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What Is a Chemical Reaction? - Definition & Effects

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  • 0:01 Change in Composition
  • 0:58 Permanent Change
  • 2:15 Examples
  • 3:28 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Sometimes molecules look different on the outside, but only their physical appearance has changed. Other times, through chemical reactions, molecules change both inside and out, meaning both their physical and chemical properties have changed.

Change in Composition

When you let ice melt into liquid water, you end up with a very different looking substance than what you started with. But even though these two look very different they're actually still the same thing. You started with frozen (or solid) water and ended up with liquid water, but throughout the whole process you still had water, even though the two phases look different. When a substance goes through a physical change that does not change its chemical composition we call this a physical change. Melting, freezing, and boiling all fall into this category.

Other times, though, when things look different it's because they've gone through something other than a phase change. Sometimes the composition of a substance is changed, which is what happens during chemical reactions. During chemical reactions, the molecules of a substance are broken apart and rearranged into new and different molecules. It would be like taking apart the hydrogen and oxygen atoms of water, and either leaving them broken apart, or putting them back together differently.

Chemical Properties Permanently Change

To better understand chemical reactions, it will help if we first work through the various parts of the reaction itself. First, what we start with are the reactants. These are the substances that react. You can think of these as the ingredients of the reaction - what you put in.

On the other end are the products. These are the newly formed substances, or what you produce from the reaction. Since the products have different chemical compositions than the reactants, we can expect the products to have different physical properties as well. The key is that these changes from a chemical reaction are permanent; once you've rearranged the components of the reactants into products, that's what you're stuck with.

For example, let's say you start your breakfast with raw eggs and then cook them in a skillet. In the end you have very different eggs than what you started with: not only do they look different physically, but you have also permanently changed the structure of the eggs' proteins. No longer a runny clear liquid, your eggs are solid, bright white, and very tasty!

Remember our ice cube from before? Melting it does nothing to change the chemical properties of the water itself, like we just saw with your eggs. It might be in a different phase, but you can easily re-freeze the water back into ice, which should be the same as the original ice you started with. No permanent physical changes occurred in your water.

Examples of Chemical Reactions

It can be difficult to tell if something has undergone a chemical reaction because just like during a phase change, there are changes to the physical properties of substances. One way to tell is if you can turn your substance back into its original form. If so, you probably just have a physical change. If not, you likely have a chemical reaction on your hands!

Just think about the water and the egg. You can go back and forth between frozen and liquid water as much as you like. But once you cook that egg, you're stuck with it like that.

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