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C5 - Physical and Chemical Changes

I can explain the differences between a chemical change and a physical change

Chemists Study of How and Why Matter Changes

While the properties of matter can tell us a lot, so too can the changes that matter undergoes. Suppose you’ve been served a slice of cake that you’ve noticed is cold to the touch (Figure below). You might guess that you’re dealing with an ice cream cake. But then again, maybe it’s just a normal cake that’s been kept in the freezer. Can you think of some way to tell between a frozen slice of ice cream cake and a frozen slice of regular cake? Well, one possibility is to wait for a while and see whether your slice melts. If the slice melts, it was an ice cream cake, and if it doesn’t, it was just regular cake. In this case, you aren’t observing a property, but rather a change in a property. The property being changed in the example is state.

Similarly, chemists learn a lot about the nature of matter by studying the changes that matter can undergo. Chemists make a distinction between two different types of changes that they study – physical changes and chemical changes. Physical changes are changes that do not alter the identity of a substance. Some types of physical changes include:

  • Changes of state (changes from a solid to a liquid or a gas and vice versa)
  • Separation of a mixture
  • Physical deformation (cutting, denting, stretching)
  • Making solutions (special kinds of mixtures)

When you have a jar containing a mixture of pennies and nickels and you sort the mixture so that you have one pile of pennies and another pile of nickels, you have not altered the identity of either the pennies or the nickels – you’ve merely separated them into two groups. This would be an example of a physical change. Similarly, if you have a piece of paper, you don’t change it into something other than a piece of paper by ripping it up. What was paper before you starting tearing is still paper when you’re done. Again, this is an example of a physical change (Figure below).

You might find it a little harder to understand why changes in state are physical changes. Until we discuss chemicals in terms of the smaller units (atoms and molecules) that make them up, it probably won’t be clear to you why freezing a substance or boiling a substance is only a physical change.

For now, though, you just have to trust that changes in state are physical changes. If you’re ever in doubt, remember this: when a lake freezes in the winter, the water doesn’t disappear or turn into something else – it just takes on a new form. Liquid water and solid water (ice) are just different forms of the substance we know as water. For the most part, physical changes tend to be reversible – in other words, they can occur in both directions. You can turn liquid water into solid water through cooling; you can also turn solid water into liquid water through heating.

The other type of change that chemists are concerned with is chemical change. A chemical change occurs when one substance is turned into an entirely new substance as a result of a chemical reaction (Figure below). Again, as we learn more about chemicals, and what chemicals look like, the meaning of a chemical change and the distinction between a chemical change and a physical change will become more obvious. For now, realize that chemicals are made up of tiny units known as atoms. Some of these atoms are bonded (or “glued”) together, but during a chemical change, some of the bonds are broken and new bonds are formed.

You’re probably wondering how you know when a chemical change has occurred. Sometimes it can be pretty tricky to tell, but there are several evidences of chemical changes to look for. There has probably been a chemical change if:

  • A change in color has occurred
  • Light, heat or sound has been given off from the material itself
  • A precipitate (a solid formed when two liquids are mixed) has appeared
  • A gas has been produced (detected by bubbling or a new odor)

Chemical changes are frequently harder to reverse than physical changes. One good example of a chemical change is burning paper. In contrast to the act of ripping paper, the act of burning paper actually results in the formation of new chemicals (carbon dioxide and water, to be exact). Notice that whereas ripped paper can be at least partially reassembled, burned paper cannot be “unburned.” In other words, burning only goes in one direction. The fact that burning is not reversible is another good indication that it involves a chemical change.

Lesson Summary

  • Chemists study the changes that different materials undergo; this can give them valuable information about the chemicals involved.
  • There are two types of changes that are important in chemistry – physical changes and chemical changes.
  • Physical changes are changes that do not alter the identity of a substance; they are usually reversible.
  • Chemical changes are changes that occur when one substance is turned into another substance as a result of a chemical reaction. They are usually difficult to reverse.
  • It is also possible to change matter into energy and energy into matter.

Review Questions

  1. Name the two types of changes that chemists are primarily interested in.
  2. Decide whether each of the following statements is true or false.
    1. Physical changes are typically accompanied by a color change
    2. A burning campfire is an example of a chemical change
    3. When you heat your house with coal, the coal undergoes a chemical change
    4. When you drop a plate, and it breaks, the plate undergoes a physical change
  3. In each of the following examples, determine whether the change involved is a physical change or a chemical change.
    1. Flattening a ball of silly putty
    2. Combining a bowl of cherries and a bowl of blueberries
    3. Boiling water
    4. Cooking an egg

Vocabulary

physical change
Changes that do not alter the identity of the substance.
chemical change
A change that occurs when one substance is turned into an entirely new substance as a result of a chemical reaction.