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Question: Why don't we run our motor vehicles on water instead of gas?
So why don't we burn water instead of gas to run our cars? This is simple. Water doesn't burn. This property makes water useful for putting out some fires as it can't combine with oxygen from the air to produce energy. This question falls into the field of chemistry, which is the field that studies how the building blocks of matter (atoms) can be rearranged to make new things and how energy is either given off or taken in as part of the process.
Atoms combine to form molecules, which are the smallest particles of a substance that have the properties that make that substance unique. Water is a good example as it is so common and essential. It is composed of two hydrogen atoms connected to one oxygen atom. Oxygen comprises about 20% of the air around us and we need to breathe it in order to make energy to run our body. Our bodies burn food in a controlled way so we can do things like move around, pump blood and even think. In essence, we take in food and oxygen and give off carbon dioxide and water. The carbon dioxide comes out when we exhale as does some of the water. The rest of the water leaves our body as urine and sweat.
Cars need oxygen too. They take in oxygen and when it reacts with gasoline, energy is given off that pushes our cars down the road. What comes out of a car's tail pipe is mostly carbon dioxide and water. If you hold a glass under the tail pipe of a car with its engine on you should be able to collect some of the water that is given off. Do not drink this water under any circumstances as it contains dissolved substances that are poisonous.
In a sense, water is burnt hydrogen. When the earth was very young our atmosphere contained hydrogen. As time went by it reacted with oxygen to form the water that makes up our oceans, lakes, rivers, ground water, and precipitation. Early automobiles did use water in the form of steam to make them go. In order to make the steam from liquid water, which is a physical change, coal or wood had to be burned. In gas powered cars water is the product of a chemical change as it is formed when gasoline is burned.
The gasoline that powers our cars is composed of carbon and hydrogen. On the average a molecule of gasoline contains eight carbon atoms and eighteen hydrogen atoms. The word octane is the name for this common gasoline molecule. The gasoline is the product of energy from the sun that plants use to turn water and carbon dioxide into molecules we call carbohydrates. When large masses of plant material are compressed underground for a long period of time they turn into oil. Oil is a mixture of hydrocarbons like gasoline that are separated from each other by oil companies at their refineries.
Some scientists have experimented with cars that run on hydrogen. This requires that you use energy from some other source to break hydrogen free from water molecules. Hydrogen's advantage over gasoline is that it burns clean to form water. Unfortunately hydrogen is a gas which makes storage and transportation tricky.
Other scientists are working on cars that run on electricity. While this seems clean as far as the cars are concerned, the electricity has to be made somehow which requires an energy source such as coal, natural gas, or nuclear power. Some cars also run on plant oils and alcohol made from plants. The advantage to these cars is that we can make these products here in the United States unlike much of our oil that we have to import. Just like gasoline, however, when you burn plant oil or alcohol you still get carbon dioxide and water.
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