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Comparing Reversible & Irreversible Physical & Chemical Changes

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  • 0:01 Understanding Change
  • 0:35 Physical Properties & Changes
  • 1:59 Reversibility of…
  • 3:00 Chemical Properties & Changes
  • 4:03 Reversibility of…
  • 4:43 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

A substance that changes form has undergone a physical change, which might be reversible. A substance that transforms into a different substance has undergone a chemical change, which is harder to reverse. Learn about reversible and irreversible changes.

Understanding Change

For this lesson we will be baking a cake. Don't worry; you didn't stumble on to a cooking video by accident. We will be using the cake to help you understand physical and chemical changes, and show you which of these changes can be reversed and which cannot.

For our demonstration we will need some ingredients, like eggs, sugar, butter, flour and, of course, chocolate frosting. As for tools, all we really need is a baking pan, bowl and a wooden spoon to stir everything together.

Physical Properties & Changes

Let's start with the eggs. As you can see, each of our eggs is white and has some mass, which we know because it's an object that we can hold in our hand. Characteristics like color and mass are called physical properties, which we can define as characteristics that can be seen and measured. We can identify physical properties for all types of matter, including all of the ingredients and tools we will be using to bake our cake. For instance, our wooden stirring spoon has density, which we can measure by determining its mass in relation to its volume. Because density can be measured, we know that it's a physical property.

Changes that affect physical properties are called physical changes. With a physical change we see that a substance changes form but does not transform into a different substance. For example, to use an egg in our recipe, we need to crack it open. This is a physical change because the egg is now in two parts, but the egg did not transform into a chicken, or into any other substance for that matter; it is still an egg. If we were to take our wooden spoon and grind it down to sawdust, we could see another example of physical change. The spoon would change form, but it would still be wood.

Reversibility of Physical Changes

Sometimes physical changes can be reversed, and sometimes they cannot. We just looked at two examples of physical changes that could not be reversed, namely the cracked egg and the ground-up wooden spoon. We learned from the Humpty Dumpty nursery rhyme that you cannot put a broken egg back together again. The same goes for the wooden spoon; it would be impossible to change the sawdust back into a wooden spoon.

But, don't get fooled into thinking that all physical changes are irreversible. In fact, many physical changes can be reversed. Remember that when a substance changes physically, it is still the same substance, just in a different form. Water turning into ice is a great example of a reversible physical change. Water, which we know is made up of two hydrogen molecules and one oxygen molecule (H2O), can be frozen, but that doesn't change the fact that it is still H2O. If you let it thaw, you could drink it again.

Chemical Properties & Changes

Let's keep baking so we can learn about chemical properties, which are not as easy to see as physical properties. Chemical properties are characteristics of a substance that can only be observed during a reaction. For example, our wooden spoon will burn, but we can only know that for sure if we light it on fire and watch it burn. Once the wood burns, it transforms into ash. So, we see that with chemical changes, a substance transforms into a different substance. In other words, the identity of the object that undergoes a chemical change gets altered; wood changed into ash, which is an entirely new substance with new chemical properties.

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