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What are the differences between physical and chemical changes?

After a substance undergoes a physical change, the resulting product is still the same substance you started with. For instance, if you tear a piece of paper into hundreds of smaller pieces, you have only changed it physically. It is still the same paper—only the size of the pieces have changed. When water is found as ice (solid), liquid water (liquid) or water vapor (gas), it is still the substance water. These changes are merely physical changes which have changed the state the matter is in, but the water is not changed in any way chemically because it is still has the formula H2O. Mixtures are another example of a physical change, like saltwater. By boiling it, the salt remains in the pot but the water evaporates and can be recaptured as pure water.


A chemical change involves a chemical reaction. Here, one set of chemicals undergo a reaction to form a different set of chemicals which are known as the products. Sometimes, a chemical reaction may require an input of energy known as activation energy and other chemical reactions may occur spontaneously.


An example of a chemical change is when sodium atoms combine with chlorine atoms to form table salt (sodium chloride). The properties of the reactants—sodium and chlorine—are much different than the properties of the product, table salt. Many times, chemical reactions need enzymes to function as catalysts, which help speed along a chemical reaction.


Another example of a chemical change is when the reactant hydrogen peroxide is converted into the products water and oxygen gas. This would normally occur at a very slow pace until the enzyme catalase is added. One can see hydrogen peroxide decompose to water and oxygen because oxygen bubbles are released during this chemical change. The properties of hydrogen peroxide—the reactant—are chemically different from the products, water and oxygen gas. This is another example of a chemical change.


I have included a link to show how to perform the hydrogen peroxide decomposition experiment.

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