Everest in syringe – an experiment you can make at home

April 25, 2015 , Vassili Philippov

The weekend is coming. Here is a simple idea of how to make an amazing experiment at home, along with your kids and using makeshift materials. Do you remember the question of whether it takes more or less time to cook an egg on top of Mount Everest? Let’s check it out right now! —

The Answer

Let me start right with the answer: it will take more time to cook an egg on the top of Mount Everest as the boiling point is lower.

But Why?

As soon as we give such an answer, the following question immediately follows:

Why is it lower?
Because atmospheric pressure there is lower too.
But why is the boiling point lower when atmospheric pressure is lower?

The question “why …” is a very powerful and correct question. This is the key for a child and an adult to understand the world. I know a very few people who can stand over five consecutive clarifying questions “why …” in a row. But we will talk about it later.

Cold Boiling Water at Home

And now let’s get back to the experiment that you can do with your children this weekend. At home, we will now create conditions like those atop Everest and see with our own eyes how not even the hottest water boils with little pressure. All you need is a conventional syringe – the bigger, the better. Next:

Fill the syringe with some warm water
Let all the air out
Plug with a finger
Pull the piston aside to lower the pressure
Yay! The water has boiled!

In fact, this way we can get a very low pressure, hundreds of times lower than the atmospheric pressure. We can boil even noticeably-colder water than on Everest. At the top of Mount Everest, the pressure is about one third of the normal atmospheric pressure.

Why does water evaporate?

So, why exactly does warm water boil when the pressure decreases? To answer this question, it is reasonable to ask why water boils in general in the first place?

Water evaporates at all temperatures. Among the trillions of water molecules present, there are always some that are faster. The speed of these molecules is enough to overcome the attraction of other molecules, and fly away in the form of steam just as they approach the surface. We should note that the fastest molecules fly in the form of steam; so, the average speed of the rest of the molecules decreases. Generally, we say that water gets colder when it is evaporating. For example, if all the best pupils of a class were shifted to another school suddenly, the average class level is bound to decrease a little.

Higher the water temperature, faster the molecules fly on average. Greater the percentage of molecules that has sufficient speed to break and fly away while approaching the surface, faster the water evaporates.

Why does water boil?

But, why does water begin to boil at a certain temperature? What is so special in this particular temperature?

To understand this in a better way, it is necessary to realize that water evaporation from the surface is not the only contributing factor. There is another parallel process where water vapor condenses. Larger the amount of water vapor above the surface, more the probability of vapor molecules going into the water and “sticking” there.

As water evaporates, water vapor keeps increasing to a point where equilibrium is reached between the rate of evaporation of water and the rate of condensation of vapor. In other words, the number of water molecules that fly away from the surface in a second is equal to the number of molecules that fly back. Generally, we say that vapor becomes saturated. Warmer the temperature of the water is, larger the number of molecules leaving the surface. Consequently, similar numbers of molecules return from the gas form to meet the equilibrium. Number of molecules of water in gas is directly proportional to the saturation of the vapor pressure.

There is a special vapor pressure for each temperature of water, at which the vapor becomes saturated; as we have seen, pressure increases when the temperature increases. Precisely here is the moment of truth! What will happen if the vapor pressure surpasses that of the atmosphere? That could happen: if a small bubble were to form in the water somehow, evaporation could start inside that bubble. Since the pressure of the saturated vapor is greater than the atmospheric pressure, the bubble will grow, which, otherwise, will collapse by the atmospheric pressure. The larger size of the bubble would mean greater surface area, which will consequently mean faster water evaporation into the bubble. This bubble has risen, floated and then flown away.

This is what we call boiling of water. To get water to boil, it is necessary that the atmospheric pressure doesn’t collapse the formation of small bubbles. This goes to mean that the saturation vapor pressure for the current water temperature is greater than that of the atmosphere.

That would be all. That is exactly why water boiled in our syringe. We had removed the atmospheric pressure; so, nothing prevented the little bubbles from growing due to the evaporation of water inside them.

Experiments with your children at home

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