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Physical Change Versus Chemical Change

You may not be aware of it, but matter is constantly changing. There are two types of changes: physical changes and chemical changes.

Video segment. Assistance may be required. Watch this video for a brief introduction on physical and chemical changes.

The chart below summarizes the differences between physical and chemical changes.

Physical Change

Chemical Change

Examples:

  • alters the form or appearance but does not result in the formation of a new substance
  • involves change in size, shape or state
    • freezing
    • melting
    • cutting

Examples:

  • results in the formation of new chemical substances
  • involves making or breaking of bonds between atoms
    • rusting
    • burning
    • cooking

In the Check Your Understanding exercise, drag and drop the images to either the physical or the chemical change column.

Evidences of Chemical Change/Reactions

In a chemical change or reaction, the original substances' chemical composition is changed. You cannot get back your original substances. This would be like un-baking a cake.

Video segment. Assistance may be required. Watch the following video to learn about the evidences of chemical reactions.

Some signs (or evidence) of chemical change are

  • production of a gas (bubble formation),
  • change in temperature,
  • formation of a precipitate (a solid),
  • change in color, and
  • production of a new odor.

Video segment. Assistance may be required. Watch the following video and use your notes to list the evidence of a chemical reaction that you see.

Formation of New Substances with Different Properties

When a chemical reaction takes place, a new substance is formed. This new substance has different properties than the original substances.

Let's look at an example. Table salt is also known as sodium chloride (NaCl). As you can see from the chemical formula, it is made up of sodium and chlorine. The table below shows the properties of sodium and chlorine.

Properties of Sodium

Properties of Chlorine

  • soft
  • solid
  • silvery-white in color
  • highly reactive
salt crystal
  • gaseous
  • green-yellow in color
  • extremely poisonous
  • reactive with many compounds containing hydrogen
image of chlorine

 

Source: 
Sodium, DNN87, Wikimedia Commons; Chlorine, Wikimedia Commons

Video segment. Assistance may be required. Watch this video to see what happened when sodium and chlorine combine.

When sodium and chlorine combine, they form salt. Use your notes to list the properties of salt. Explain how the properties of salt are different from sodium and chlorine.

image is of salt

Source: 
RealSalt, Tricia Simpson, Wikimedia Commons