Science Stars: 8th grade Lesson Plan

Determining pH

Standards: 5.e. Students know how to determine whether a solution is acidic, basic, or neutral

Suggested time: 50 minutes

Anticipatory Set (Engage): If I gave you the opportunity to eat a lemon right now, who would do it? Why wouldn’t you eat a lemon? Would you eat an orange? If you will eat an orange but not a lemon, what is the different between the two? The lemon is sour, or more acidic. All chemicals can be distinguished as being an acid or a base. Each of these chemicals can be put on a scale called a pH scale depending on how acidic or basic they are. If we take a look up here on the board, you will see the pH scale. Today we are going to test a few chemicals to see if they are acidic, basic or neutral.

Objective: Students will use litmus paper to establish a liquid as an acid or base based on color change. Students will determine the pH of known and unknown acids and bases using pH indicator paper.

Materials: For each pair of students: Container (or multiple containers) with a total of 13 wells 3 pieces of red litmus paper 3 pieces of blue litmus paper 8 pieces of pH indicator paper One dropper with Sodium Hydroxide One dropper with Hydrochloric acid 4 plastic droppers with unknown solutions (milk, ammonia, water, soda) Tables 1, 2, and 3

Prep: 1. Cut pieces of litmus and indicator paper 2. Fill bottles with unknown solutions 3. Label well containers

Background: pH is the measure of acidity . The pH scale ranges from 0-14. The “neutral” center mark of the scale is 7. Solutions with a pH of less than 7 are considered acidic, while solutions with a pH greater than 7 are considered basic. Since pH numbers are based upon a logarithm of base 10, each pH number varies from the next by a factor of 10. For example, an acid with a pH of 5 is 10 times more acidic than an acid with a pH of 6, and a pH of 4 is 100 times more acidic than one with a pH of 6.

Vocab: • Acid • Neutral • Base • pH

Modeling: 1. Explain that all solutions are acidic, basic or neutral. 2. Introduce the methods used to determine pH. One indicates whether a solution is acidic or basic. Others identify the number on a pH scale. 3. Establish the concept of a baseline or control by using temperature as an example. How do we know the temperature has changed? 4. Introduce the litmus paper and demonstrate how it’s used. 5. Explain how each well will be used.

Guided Practice: 1. Students will place one a piece of red litmus paper in three wells (1,2, and 3) and a piece of blue paper in three wells (4,5, and 6). 2. Students should record the “original” color of the paper in table #1. This is the control. 3. Students will add one drop of Hydrochloric acid to 2 wells; one with red paper (#2) and one with blue paper (#5). Leave 1 and 4 as the control. 4. Record results in table #1. 5. Add one drop of Sodium Hydroxide to 2 wells; one with red paper (#3) and one with blue litmus paper (#6). 6. Record results in table #1. 7. Discuss the results.

Check for understanding: What happened when you added an acid to red and blue litmus paper? What happened when you added a base to red and blue litmus paper? (Litmus paper turns the solution red if it has a pH of 4.3 or less. In the presence of a basic solution above pH 8.3, the litmus paper turns blue.)

Guided Practice: 1. Students will place a piece of indicator paper in 3 different wells (#7, 8, and 9). 2. Students should record the “original” color of the paper in table #2. This is the control. 3. Students will add one drop of hydrochloric acid to one well with the indicator paper (well #8). Students will use the pH chart to determine the pH and record it in table #2. 4. Repeat the previous step using sodium hydroxide in well #9.

Check for understanding: What was the pH of the acid? What is the range for acidic solutions on the pH scale? What was the pH of the base? What is the range for basic solutions on the pH scale? If a solution is not acidic or basic, what is it?

Guided Practice: 1. Students will place one piece of indicator paper in four different wells (#10, 11,12, and 13). 2. The students will test four unknown solutions. The pH should be recorded in table #3.

Check for understanding: What is the pH of unknown #1? Does that make it an acid or base? Continue questioning with all four unknown solutions.

Determining pH

Table #1: Litmus Paper

Original color Color with acid Color with base (Control) (hydrochloric (sodium acid) hydroxide )

Red Litmus Paper

(well #1) (well #2) (well #3)

Blue Litmus Paper

(well #4) (well #5) (well #6)

Table #2: Indicator Paper Original Color Color with acid: Color with base: (Control) (hydrochloric acid) (sodium hydroxide)

pH reading:

(well #7) (well#8) (well# 9)

Table #3: Unknowns #1 #2 #3 #4 pH:______pH:______pH:______pH:______

Acid or Base Acid or Base Acid or Base Acid or Base

(well #10) (well #11) (well #12) (well #13)