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Physical versus Chemical Change

This assignment is past due and can no longer be submitted.

Here is an old exam question on physical and chemical changes of matter. How might you approach answering this question? The original question and multiple choice answers are in black and the thought processes used to answer the question are in blue.

Which of the following is a chemical property (or change)?

  a. oil floating on water
    When mixing oil and water, neither substance changes and no new substance is formed. The oil is still oil and the water is still water. They retain their physical properties like melting point, boiling point, slipperiness, color, odor, and density. Since a chemical change did not occur, choice A is not the correct answer.
  b. wax melting
    Melting is changing from the solid to the liquid state. Changes of state are physical changes. The composition of the wax did not change and no new substance was formed. Simply cooling the hot wax returns it to its original state. Choice B is not the correct answer.
  c. a candle burning
    When a candle burns, the wax is consumed and becomes soot, heat, and other stuff. When new materials (soot) are produced that have different physical properties than the original substance (wax), a chemical change has taken place. C is the correct answer.
  d. sea water drying up to leave a residue of salt
    When two substances can be separated by a physical means (in this case evaporation), no chemical change has occurred. Notice, if you added water to the residue, you would get back to the original stuff. D is not the correct answer.
  e. mixing together rust (iron oxide) and metallic iron
   

In everyday language we talk about mixing. Sometimes mixing produces mixtures. Other times mixing results in a chemical reaction. How can we tell? If a chemical reaction happened, new substances form and and we may see a change in color, bubbles indicating the formation of a gas, or other tell-tale signs. Here, simpling passing a magnet near the sample would separate the iron and iron oxide, so the iron and the iron oxide were chemically unchanged. E is not the correct answer.

Now it is your turn. Tell me if each of the following is a chemical or a physical change and justify your answer by telling me your thought process.

You may change your mind as often as you wish. When you are satisfied with your responses, click the SUBMIT button at the bottom of this page.

Question 1. Burning aspirin produces carbon dioxide and water vapor.

Chemical change?
Physical change?

Explain your choice.

 

Question 2. Aspirin tablets are made by pressing together starch and aspirin.

Chemical change?
Physical change?

Explain your choice.

 

Questions 3. A sidewalk is formed when concrete sets (hardens).

Chemical change?
Physical change?

Explain your choice.

 

 


This assignment is past due and can no longer be submitted.



 

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