Arrange students in groups of 2.

Tell students that they will look at how acidic different liquids are and how acidity is typically measured. Before explaining further and if practical, consider demonstrating the use of pH strips to test the acidity of a few liquids such as water, liquid soap, or salt water.

Then, explain to students that the pH scale is a way of measuring the acidity of different liquid solutions. Roughly speaking, it measures the concentration of positive hydrogen ions () in the solution. A small pH means that there are a lot of positive hydrogen ions and such a solution is called acidic. A large pH means that there are relatively few positive hydrogen ions and such a solution is called basic.

Instruct students to look over the list of liquids and ask, “Which drink has more hydrogen ions: coffee or water? How do you know?” Make sure students recognize, for example, that is greater than .

Use Three Reads to support reading comprehension and sense-making about this problem. Display only the problem stem and the table, without revealing the instructions or questions. 

• For the first read, read the problem aloud, and then ask, “What is this situation about?” (measuring the pH of different liquids) Listen for and clarify any questions about the context. 
• After the second read, ask students to list any quantities that can be counted or measured. (the hydrogen ion concentration and the pH)
• After the third read, reveal the instructions to complete the table and the next question and ask, “What are some ways we might get started on this?” Invite students to name some possible starting points, referring to quantities from the second read. (look for a connection between the given columns of the table, then use the definition of acidity to figure out which drink is the most acidic)
   

Ask students to look at the table in this activity and the one in the previous activity. Briefly discuss how they are alike and different, and whether the table still shows the same relationship between hydrogen ion concentration and pH, as in the previous task. Make sure students see that the relationship hasn’t changed and that the hydrogen ion concentrations can be written as powers of 10.

Arrange students in groups of 2.

Explain to students that a scale called the Richter scale is used to report the magnitude of earthquakes. The scale was initially read from a machine called a “seismometer” which included a writing instrument and paper. The writing instrument records how much it moves back and forth when affected by an earthquake.The movement of the writing instrument is called the “displacement” and the stronger the quake, the greater the displacement. 

The displacement measurement is then converted into the Richter scale, taking into account the distance between the seismometer and the earthquake’s “epicenter” (or point on the surface of the earth directly above the main focus of an earthquake). Modern seismometers function in a different way, so they are able to record physical displacements that are too large to realistically measure with a classical seismometer.

Display the table from the activity. Give students 1 minute of quiet think time, and ask them to be prepared to share at least one thing they notice and one thing they wonder. After inviting students to share some things they noticed and wondered, help students make sense of the table by discussing questions such as:

• “What displacement, in meters, is recorded when an earthquake is given a Richter rating of 7?” (1 m)
• “What about 3?” ( meters, 0.0001 meters, or 0.1 millimeters)
  “During an earthquake, a seismograph records a displacement of 10 millimeters. What is the Richter rating for that quake? Explain your reasoning.” (5. 10 millimeters is the same as 0.01 meters, or meters.)
  “What about when the displacement is 0.01 mm? Explain your reasoning.” (2. This is 1,000 smaller than the answer for the previous question, so it is 3 multiples of 10 less. That means the Richter rating is 3 less, and .)

If needed, ask students to pause for a class discussion after the second question so students can see more clearly the relationship between seismometer displacement and Richter rating, before answering the last question.