NRC Science 101 – What are Chemicals?

Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors
 

science_101_squeakychalkIn 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.

chemistrylab2Here 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.

NRC Science 101 – What is an Atom?

Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors

science_101_squeakychalkWelcome to the NRC’s new blog series, Science 101. Over the course of this series, NRC experts will discuss various scientific principles, with some of the later posts relying on principles and ideas discussed in earlier ones. So, in that sense, this blog series will play out much like a textbook, with each post (or chapter) building upon the previous one.

We hope the information in this series will be helpful to teachers, students and the public who want to better understand the science behind the NRC.

So where do we begin? The topic for today’s post is the atom. It’s considered the basic building block of matter.

Anything that has a mass—in other words, anything that occupies space—is composed of atoms. While its name originally referred to a particle that couldn’t be divided any more—the smallest thing possible—we now know that each atom is generally made up of smaller particles. Given that these particles make up atoms, they are often referred to as subatomic particles. There are three subatomic particles: protons, neutrons and electrons.

Two of the subatomic particles have electrical charges: protons have a positive charge while electrons have a negative charge. Neutrons, on the other hand, don’t have a charge. A fundamental rule is that particles with the same charge are repulsed from each other, while particles with opposite charges are attracted to each other. So, much like opposite ends of a magnet, protons and electrons are attracted to each other. Likewise, just as when you experience resistance trying to push the same ends of two magnets together, protons are repelled from other protons and electrons are repelled from other electrons.

atom1The nucleus (or center) of an atom is made up of protons and neutrons. The number of protons in the nucleus, known as the “atomic number,” primarily determines where that atom fits on the Periodic Table. The number of protons in the nucleus also defines in large part the characteristics of an atom—is it a gas or a metal, for example.

Two atoms with an identical number of protons in their nuclei belong to the same element. An element, like hydrogen, oxygen or iron, is a substance that cannot be broken down—outside of a nuclear reaction—into anything else. In other words, one element cannot be transformed into another (again, with the exception of nuclear reactions).

Now, while the protons are the same in an element, the number of neutrons may vary from atom to atom. The number of neutrons determines what isotope an atom is. This is important to the NRC because the number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay. While radioactive decay can occur in a variety of ways, it is, simply put, the process by which unstable atoms break down, releasing particles (and energy).

Generally speaking, atoms with roughly matching numbers of protons and neutrons are more stable against decay. Given how important it is to the work of the NRC, the concept of radioactive decay will be taken up as part of a future blog post.

The nucleus of an atom is surrounded by a cloud of electrons. Remember, electrons are negatively-charged and are attracted to the positively-charged protons in the nucleus. An atom is considered to be electrically neutral if it has an equal number of protons and electrons. If an atom has a different number of electrons and protons, it is called an ion.

An important principle to know is electrons may be transferred from one atom to another or even shared between atoms (allowing atoms to bind together). These bonds allow for the formation of molecules, combinations of atoms (including those of different elements). Just as several atoms make up a molecule, many molecules make up a chemical. Chemicals will be the subject of our next Science 101 post.

The author has a bachelor’s degree in Nuclear Engineering and a master’s in Reliability Engineering.