In the last Science 101 blog post, we talked about the atoms, the basic building blocks of matter, and molecules. In this post, we’ll talk about chemicals, which are made up of a collection of molecules.
A chemical is any substance that has a defined composition. In other words, a chemical is always made up of the same “stuff.” Some chemicals occur in nature, such as water. Other chemicals are manufactured, such as chlorine (used for bleaching fabrics or in swimming pools). Chemicals are all around you: the food you eat, the clothes you wear. You, in fact, are made up of a wide variety of chemicals.
A chemical reaction refers to a change in a chemical. More generally, a chemical reaction can be understood as the process by which one or more substances change to produce one or more different substances. Chemical changes are different from physical changes, which don’t result in a change in substances. One example of a physical change is when water freezes into ice. While ice may have different physical properties, it is still just water. Another example is when you dissolve salt into a cup of water. While the salt may appear to disappear into the water, you still have water and salt—no substance changed into a completely new substance.
Here is one example of a chemical reaction: Iron + Oxygen → Iron Oxide
Iron oxide, also known as rust, cannot become iron or oxygen again. It is a completely new substance. In the equation, the substances on the left-hand side of the arrow are considered reactants (the substances that participate in a chemical reaction). The substance on the right-hand side of the arrow is considered a product (a substance that results from a chemical reaction). It’s important to note from this example that no material is “lost” in the reaction. On one side of the equation you have iron and oxygen; on the other you still have iron and oxygen (now just combined into one chemical).
In that sense, this example illustrates what is known as the law of conservation of mass. By “law,” we mean a general rule of how something works or how something occurs. This description is considered to be extremely reliable due to a large amount of supporting experimental testing and observation. Considering the given example, the law states the products of a chemical reaction have the same mass (“stuff”) as the reactants. In other words, while things are rearranged, nothing is created or destroyed.
Here are some ways to tell if a chemical change is occurring:
1. You might notice bubbling or a change in odor, indicating the production of a gas. Such is the case when baking soda is mixed with vinegar.
2. When two clear solutions are mixed together and the resulting mixture is cloudy (due to the presence of some solid substance now in the liquids). This is known as the formation of a precipate.
3. A change of color (like in our rust example).
4. A change in temperature or if light is produced, such as with fire.
While any of the above may be evidence of a chemical change, physical changes can have some of the same effects. One way to determine the difference between the two is to think about whether the new substance could be physically separated back into its original parts—in other words, if the involved matter could “go back” to how it originally was.
The author has a bachelor’s degree in Nuclear Engineering and a master’s degree in Reliability Engineering.
U.S. NRC Blog 
Thank you. We fixed it!
Moderator
typo in the word “just”
We can simply put the idea of chemical in this way. Chemical can be gas, liquid, and solid. It cannot be separated easily.
Chlorine as a basic element is gaseous and poisonous. It is also doesn’t exist in its basic elemental state naturally on Earth. Chlorine (as a compound and as a basic element) is manufactured. (Reference http://www.bt.cdc.gov/agent/chlorine/basics/facts.asp) Just because something is an element, does not mean that we can’t create it. In fact, at least 20 elements on the periodic table do not exist on Earth, but are considered “synthetic” elements, as they were created artificially. Actually just last month, a new element was created/discovered. (http://www.cnn.com/2013/08/28/world/europe/new-chemical-element/index.html) Oftentimes, isotopes used for medical purposes are created at reactors, such as the Missouri University Research Reactor, because the natural occurrence of isotopes is low here on Earth.
I do concede, though, that the more correct term, for the examples given, would be chlorine compound (such as calcium hypochlorite, which is manufactured for use in swimming pools or as a bleaching powder).
Suzanne Schroer
Great point. You are correct. We didn’t mean to perpetuate any myths. Salt into water is considered a chemical reaction because of the ionic dissociation. It is one of the trickier reactions to identify because one of the common tests to differentiate between physical and chemical changes (can it revert back to its original state) does not hold true for salt water because the ions recombine. Thanks for making sure we had this straight!
Suzanne Schroer
Dissolving salt into water, while a useful explanation, is not quite accurate, as ionic dissociation occurs as solid NaCl becomes free Na+ and Cl- ions. Perhaps a more useful example may have been dissolving sugar into water, as the hydrophilicity of the -OH groups enable sugar’s solubility.
This sort of nuance is actually what you state in 4, regarding temperature change – many chemical species have associated enthalpies of solution. A simple one is the reaction between acid and water – if you remember back to general chemistry lab, and they mention to always add acid to water, and not the other way around. This is because many acids have highly exothermic reactions when dissociating in water.
Excellent article!
Sorry, I worded my reply poorly.
An element is in fact a chemical. It is, however, incorrect to say that chlorine is “manufactured,” as it’s a basic element.
Steven Dolley
Managing Editor, Inside NRC
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Chlorine is not a “chemical” by your definition. Compounds containing chlorine might be manufactured, but chlorine per se is a basic element. See periodic table, Cl.
Steven Dolley
Managing Editor, Inside NRC
Platts
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202-383-2166 Office
202-383-2187 Fax