1.3 Chemical Reactions Study Guide

1.3 Chemical Reactions (8.1.3)

Explore this Phenomenon

The photo shows a piece of iron that has been outside in the weather and has “rusted”

1. What do you observe about the iron’s appearance?
2. How has the piece of iron changed over time?
3. How do you explain what happened?

8.1.3 Signs of Chemical Reactions

Plan and conduct an investigation and then analyze and interpret the data to identify patterns in changes in a substance's properties to determine if a chemical reaction has occurred. Examples could include changes in properties such as color, density, flammability, odor, solubility, or state. (PS1.A, PS1.B)

In this section, focus on patterns. The patterns observed in chemical changes are related to what happens to structures at the atomic level.

Chemical Reactions

Did you ever make a "volcano," like the one in the next figure, using baking soda and vinegar? What happens when the two substances combine? They produce an eruption of foamy bubbles. This happens because of a chemical reaction. A chemical reaction  occurs when matter changes chemically into an entirely different substance with different properties. When vinegar and baking soda combine, they form carbon dioxide, the gas that fills the bubbles. It’s the same gas that gives soft drinks their fizz and that we exhale during breathing.

Not all reactions are as dramatic as this "volcano." Some are slower and less obvious. The following video shows examples of chemical reactions. http://go.uen.org/aZq

Signs of Chemical Reaction

How can you tell whether a chemical reaction has occurred? Often, there are clues. Several are demonstrated in the following video http://go.uen.org/aZw (9:57)

To decide whether a chemical reaction has occurred, look for these clues or signs:

• Gas bubbles are released. (Example: Baking soda and vinegar mixed produce bubbles.)
• Something changes color for a reason other than simply that two colors were mixed. (Example: Leaves turn from green to other colors or eggs turning from clear to )
• A new odor is produced. (Example: Logs burn and smell smoky.)
• A solid comes out of a solution. This is called the formation of a precipitate. (Example: Eggs cook and a white solid comes out of the clear liquid part of the egg.)
• A change in energy. (Example: A firework produces heat, light, and sound)

Changes in a substance’s properties indicate that a chemical reaction may have occurred. When there are changes in properties such as color, density, flammability, odor, solubility, or state it is likely that a chemical reaction has occurred. The only way to be positive that a chemical reaction has occurred is to know if a new substance has formed.

Reversing Chemical Reactions

Because chemical reactions produce new substances, they often cannot be undone. For example, you can’t change a fried egg back to a raw egg. Some chemical reactions can be reversed, but only by another chemical reaction. For example, to undo the tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar reacts with the tarnish. This is a chemical reaction that makes the pennies bright and shiny again. You can try this yourself at home to see how well it works.

Your cell phone battery provides energy to your cell phone but goes down after a while and needs to be recharged. When you plug it in, it causes the chemical reaction in the battery to reverse so it is then available, once again, to power your phone.

Cooking and Chemistry

Whether it is fixing a simple grilled cheese sandwich or preparing an elaborate meal, cooking demonstrates some basic ideas in chemistry. When you make bread, you mix flour, sugar, yeast, and water together. After baking, this mixture changes into what we consider bread, a new substance that has different properties than the original  materials. The process of baking causes chemical reactions in the ingredients that result in a new product: bread.

To practice and learn more about chemical reactions, go to this virtual lab: https://phet.colorado.edu/en/simulation/reactants-products-and-leftovers

Putting It Together

1. Explain your understanding of what is happening to this pipe. How has it changed over time?
2. Think of another phenomenon that applies the concept of the signs of chemical reactions.
3. Explain what is happening to the pipe based on what you have learned in this section.