In this activity, students will be introduced to the fundamentals of the pH scale and how it is used to distinguish between acids and bases. They will be shown everyday examples of acids and bases and where they fall on the pH scale. The logarithmic nature of the pH scale is explained, and universal indicator is introduced as a way of identifying the pH of a substance. There is also a brief overview of the chemistry of acids and bases.
Middle School, High School
By the end of this activity, students should be able to:
- Identify whether substances are acidic, basic, or neutral based on their pH.
- Explain how logarithmic scales work.
- Identify the pH of a solution based on the color change produced by the addition of universal indicator.
This activity supports students’ understanding of:
- The pH scale
- Acids and bases
Teacher Preparation: minimal
Lesson: 10-30 minutes
- Computer and projector with internet access
- Student handout
- No specific safety precautions need to be observed for this activity.
- All of the animations that make up the AACT Animation collection are designed for teachers to incorporate into their classroom lessons. Intentionally, these animations do not have any spoken explanations so that a teacher can speak while the animation is playing, and stop the animation as needed to instruct.
- We suggest that a teacher pause this animation at several points, including when questions are posed before the answers are revealed, or watch it more than once to give students the opportunity to make notes, ask questions, and test their understanding of the concepts presented. The student activity sheet can help activate students’ prior knowledge, guide them through the animation, and provide a chance for after-viewing reflection and optional extension questions.
- If you assign this to students outside of class time, you can create a Student Pass that will allow students to view the animation (or any other video or ChemMatters issue on the AACT website).
- This activity and the accompanying animation are meant as an introduction to the pH scale, and not an in-depth explanation of acid-base chemistry. The definition of acids and bases at the end of the animation is somewhat limited – “substances that produce H+ or OH– in water” – as the animation was designed to be accessible to both middle school and high school audiences. For an overview of acid-base theories, the Acid & Base Guys Video in the AACT multimedia collection is a good starting point.
- Similarly, this animation does not reference specific concentrations of H+/OH–, discuss the autoionization of water, or use exponents. It also does not mention that both H+ and OH– are present in both acids and bases, but acids have more H+ and bases have more OH–. For a resource that dives deeper into calculations with pH, pOH, and concentrations, see the AACT lesson plan Calculating pH, A Look at Logarithms.
- Logarithmic scales can be very confusing to students, especially younger students, or those with a weaker math background. It could be helpful to have students view the segment introducing logarithmic scales several times and ask them to apply that information to the pH scale. An example is given with coffee (pH = 5) and orange juice (pH = 3) – since the orange juice is 2 steps further away from neutral (pH = 7), it is 10 x 10 = 100 times more acidic than the coffee. The student handout asks students to do a similar comparison between the two acids and two bases presented in the “Test Your Knowledge” section. Be sure students know that the same logic would apply to bases – pH = 9 is 10 times more basic than pH = 8, recalling that the further away from 7 (neutral) a substance is (in either direction), the more acidic or basic it is.
- It might be beneficial to discuss with students, particularly if they are going to do pH calculations in subsequent lessons, that it is not just the chemical formula of a substance, but also its concentration that determines how acidic or basic something is. So, for example, not all hydrochloric acid solutions will have a pH of 2, as presented in the “Test Your Knowledge” section. More or less concentrated hydrochloric acid solutions will have lower or higher pH values, respectively.
- The final segment of the animation places a number of household substances on the pH scale. The colors used for the pH scale in this segment do not align exactly with the universal indicator colors presented earlier in the animation. They are more similar to the colors you would get from using pH paper strips, so the colors may not exactly match up with the earlier segments of the animation. The universal indicator added to solutions works well between a pH of 3 and 10, but does not distinguish well between pH values outside of that range. However, pH paper strips can usually distinguish from 0 to 14, so this layout allowed for substances with more extreme acids/bases to be presented.
- Though most substances fall between 0 and 14 on the pH scale, some extremely acidic or basic substances do fall outside that range, and pH below 0 or above 14 are possible, both in nature and in commercially available chemical solutions. The last extension question asks students to research an example of one of these substances.
- Classroom resources from AACT Library that may be used to further teach pH, acids and bases, indicators, and other related topics include:
- Lesson Plan: Calculating pH, A Look at Logarithms
- Lab: Indicators of Acids and Bases
- Lab: Meet the Indicators
- Lab: Exhaling Acid
- Demonstration: A Secret Message
- Video: Acid & Base Guys Video
- Lab: Detecting the pH of Commonly Used Acids and Bases
- Lab: Exploring the Properties of Acids and Bases
- Lab: Hydrolysis of Salts
For the Student
As you view the animation, answer the questions below.
- Where do acids fall on the pH scale? Where do bases fall?
- What does it mean that the pH scale is logarithmic?
- Which substance is more acidic: hydrochloric acid, HCl (aq), or carbonic acid, H2CO3 (aq)?
- How many times more acidic is the more acidic substance? (Remember, pH is logarithmic!)
- Which substance is more basic: calcium carbonate, CaCO3 (aq), or sodium hydroxide, NaOH (aq)?
- How many times more basic is the more basic substance? (Remember, pH is logarithmic!)
- When using universal indicator, which color indicates very acidic solutions? Which color indicates very basic solutions?
- Based on the color the solutions turned when universal indicator was added:
- What is the approximate pH of vinegar? Is it an acid or base?
- What is the approximate pH of baking soda solution? Is it an acid or base?
- When acidic substances are present in water, what ion is present?
- When basic substances are present in water, what ion is present?
- The animation gave several examples of household materials and their pH – orange juice, coffee, eggs, baking soda, etc. Select 3 more household materials not mentioned in the animation and research whether they are acids or bases, their pH level, and if/how their acidity level affects their function or use. Be sure to use reliable sources and cite them properly.
- Most substances fall between 0 and 14 on the pH scale. However, some extremely acidic or basic substances fall below 0 or above 14. Research one of these substances – list its pH level and describe what it is used for or where it is found if it is naturally occurring. Be sure to use reliable sources and cite them properly.