Big Question

How can we use pH to figure out what is wrong with the water park water?

What You Will Do

1. Make a red cabbage indicator

2. Collect 6 mystery water samples

3. Test each sample

4. Record your results on your handout

5. Decide what the pH suggests about the water

6. Recommend what should be added to bring the water back toward neutral

Step 1: Make Your Indicator

Materials

• Red cabbage leaves in a baggie

• Water

• Cheesecloth

• Rubber band

• Cup

Directions

1. Squish the cabbage leaves until the water becomes dark purple.

2. Put the cheesecloth over the cup.

3. Secure it with the rubber band.

4. Pour the cabbage mixture through the cloth.

5. Save the purple liquid. This is your indicator.

Think

• What do you predict this purple liquid will help you discover?

• Why might color be useful in science?

Step 2: Collect Your Mystery Samples

You will get 6 numbered test tubes.

Each test tube matches one sample station.

Directions

1. Go to stations 1-6.

2. Add about 5-6 mL from each station into the matching numbered tube.

3. Return to your table carefully.

Important

• Keep tube numbers matched to station numbers.

• Do not mix samples together unless your teacher tells you to.

Step 3: Test the Samples

Test each sample with a pH strip first.

Then add cabbage indicator and observe the color.

Use your handout to record:

• Sample location

• Observed color

• Estimated pH

• Actual pH

• What the pH suggests about the water

• What you would add to get the sample back to neutral

Think

• Which samples seem safest so far?

• Which samples seem like they could be a problem for swimmers?

Step 4: Look for Patterns

As you test, compare your evidence.

Ask

• Which samples look acidic?

• Which samples look basic?

• Which sample seems closest to neutral?

• Do the cabbage colors match the pH strip results?

Use Evidence

Do not guess.

Use both:

• the pH strip result

• the cabbage indicator color

Think

• Which sample changed the most?

• Which sample stayed closest to purple?

• Do the pH strip results match the cabbage colors?

Step 5: Make a Claim

Talk with your partner.

Discuss

• Which water features seem normal?

• Which water features seem like a problem?

• Why would water near neutral usually be better for guests?

Use evidence from your handout before making your claim.

Step 6: Recommend a Fix

For each sample, decide what should be added to move it back toward neutral.

Think Before You Answer

• If the sample is too acidic, what type of substance might help?

• If the sample is too basic, what type of substance might help?

• What evidence supports your recommendation?

Complete the final column on your handout.

Final Reflection

• Which location seemed the most unsafe?

• Which location seemed the closest to normal?

• Why is it risky to assume clear water is safe water?

• How did the pH strips and cabbage indicator help you make decisions?

Science Words

• pH: a measure of how acidic or basic something is

• Acid: a substance with a lower pH

• Base: a substance with a higher pH

• Neutral: in the middle of the pH scale

• Indicator: a substance that changes color to give information

• Evidence: observations or data that support an idea

Reminder

Scientists do not guess first.

They observe, test, compare, and use evidence.

How does it work?

Chemists classify substances as acids or bases. Lemon juice and vinegar are both examples of acids. On the other end of the spectrum are bases. An example of a base is baking soda, which you might have used in the kitchen to make cookies and cakes. Many soaps are bases. Some substances are neutral, meaning they are neither an acid nor a base, like water.

How can you tell if something is an acid or a base? Acids and bases can change the color of substances called acid-base indicators.

Red cabbage contains a chemical called anthocyanin. This pigment is a natural acid-base indicator. It is blue in neutral substances, like plain water. When an acid like lemon juice gets in the water, a reaction makes the indicator molecule change shape and it looks pink. When instead a base gets in the water, a different reaction happens that changes the indicator molecule and it looks green.

https://www.acs.org/education/activities/red-cabbage-indicator.html