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Water is the common name for dihydrogen monoxide or H2O. The molecule is produced from numerous chemical reactions, including the synthesis reaction from its elements, hydrogen, and oxygen. The balanced chemical equation for the reaction is:
2 H2 + O2 → 2 H2O
How to Make Water
In theory, it's easy to make water from hydrogen gas and oxygen gas. Mix the two gases together, add a spark or sufficient heat to provide the activation energy to start the reaction, and presto-instant water. Merely mixing the two gases at room temperature, however, won't do anything, like hydrogen and oxygen molecules in the air don't spontaneously form water.
Energy must be supplied to break the covalent bonds that hold H2 and O2 molecules together. The hydrogen cations and oxygen anions are then free to react with each other, which they do because of their electronegativity differences. When the chemical bonds re-form to make water, additional energy is released, which propagates the reaction. The net reaction is highly exothermic, meaning a reaction that is accompanied by the release of heat.
One common chemistry demonstration is to fill a small balloon with hydrogen and oxygen and to touch the balloon-from a distance and behind a safety shield-with a burning splint. A safer variation is to fill a balloon with hydrogen gas and to ignite the balloon in the air. The limited oxygen in the air reacts to form water but in a more controlled reaction.
Yet another easy demonstration is to bubble hydrogen into soapy water to form hydrogen gas bubbles. The bubbles float because they are lighter than air. A long-handled lighter or burning splint at the end of a meter stick can be used to ignite them to form water. You can use hydrogen from a compressed gas tank or from any of several chemical reactions (e.g., reacting acid with metal).
However you do the reaction, it's best to wear ear protection and maintain a safe distance from the reaction. Start small, so that you know what to expect.
Understanding the Reaction
French chemist Antoine Laurent Lavoisier named hydrogen, Greek for "water-forming," based on its reaction with oxygen, another element Lavoisier named, meaning "acid-producer." Lavoisier was fascinated by combustion reactions. He devised an apparatus to form water from hydrogen and oxygen to observe the reaction. Essentially, his setup employed two bell jars-one for hydrogen and one for oxygen-that fed into a separate container. A sparking mechanism initiated the reaction, forming water.
You can construct an apparatus the same way as long as you are careful to control the flow rate of oxygen and hydrogen so that you don't try to form too much water at once. You also should use a heat- and shock-resistant container.
Role of Oxygen
While other scientists of the time were familiar with the process of forming water from hydrogen and oxygen, Lavoisier discovered the role of oxygen in combustion. His studies eventually disproved the phlogiston theory, which had proposed that a fire-like element called phlogiston was released from matter during combustion.
Lavoisier showed that a gas must have mass for combustion to occur and that the mass was conserved following the reaction. Reacting hydrogen and oxygen to produce water was an excellent oxidation reaction to study because nearly all the mass of water comes from oxygen.
Why We Can't Just Make Water?
A 2006 report by the United Nations estimated that 20 percent of people on the planet don't have access to clean drinking water. If it's so hard to purify water or desalinate sea water, you might be wondering why we don't just make water from its elements. The reason? In a word-BOOM!
Reacting hydrogen and oxygen is basically burning hydrogen gas, except rather than using the limited amount of oxygen in the air, you're feeding the fire. During combustion, oxygen is added to a molecule, which produces water in this reaction. Combustion also releases a lot of energy. Heat and light are produced so quickly that a shock wave expands outward.
Basically, you have an explosion. The more water you make at once, the bigger the explosion. It works for launching rockets, but you've seen videos where that went horribly wrong. The Hindenburg explosion is another example of what happens when a lot of hydrogen and oxygen get together.
So, we can make water from hydrogen and oxygen, and chemists and educators often do-in small quantities. It's not practical to use the method on a large scale because of the risks and because it's much more expensive to purify hydrogen and oxygen to feed the reaction than it is to make water using other methods, to purify contaminated water, or to condense water vapor from the air.