rising in a pot. It involves how heat moves, how the liquid flows, and how bubbles start to form. There are different types of boiling. One type is called nucleate boiling, where tiny bubbles form on special spots on a hot surface. Another type is film boiling, where a thin layer of vapor forms all over the surface and stops the heat from moving into the liquid easily. The switch between these types depends on things like how rough the surface is, how hot it is, and what kind of liquid is used. Knowing about these boiling types is really important for making machines like heat exchangers and nuclear reactors, where it is important to remove heat safely and quickly.

Scientists study boiling using different areas of science like thermodynamics, fluid mechanics, and heat transfer. These fields help explain how heat and liquids behave when boiling happens. One important idea is called the Clausius-Clapeyron relation. It is a math formula that shows how the boiling point of a liquid changes when the pressure around it changes. This helps us understand when a liquid will turn into gas. Boiling starts with tiny bubbles forming inside the liquid. The way these bubbles form is explained by something called nucleation theory. Sometimes bubbles form inside the liquid itself (this is called homogeneous nucleation), and other times they form on surfaces or tiny particles in the liquid (this is called heterogeneous nucleation).

The environment around a liquid can change how it boils. For example, at high places like mountains, the air pressure is lower. This means liquids boil at lower temperatures, so cooking might take longer or be different than at sea level. On the other hand, if you increase the pressure, like inside a pressure cooker, the boiling point goes up. This lets food cook faster because the liquid gets hotter before it boils. Scientists and factories sometimes boil liquids in places with very low pressure, called a vacuum. This helps the liquid turn into gas at lower temperatures, which is useful when they want to protect delicate materials that could be damaged by heat.

Uses for boiling

• Food can be cooked by boiling.
• Germs in a liquid (for example, water) can be killed by boiling the liquid.
• Germs on an object can be killed by putting the object in boiling liquid.

Related pages

• Evaporation