Boiling Definition in Chemistry

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn what boiling means in chemistry and how it changes depending on atmospheric conditions. We will also learn how the boiling point changes based on what element is being boiled.

Boiling Point

Even some of the most basic recipes requires you to boil water. So most people are familiar with the phenomena when bubbles are violently popping in the water. We can use our understanding of water boiling in order to better understand the definition of boiling.

Boiling or the boiling point is the temperature when a liquid will have vaporization occurring through the entire liquid (not just on the surface). This occurs when the liquid's vapor pressure equals the atmospheric pressure.

Bubbles in the water are full of water in the gaseous state, once they reach the surface they are able to escape into the air
Boiling water

Liquids are continually vaporizing (going from a liquid state to a gaseous state) on the surface of the liquid. But this doesn't make the liquid all vaporize away because typically condensation (going from a gaseous state to a liquid state) is occurring just as quickly. If you've ever picked up a glass of water and the outside of the glass is wet it is due to this vaporization and condensation.

As temperature is increased then the vapor pressure of a liquid increases. So more vaporization can occur. Yet until boiling occurs this can still only occur at the surface of a liquid. The violent bubbles with boiling water is simply when the vapor pressure in the water has reached a point that the gaseous water needs to escape so it forms bubbles until it can escape into the atmosphere.

Boiling Point and Pressure

Remember that the boiling point of a liquid requires the liquid's vapor pressure to equal the atmospheric pressure. Yet the atmospheric pressure isn't always the same. The weather can slightly change the atmospheric pressure - a cloudy day may have a slightly lower atmospheric pressure than a clear day so the boiling point can be slightly lowered as well.

The biggest changes in atmospheric pressure comes from elevation. The higher your elevation the lower the atmospheric pressure becomes.

This is important to know because we typically state the boiling point for one atmospheric pressure. This is typically the atmospheric pressure that we find at sea level. Yet if we increase our elevation then the atmospheric pressure will decrease. Since the liquid no longer needs to increase its vapor pressure as high a lower temperature is required to reach the boiling point.

For example if we were to go to the highest point on the Earth - Mt. Everest - water would boil as low as 70 degrees Celsius. Normally water boils at 100 degrees Celsius. So the elevation can change the point boil of water by 30 degrees!

Boiling Point of Different Elements

Every element has a boiling point. For example the boiling point of oxygen is -183 degrees Celsius while the boiling point of carbon is 4827 degrees Celsius. These aren't temperatures that we can easily obtain. But it is possible to boil these elements.

Since the boiling point of oxygen is so low (well below 0) we know that any oxygen that we find in the world has already been boiled and is in the gaseous state. This is why oxygen in the atmosphere is in the gaseous state.

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