Acid Oceans Part 1

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SWBAT explain what is meant by pH, create a pH indicator and use the indicator to explain how to measure the pH of solutions.

Big Idea

Introduce your students to the acids, bases and the pH scale in the context of ocean acidification.

Getting Started

I use this lesson as an introduction to acids and bases. It is appropriate for any of grades 6-8. Prior to talking about ocean acidification and seeing a demonstration in the second lab I have on acids and bases, we learn about the basics of pH and what we mean when we talk about an acid or a base. 

You will make and use a pH indicator with red cabbage.

Note: Be prepared for the smells that accompany this lab. :)

Also, be very aware of the smells of all of the other ingredients. I opened windows to keep fresh air flowing as we used bleach, ammonia, etc. Tell your class ahead of time to not smell the liquids directly as the fumes are quite pungent.

Safety goggles are a must!  I discuss safety issues related to this lab in my reflection.


10 minutes

Use the following prompts as a Do Now/Bellringer activity to begin this lesson.

  • List a property of an acid and an example of an acid.
  • List a property of a base and an example of a base.
  • Give an example of a pH value of the strong acid.
  • Give an example of a pH value of a strong base.
  • What does the pH scale tell you?
  • What does pH stand for?

Ask students to write down their idea in their science journals, then turn and talk to their table group about their ideas.

Bring the class back together and elicit some responses. Copy down the student ideas to return to after the lesson to clarify what they learned. 

Hand out the reading Acids and Bases: What are they?

Ask student to read and then refer back to their ideas. Clarify what they learned from the reading.

You could flip this lesson. Assign the reading the night before with the questions and review at the start of class.


45 minutes

I begin this lesson with an overview of acid and bases using the slides Acids & Bases. These end with a slide about indicators with a reference to the lesson, making and using a pH indicator with red cabbage juice. 

Next I ask my students if they'd be interested in making and testing pH indicator, to which there is an enthusiastic YES!

If you choose to proceed by making the indicator with your class continue reading here. If you would rather make it yourself ahead of time and have it ready, move on the testing activity below. 

For the activity you will need the following materials to make the indicator:

  • Red cabbage
  • Coffee filter or paper towel
  • 250 ml beaker for water
  • Three 100 ml  transparent cups or other similar containers
  • Hot water 
  • Thermometer
  • Vinegar
  • Baking soda
  • Safety goggles
  • Tongs or fork
  • Eyedropper 
  • Craft stick or toothpick
  • Extra sticks or drinking straws for stirring

Follow How to make a pH Indicator for making the indicator with your class. For a MUCH FASTER version, place 2 cups of chopped cabbage into a blender, cover with boiling water, blend, strain into a beaker and VOILA! its ready. 

Acid Breath - Cool Science Experiment by Steve Spangler will help you out:

Once it is prepared, move to testing it with vinegar and baking soda. 

Using an eyedropper, transfer vinegar from its container to one cup of cabbage water, five drops at a time. Stir the cabbage water after each vinegar transfer. Note any color change. Continue transferring vinegar five drops at a time until the color of the cabbage water has changed to a color different from that of the original cabbage water.

Next, Use a craft stick or toothpick to transfer baking soda from its container to another cup of cabbage water, just a few grains at a time. Stir the cabbage water after each baking soda transfer. Note any color change. Continue transferring baking soda a few grains at a time until the color of the cabbage water has changed to a color different from that of the original cabbage water.

Have on hand some other household liquids and dissolvable solids to test and see how they change the color of the indicator such as

  • Household ammonia (NH3)
  • Baking soda (sodium bicarbonate, NaHCO3)
  • Washing soda (sodium carbonate, Na2CO3)
  • Lemon juice (citric acid, C6H8O7)
  • Vinegar (acetic acid), CH3COOH)
  • Cream of tartar (Potassium bitartrate, KHC4H4O6)
  • Antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide)
  • Seltzer water (carbonic acid, H2CO3)
  • Muriatic acid or masonry's cleaner (hydrochloric acid, HCl)
  • Lye (potassium hydroxide, KOH or sodium hydroxide, NaOH)

Have students create a data table to record their results. 

Remember to follow you safety protocols!!



20 minutes

To evaluate this lesson, ask students to complete the following Writing Prompt:

Essential Question: What are the properties of acids and bases?

Learning Objective: The student will differentiate among acids, bases, and salts based on their properties.

Your friend had an upset stomach caused by indigestion.  His mother explained to him that indigestion is when the contents in the stomach are too acidic.  She also told him to drink soda to settle his stomach.  The soda did not help.  You tell him that you would not have expected the soda to help with the indigestion based on what you know about acids and bases.

Explain why soda would not help settle your friend’s stomach.  In your response, be sure to include:

  • A better alternative to drinking soda.
  • How each substance above would react with the stomach acid.
  • Defining characteristics of acids and bases, besides how they react.

Be sure to consider the completeness of your response, supporting details, and accurate use of terms.