Water chemistry: acids and bases (pH and alkalinity)

In this mini-lesson learners will explore acids and bases in water quality, with a focus on their impact on aquatic organisms and ecosystems. Students will use Water Rangers test strips to investigate the properties of acids and bases and connect it to pH and alkalinity, exploring how pH changes with everyday household products and testing pH levels in their local environment.

Recommended grades

This activity is recommended for grades 8 to 10. We’ve included tips and tricks to adapt the activity for a wide range of students throughout the lesson.

Learning outcomes

  • Use water testing strips to interpret and accurately record pH results.
  • Identify qualitative and quantitative properties of acidic and basic substances.
  • Apply knowledge of acids and bases to associate their characteristics with their positions on the pH scale.
  • Recognize the significance of pH in determining water conditions for supporting aquatic life.
  • Evaluate the potential impact of household products and pollution on water quality.

How to use Water Rangers test strips

In this video, our founder Kat explains how to use and read the measurements on Water rangers test strips.

Background

Acids and bases

Acids and bases play crucial roles in water quality assessment, influencing the conditions that support aquatic life. Acids are often described as having a sour taste, while bases can be characterized as soapy or bitter. At an atomic level, acids release hydrogen ions into water and lower pH levels. Bases, on the other hand, accept hydrogen ions, raising pH levels and influencing alkalinity — bases are also referred to as alkaline.

pH and the pH scale

pH stands for “potential of Hydrogen”, and the pH scale is a measure of how acidic or basic a substance is. It ranges from 0 to 14, with 7 being neutral. pH sets up the conditions for how easy it is for nutrients to be available for aquatic organisms, and how easily toxic substances can dissolve in the water. Different species thrive within different pH ranges, and waterbodies can range between 5 (acidic) and 9 (basic). You’ll need to create a baseline to determine what’s a normal pH level for your local waterbody!

Teacher tip!

For younger audiences, use relatable language and sensory examples to explain pH! Acids usually taste sour and bases feel soapy — you can get them to draw their own pH scale based on the household items they test or foods they are familiar with in the classroom activity.

This image shows a scale of pH, going from zero which is very acid, to 14 which is very basic. The neutral is shown at 7. Stream water usually has a pH between 6 and 8.

Alkalinity

Alkalinity is water’s ability to neutralize acid or resist decreases in pH. It acts as a buffer against rapid pH changes and is important for maintaining a stable pH range. Natural water’s alkalinity is affected by inorganic minerals like rocks and soil, as well as organic matter from plants, bacteria, and fungi. Limestone, for example, can elevate alkalinity. Alkalinity’s measurement is expressed as milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3) in a water sample. While high alkalinity doesn’t necessarily imply poor water quality, each body of water has its unique normal level shaped by environmental factors and influences.

In the classroom: Changing pH with household products

Duration: 30 minutes

Objective: Use test strips to observe changes in pH when adding common household products to water samples, investigating the potential impact on water quality.

Groups: We recommend groups of 5 for this activity.

Materials needed

  • Paper and pencil
  • Water Rangers test strips
  • 2-4 sample cups per group (drinking cups or vials)
  • Water to sample (tap is fine!)
  • At least 3 household liquids and substances. We recommend lemon juice, vinegar, dish soap, baking soda — but you can get creative!

Steps

1. Testing your control sample

Teacher tip!

For older students, encourage them to delve deeper into the chemistry behind ph. Discuss the role of hydrogen ions (H+) and hydroxide ions (OH-) in determining acidity and alkalinity. For each household product, introduce simple chemical equations representing their reactions in water. 

  • Instruct each group to take a water sample and dip a test strip to it. 
  • Record the initial pH reading. This will serve as their control.

2. Changing pH

  • Each group should take another water sample and add a small amount of one household product. 
  • Instruct them to test the pH of the modified sample and record their results.
  • Repeat this step for each household product provided.
Student example of the pH of household products recorded

3. Drawing and reflection

  • Ask students to draw a simple pH scale and label it.
  • Encourage students to think about other factors that could change pH.

Teacher tip!

The discussion and reflection section can be used to evaluate the student’s knowledge.

Discussion questions

  • Which household product changed the pH the most? 
  • Did different products result in higher or lower pH readings? What do higher/lower pH readings mean?
  • Given what you know about these household products, how would you describe an acid? How would you describe a base? 
  • How might water with high/low pH affect the ecosystem within it?

Outdoors: Exploring natural pH sources

Duration: 30 minutes

Objective: To make physical observations in the outdoor environment, predict potential pH values based on these observations, and test the actual pH using test strips.

Groups: We recommend groups of 5 for this activity.

Materials needed

  • Test strips
  • Sample cups (one per group)
  • Reacher sticks
  • Notepads and pens

Do you have your Education testkit?

All materials for outdoor activities can be found in the Water Rangers education testkit.

Education testkit

Always refer to the safety tips in your educator’s guide, as well as our testing location guide and general tips to choosing a sample location when testing outdoors.

Steps

1. Physical observations

  • Distribute note pads and pens to each group to record their observations.
  • Ask each group to make physical observations of the outdoor environment. Encourage them to note any natural sources that could potentially influence the pH of the water. This could include rocks, soil, vegetation, or other elements.
  • Instruct students to consider the landscape and surrounding features. Are there areas with a lot of decaying vegetation? Is the soil sandy or rocky? These observations will serve as clues for predicting pH.

2. Prediction

  • Based on their physical observations, have each group make predictions about the pH of the water. Do they think the environment is more likely to have acidic or basic conditions? Ask them to provide reasoning for their predictions.

3. pH testing

Suggestions for enrichment

Check out this chart of critical pH levels for aquatic organisms. Discuss why certain organisms may be more sensitive to pH changes than others and what happens to them at certain pH levels. For example, snail shells are made of calcium carbonate, and low pH will cause their shells to deteriorate. Also, low pH levels can affect fish’s gills and their ability to reproduce. 

Discussion questions

How do different pH levels affect aquatic organisms?

What does this mean for the local aquatic organisms in your environment? 

  • Distribute the test strips, reacher sticks and sample cups to each group.
  • Remind students to rinse the sample cup three times and ensure their hands are dry before using the strips.
  • Get students to take a sample of water using the reacher sticks and sample cups.
  • Instruct them to dip the entire strip into the sampled water for 2 seconds and then wait for 20 seconds before reading the results.
  • In pairs, compare the colors on the strips with the guide on the side of the bottle to determine the pH value.

4. Reflection and discussion

  • After testing, give each group time to reflect on their predictions and compare them with the actual pH results.
  • Facilitate a class discussion where groups can share their observations, predictions, and test results. Discuss any surprises or patterns noticed across different areas tested.
  • For an extra challenge, you can encourage students to explore the connection between their pH observations and the potential impact on aquatic life. How might the observed pH levels influence the types of organisms that could thrive in these environments?

Discussion questions

  • Were there any unexpected results in your pH testing? 
  • What are some natural sources that affect the pH of water? 
  • What are some human caused sources that affect the pH of water?

Real world connections

Saskatchewan is known for its saline lakes, like Little Manitou, that are also known as “soda lakes”. These lakes have an alkaline pH, meaning that the lakes are basic rather than acidic. Because of the connection between pH and CO2 levels, Saskatchewan’s lakes can act as carbon sinks. Read more about Saskatchewan’s alkaline lakes and carbon sinks in our blog post.

Ocean connections

Ocean acidification 

The ocean absorbs about 30% of carbon dioxide (CO2) that is released into the atmosphere. Since the beginning of the industrial revolution, as levels of carbon dioxide has increased in the atmosphere because of human activities, the pH of surface ocean waters has decreased by 0.1 pH units. Although it doesn’t seem like much, the pH scale is logarithmic so this change means approximately a 30% increase in acidity in the ocean (NOAA). The ocean’s average pH is now around 8.1 (basic), but as the ocean continues to absorb more CO2, the pH will continue to decrease and the ocean will become more acidic.

Curriculum links

Check out our curriculum connections to see how these activities fit in the curriculum of various provinces.

Additional resources

Lesson resources

National Oceanic and Atmospheric Association (NOAA): Ocean acidification 

United States Environmental Protection Agency (EPA): The effects of acid rain on ecosystems

Additional activities

ThoughtCo: How to make a red cabbage pH indicator

Glossary

Alkaline: Having a pH greater than 7, indicating a basic or non-acidic substance.

Control sample: In an experiment, a control is something that stays unchanged on purpose. It’s used as a standard to compare other results with.

Inorganic: Composed of non-living matter.

Ocean acidification: The process where the ocean absorbs carbon dioxide from the atmosphere, leading to a decrease in pH and increased acidity in the ocean. This can have negative effects on marine life and ecosystems.

Organic: Composed of living matter.