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3.3: Physical and Chemical Changes

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    177507
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    Learning Objectives

    • Label a change as chemical or physical.
    • List evidence that can indicate a chemical change occurred.

    Change is happening all around us all of the time. Just as chemists have classified elements and compounds, they have also classified types of changes. Changes are either classified as physical or chemical changes. 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 Change

    Physical changes are changes in which no bonds are broken or formed. This means that the same types of compounds or elements that were there at the beginning of the change are there at the end of the change. Because the ending materials are the same as the beginning materials, the properties (such as color, boiling point, etc) will also be the same. Physical changes involve moving molecules around, but not changing them. 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)

    As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change. A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. When we heat the liquid water, it changes to water vapor. But even though the physical properties have changed, the molecules are exactly the same as before. We still have each water molecule containing two hydrogen atoms and one oxygen atom covalently bonded. 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 started tearing is still paper when you're done. Again, this is an example of a physical change.

    Figure \(\PageIndex{1}\): Ice Melting is a physical change. When liquid water (\(H_2O\)) freezes into a solid state (ice), it appears changed; However, this change is only physical as the the composition of the constituent molecules is the same: 11.19% hydrogen and 88.81% oxygen by mass. Image used with permission (Public Domain; Moussa).

    Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible. Other changes of state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state. The salt may be regained by boiling off the water, leaving the salt behind.

    Chemical Change

    Chemical changes occur when bonds are broken and/or formed between molecules or atoms. This means that one substance with a certain set of properties (such as melting point, color, taste, etc) is turned into a different substance with different properties. Chemical changes are frequently harder to reverse than physical changes.

    One good example of a chemical change is burning a candle. The act of burning paper actually results in the formation of new chemicals (carbon dioxide and water, to be exact) from the burning of the wax. Another example of a chemical change is what occurs when natural gas is burned in your furnace. This time, on the left we have a molecule of methane, \(\ce{CH_4}\), and two molecules of oxygen, \(\ce{O_2}\), while on the right we have two molecules of water, \(\ce{H_2O}\), and one molecule of carbon dioxide, \(\ce{CO_2}\). In this case, not only has the appearance changed, but the structure of the molecules has also changed. The new substances do not have the same chemical properties as the original ones. Therefore, this is a chemical change.

    Figure \(\PageIndex{2}\): Burning of wax to generate water and carbon dioxide is a chemical reaction. Image used with permission (CC-SA-BY-3.0; Andrikkos )

    We can't actually see molecules breaking and forming bonds, although that's what defines chemical changes. We have to make other observations to indicate that a chemical change has happened. Some of the evidence for chemical change will involve the energy changes that occur in chemical changes, but some evidence involves the fact that new substances with different properties are formed in a chemical change.

    Observations that help to indicate chemical change include:

    • Temperature changes (either the temperature increases or decreases)
    • Light is given off
    • Unexpected color changes (a substance with a different color is made, rather than just mixing the original colors together)
    • Bubbles are formed (but the substance is not boiling - you made a substance that is a gas at the temperature of the beginning materials, instead of a liquid)
    • Different smell or taste (do not taste your chemistry experiments, though!)
    • A solid forms if two clear liquids are mixed (look for floaties - technically called a precipitate)

    Example \(\PageIndex{1}\)

    Label each of the following changes as a physical or chemical change. Give evidence to support your answer.

    1. Boiling water
    2. A nail rusting
    3. A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color.
    4. Two colorless solutions are mixed. The resulting mixture has a yellow precipitate.

    Solution:

    1. Physical: boiling and melting are physical changes. When water boils no bonds are broken or formed. The change could be written: \(\ce{H_2O} \left( l \right) \rightarrow \ce{H_2O} \left( g \right)\)
    2. Chemical: The dark grey nail changes color to form an orange flaky substance (the rust); this must be a chemical change. Color changes indicate chemical change. The following reaction occurs: \(\ce{Fe} + \ce{O_2} \rightarrow \ce{Fe_2O_3}\)
    3. Physical: because none of the properties changed, this is a physical change. The green mixture is still green and the colorless solution is still colorless. They have just been spread together. No color change occurred or other evidence of chemical change.
    4. Chemical: the formation of a precipitate and the color change from colorless to yellow indicate a chemical change.

    Exercise \(\PageIndex{1}\)

    Label each of the following changes as a physical or chemical change.

    1. A mirror is broken.
    2. An iron nail corroded in moist air
    3. Copper metal is melted.
    4. A catalytic converter changes nitrogen dioxide to nitrogen gas and oxygen gas.
    Answer a:
    physical change
    Answer b:
    chemical change
    Answer c:
    physical change
    Answer d:
    chemical change

    Summary

    • 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.
    • Chemical changes are changes that occur when one substance is turned into another substance.
    • Chemical changes are frequently harder to reverse than physical changes. Observations that indicate a chemical change occurred include color change, temperature change, light given off, formation of bubbles, formation of a precipitate, etc.

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