chemical change


Chemistry. a usually irreversible chemical reaction involving the rearrangement of the atoms of one or more substances and a change in their chemical properties or composition, resulting in the formation of at least one new substance: The formation of rust on iron is a chemical change.



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Compare physical change. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020


What is a chemical change?

A chemical change happens when one chemical substance is transformed into one or more different substances, such as when iron becomes rust.

Chemical changes occur through the process of chemical reactions, and the resulting substances have different properties because their atoms and molecules are arranged differently.

A chemical change is different from a physical change, which doesn’t rearrange atoms or molecules and produce a completely new substance. Ice melting into water is an example of a physical change.

Chemical change vs. physical change

Chemical changes are all around us. Without them, we couldn’t breathe. Plants couldn’t make food from sunlight. We couldn’t drive a car if it weren’t for chemical reactions. We couldn’t even fry an egg without a chemical change.

A chemical change is usually contrasted with physical change. So, let’s start there. A physical change involves only a change in the physical makeup of a substance, not its chemical makeup. While the substance may look very different, deep down it has the same atoms and molecules arranged together in the same way.

Ice melting into water is a common physical change. Ice and water look very different from each other, but they have the exact same chemical structure. This means that ice and water are identical chemical substances, but thanks to temperature, they are simply in different physical states of matter. The physical state of ice, of course, is solid, and water is liquid.

Now, let’s get chemical. The structure of water is represented as H2O, two hydrogen molecules bonded to one oxygen molecule. That stays the same whether water takes the form of ice, steam, or liquid—and that’s why going from ice to water is only a physical change.

A more complex physical change is the dissolving of salt into water. This seems as if it should be a chemical change, right? Where did the salt go?! But, the original salt crystals are still there, just in a different physical state. Just as we can melt solid ice to get liquid water again, so can we boil away the water from a pot of salted water to get the salt back out, recrystallized. (Take out the pasta, first!)

This uncomplicated reversibility from one state to another is typical of physical changes. Not so for chemical changes though. In a chemical change, you always get something wholly new. The atoms and molecules in the substances you started with become completely reshuffled.

Remember how water gets frozen or boiled but still contains one oxygen molecule for every two hydrogen molecules? Not so when something like water meets something like sodium, where two new substances are created. Basically, when you introduce pure sodium metal (Na) to water (H2O), you end up with sodium hydroxide (NaOH) and hydrogen gas (H2). It’s like magic.

The resulting molecules are put together differently from the original molecules. NaOH, which can badly burn your skin, and H2, which can be highly explosive, are not at all the same as Na and H2O. Clearly, chemical changes have taken place, and it would require a very complex process to separate the new molecules of sodium hydroxide and hydrogen gas back out into plain sodium and water.

Rust is the result of a very familiar chemical change. In simplified terms, rust occurs when iron (Fe) reacts with oxygen (O2), forming a new substance. Most of us call this substance rust. Scientists call it iron oxide (Fe2O3).

So how do you know if a chemical change, and not a physical change, has happened?

  1. A chemical reaction is involved. When this is happening, the changing substance(s) may give off heat and/or light, or may bubble and/or fizz. Combustion (the process of burning) is a sure-fire sign of chemical change.
  2. The resulting substance(s) have chemical properties that differ from those of the original substance(s). Such properties might include flammability or a specific reaction with water. That’s because chemical changes produce substances with different chemical makeups.

One thing to note is that chemical changes can also result in color changes. Color is very much a physical property—along with size, shape, smell, boiling point, density, etc. But, the only way to be certain that a difference in color is the result of chemical change is to determine that the new substance has a new chemical structure.

What are real-life examples of chemical change?

People have been pursuing the ability to change one substance into another for a very long time. From antiquity into the Middle Ages, alchemists attempted to change base metals into gold.

History time. The Arab scientist Jabir ibn Hayyan (also latinized as Geber) is commonly considered the father of chemistry for his influential experiments in the 700s CE. In the 1600s, with the rise of modern chemistry in the West, scientists began to understand chemical changes and discuss them in a scientific way. A noteworthy moment, in the late 1700s, was Antoine Lavoisier’s explanation of combustion in terms of chemical reactions.

The term chemical change is used throughout the many specialized fields of chemistry, including organic chemistry, inorganic chemistry, biochemistry, and electrical chemistry. Not surprisingly, chemical changes are discussed and analyzed by scientists and science teachers and written about in science textbooks and articles, but does the subject ever come up in just ordinary, everyday life? Absolutely! Check it out:

While many of us may not use the words chemical change outside of science class, these changes are occurring all around us. Keep an eye out.