Henry’s Law

Mar 9, 2018

Environmental Enlightenment #120

We use Henry’s Law to extract contaminants from soils and groundwater.

In the diagram below, the two containers have liquid in the bottom and gas above the liquid. Pressure P2 in the right-side container is greater than P1 on the left.

At P2, more gas dissolves into the liquid.

It is a dynamic equilibrium: at every instant, the number of molecules jumping into the air equates the number diving into the water.

At P2, more gas dissolves into the liquid.

It is a dynamic equilibrium: at every instant, the number of molecules jumping into the air equates the number diving into the water.

Gas is a substance that has no definite shape or volume, with high energy particles flying continually at random.

Gas is one of the three basic states of matter (solid, liquid, or gas) where matter can expand indefinitely to completely fill its container.

Oxygen, nitrogen, carbon dioxide, hydrogen sulfide, superheated steam, chlorine, and radon are gasses, to name a few.

Substances are made of atoms and molecules. These particles are in continual motion — slow in solids, faster in liquids and faster in gasses. The particles collide with each other and collide with the walls of the containers. These collisions create pressure. (PB is the pressure inside the balloon.)

In the open atmosphere the pressure of a gas is exerted by its weight over a given area. At sea level, air places a weight of 14.7 pounds on every square inch. That is 500 pounds of air pressing on your head.

The reason your head has not sunken between the shoulders is that we live in a sea of air, where the pressure equalizes on all sides. (If you should examine individual tissues of your body you’d find them squeezed at the rate of 14.7 pounds on square inch of each side).

There are a number of gases which make up air. Of these, nitrogen and oxygen make up 78% and 21% respectively. Thus, in atmospheric air, oxygen contributes 21% to the total pressure, and it will dissolve in open-face water in accordance with this pressure.

If you were to create an atmosphere of 100% oxygen above water, oxygen would dissolve 5 times as much, but as soon as you placed that oxygen-impregnated water in open air, the oxygen would escape until an equilibrium was struck between that which is dissolved in the water and that which is hovering above.

It is a dynamic equilibrium: at every instant, the number of oxygen molecules jumping out would equate the number diving in.

We say that when the above equilibrium is reached, the pressure of the oxygen inside the water equates its pressure outside in the air.

A volatile substance is one that is capable of being evaporated or changed to a vapor at a relatively low temperature. A vaporized substance behaves like gas.

Water, ether, alcohol, gasoline, turpentine, benzene, paint solvents, are all volatile. The volatility of a substance is the speed at which its molecules collide internally. Some are more volatile than others.

Volatile substances dissolve in water. The dissolved substance will stay in the water as long as the pressure it creates inside the water does not exceed the pressure of that same substance in the environment surrounding the water.

For example, a gasoline tank leaks. The gasoline trickles down to groundwater and partially dissolves in the water. Just above the water, the soil is impregnated with gasoline vapors. An equilibrium ensues. If you now suck the gasoline from the soil, the equilibrium is no more and gasoline will start moving out of the water. We use Henry’s Law to design the vacuum system.

The physical universe does not tolerate excellence. The goal is to move all present into a dispersed equality of apathetic inaction, expressed well in the laws of energy, thermodynamics and nuclear physics, and manifested in every crumbled rock, alluvial fan and fossiled organism. Henry’s Law is just another expression of same. It is only Life that makes the antidote, elevating dirt into animated states of being, capable of a predetermined effort converting dust into computer chip, space shuttle and giant metropolis.


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