Explore: Tricky Physical Changes or Chemical Changes?  


  Try This!

Look at an antacid tablet. It is white, solid, and round.

Now, break it into smaller pieces. You have just performed a physical change. It is still white and solid , but now it is in tiny pieces. You have changed how it looks but not what it is. You could grind it to powder; it would still be white, but it would still be an antacid.

Take your bits of antacid tablet and add them to half a cup of vinegar. What happens?

Fizzing and bubbling occurs, and the solid, white tablet you started with is gone. Instead, you have produced a new substance: carbon dioxide gas. The new substance and the bubbling are both signs that you have just performed a chemical change.



Carbon dioxide is a common gas and is the product of many chemical reactions. In fact, when we breathe out, we release carbon dioxide that has been produced by chemical reactions in our bodies.

Carbon dioxide is denser than air is. Because of this, it settles to the bottom of a container. It does this by forcing the air up and out of the container. This property simplifies filling bottles, beakers, or glasses with carbon dioxide.

When the air near a flame is replaced by carbon dioxide, the flame goes out. Carbon dioxide is used in some types of fire extinguishers. It pushes the oxygen needed for combustion out of the way.

A way to test for carbon dioxide is to add lime water. Lime water will change from clear to cloudy white in the presence of carbon dioxide. This is another example of a chemical change. 

Remember that, in contrast, a physical change is a change in the size, shape, form, or appearance of a substance. No new substance is formed.

You will be looking for any or all of these signs to determine if the changes in this activity are physical or chemical. However, keep in mind that exceptions occur to most rules. The best way to determine if a chemical change has occurred is to ask yourself: Was a new substance formed?

Here are some examples of exceptions in which the clues might suggest a chemical change. However, no new substance is formed.


Colouring with a crayon on paper changes the colour of the paper but it does not produce anything new. The paper and the crayon are the same but they are now combined. This is a PHYSICAL change.

(bubbles or fizzing)

Opening a can of soda pop produces fizzing and bubbling from the dissolved carbon dioxide being released from the liquid. No new substance is formed. The carbon dioxide and soda pop are the same before and after you open the can. This is a PHYSICAL change.

When you break an egg, there is no real way to restore the egg to its original shape—remember Humpty Dumpty! The broken egg is still the same substance (egg and eggshell), but it looks different now. No new substance is formed; this is a PHYSICAL change.

Rechargeable batteries

  Technology Connection

How about this? Recharchable batteries are a chemical reaction, but they are a reversible chemical reaction! Reversible and irreversible changes are shown in non-rechargeable and rechargeable batteries. A chemical reaction within a battery produces the electrical current. When a non-rechargeable battery has completed its chemical reaction, the battery no longer can be used. The chemical change producing electricity was an irreversible reaction!

On the other hand, when a chemical reaction has been completed in the rechargeable batteries, the chemical reaction can be reversed, and then allowed to react again. This reversible reaction means that electricity can be produced again!

Non-rechargeable batteries


Keep these examples in mind when you consider the changes in this activity to see if you can tell if chemical changes have occurred.


  Video

Watch this video to review some these ideas before you get started.

BrainPOP: Physical and Chemical Changes
  • Follow this link and use the following information to login: Username: 0099  Password: students

Wonderville You Bet Your Hide

In this interactive LearnAlberta activity, students explore traditional and commercial methods of hide tanning and learn the difference between physical and chemical processes along the way.  Wonderville: The Buffalo Hide Tanning Process

How can you tell a chemical change has occurred?

Materials

  • paper
  • lighter or matches
  • food colouring
  • glass or small bowl
  • glow stick
  • kettle
  • scissors
  • salt
  • baking soda
  • vinegar
  • Epsom salts
  • laundry detergent
  • lime juice
  • balloon
  • plastic water bottle
  • measuring cup
  • measuring spoons
  • funnel


  Notebook

Click here to download the instructions for this activity. Save this file to your Chemistry Notebook.

  Video

Watch this ADLC Digital Lesson: Physical and Chemical Changes to help you set up this experiment.

Physical and Chemical Changes Video

  Check Your Answers

Once you have completed this activity, check your answers below. 

Change Colour Change
Gas Produced
Precipitate formed
Heat and/or light produced
Irreversible New Substance formed
Physical or chemical?
A. Cutting up paper
       
  Physical
B. Burning Paper

   

  Chemical
C. Adding food colouring to water
            Physical
D. Breaking a glow stick
     


Chemical
E. Mixing salt with water
            Physical
F. Boiling Water
 
 
    Physical
G. Mixing baking soda with vinegar
 
   

Chemical
H. Adding Epsom salt to water
     

   Chemical
I. Mixing Epsom salt solution with laundry detergent solution
   
 
  Chemical

  1. Heat here is produced from the heating element for your kettle and not from the boiling of the water. The heat produces boiling water and causes the physical change. The boiling water does not produce heat.
  2. The new substance formed by mixing the Epsom salt and laundry detergent is not soluble in water. Because it does not dissolve in water, you see a solid formed when you mix the two solutions.
  1. Which of your hypotheses were incorrect?
    Answers will vary.
  2. Which physical changes have some signs of being a chemical reaction but did not actually produce a new substance?
    Cutting paper, adding food colouring to water, and boiling water were all physical changes that had some signs that might suggest a chemical reaction, but no new substances were formed.
  3. Based on your observations, answer your Explore Question: How can you tell a chemical change has occurred?
    • colour change
    • gas produced (shown by bubbles or fizzing)
    • precipitate formed (solid from mixing two liquids)
    • heat or light produced
    • irreversible
    • new substance formed



  Chemistry Connection

Did you know that dry ice is frozen carbon dioxide? A block of dry ice has a surface temperature of -78.5°C. Because of this, dry ice is great for refrigeration. Also, dry ice turns directly into carbon dioxide gas (ice→vapour). This is different than regular ice which turns into a liquid and then a gas (ice→water→vapour). This change from a solid to a gas is a physical change that can be used to produce some cool special effects.

View the video Giant Dry Ice Bubble Experiment.

This shows an experiment that makes a huge bubble of carbon dioxide from dry ice.