Educational Resources Lets Talk Science Challenge participants

Summary

Learn how to make a pH indicator and test water samples.

What You Need

  • Red cabbage
  • Knife or food processor
  • Cooking pot
  • Water
  • Strainer
  • Container or jar
  • Clear plastic cups
  • Drinking straws
  • Tap water
  • Club soda
  • White paper
  • Measuring cup

Safety First!

Be very careful working at the stove with hot solutions and while using a knife or food processor. Adult supervision and assistance is highly recommended.

What to Do

Preparing for the experiment:

  1. Shred half a head of red cabbage by hand or using a food processor.
  2. Put the shredded cabbage in a pot and fill it with enough water so that the cabbage is completely submerged.
  3. Bring cabbage and water to a boil and keep it boiling for 30 minutes, stirring occasionally.
  4. Remove the pot from the heat and strain water from the cabbage into a sealable container or jar. There should be approximately 2 cups of dark blue/purple liquid.
  5. Let the liquid cool and firmly seal the container or jar.
  6. The dark blue/purple liquid is the “cabbage juice indicator.” The indicator will only stay fresh for a couple of days, so it’s best to prepare it the day before or the day of the experiment.

Completing the experiment:

  1. Place 50mL of tap water into one of the cups and 50mL of carbonated water (club soda) into the other cup.
  2. Use the straw to add 10 drops of the “cabbage indicator juice” (which you have already prepared) to each cup. To add the drops, mark a line approximately 4 cm from the end of the straw. Next, insert the straw into the container of cabbage indicator as deep as the line and plug the other end of the straw with your index finger. This will capture the indicator inside the straw and let you transport it from one cup to the other. To release the liquid, simply unplug the end of straw and it will pour out.
  3. Use the straw to stir the water and cabbage indicator solution first, then stir the carbonated water and cabbage juice indicator solution second.
  4. Hold up the white piece of paper behind the two solutions so that you can compare the colours.

Discovery

What’s happening?

Scientists have a special scale to measure acidity. It’s called a pH scale. It puts numbers to the degree of acidity, so that we can compare things. Acidic materials range from a pH of 6.9 down to 1 (with 1 being the most acidic) and basic materials range from a pH of 7.1 to 14 (with 14 being the most basic). A pH of 7 is neutral and is the pH of pure water.

In the laboratory, there are special instruments to measure pH, but we can also use an “indicator” (something that changes colours when it is put in an acidic or basic solution). The indicator used in this experiment is red cabbage juice. The cabbage juice will turn blue/green if it is mixed with a base and red/pink if it is mixed with an acid.

Carbonated water is more acidic than tap water because of the carbon dioxide present in the water. This is the same gas that we exhale when we breathe. Our bodies consume oxygen from the air when we inhale (breathe in) and release carbon dioxide when we exhale (breathe out). The carbon dioxide gas that we exhale is not acidic on its own, but when it is dissolved in water, a weak acid called carbonic acid is formed.

Rain will always be slightly acidic and this is important, because the weak acid dissolves minerals in the earth that are necessary for our health. This is also why rivers and streams are never considered "pure water," even though they may be clean.

What’s happening?

Scientists have a special scale to measure acidity. It’s called a pH scale. It puts numbers to the degree of acidity, so that we can compare things. Acidic materials range from a pH of 6.9 down to 1 (with 1 being the most acidic) and basic materials range from a pH of 7.1 to 14 (with 14 being the most basic). A pH of 7 is neutral and is the pH of pure water.

In the laboratory, there are special instruments to measure pH, but we can also use an “indicator” (something that changes colours when it is put in an acidic or basic solution). The indicator used in this experiment is red cabbage juice. The cabbage juice will turn blue/green if it is mixed with a base and red/pink if it is mixed with an acid.

Carbonated water is more acidic than tap water because of the carbon dioxide present in the water. This is the same gas that we exhale when we breathe. Our bodies consume oxygen from the air when we inhale (breathe in) and release carbon dioxide when we exhale (breathe out). The carbon dioxide gas that we exhale is not acidic on its own, but when it is dissolved in water, a weak acid called carbonic acid is formed.

Rain will always be slightly acidic and this is important, because the weak acid dissolves minerals in the earth that are necessary for our health. This is also why rivers and streams are never considered "pure water," even though they may be clean.

Why does it matter?

Carbon dioxide is one of the causes of acid rain, but it is not the only contributor. Industrial plants emit (or release) large amounts of sulfur dioxide and nitrogen oxides from their smoke stacks. Like carbon dioxide, these gases undergo chemical reactions with water vapour in the atmosphere and form sulfuric acid droplets, sulfates, nitrogen dioxide and nitric acid vapours, which then return to Earth's surface in the form of acid rain.

Why does it matter?

Carbon dioxide is one of the causes of acid rain, but it is not the only contributor. Industrial plants emit (or release) large amounts of sulfur dioxide and nitrogen oxides from their smoke stacks. Like carbon dioxide, these gases undergo chemical reactions with water vapour in the atmosphere and form sulfuric acid droplets, sulfates, nitrogen dioxide and nitric acid vapours, which then return to Earth's surface in the form of acid rain.

Investigate further
  • Try collecting rain samples and measuring the acidity of the samples.
  • Use the cabbage juice indicator to measure the acidity of common household liquids such as milk, juice, pop, vinegar, etc.
  • After testing several liquids, line them up from most to least acidic.
  • Learn more here: PBS Learning Media: Acid Lake (interactive html)

For more information on this topic check out these Let's Talk Science resources:

  • What are the effects of acid rain on rocks? (Hands-on Activities) - Learn about the chemical reaction that happens between acid rain and limestone.
  • What is acid rain? (STEM in Context) - Acid rain is any precipitation that has an unusually low pH. It can be rain, snow, fog, etc. But what is a low pH and why is this a problem?
Investigate further
  • Try collecting rain samples and measuring the acidity of the samples.
  • Use the cabbage juice indicator to measure the acidity of common household liquids such as milk, juice, pop, vinegar, etc.
  • After testing several liquids, line them up from most to least acidic.
  • Learn more here: PBS Learning Media: Acid Lake (interactive html)

For more information on this topic check out these Let's Talk Science resources:

  • What are the effects of acid rain on rocks? (Hands-on Activities) - Learn about the chemical reaction that happens between acid rain and limestone.
  • What is acid rain? (STEM in Context) - Acid rain is any precipitation that has an unusually low pH. It can be rain, snow, fog, etc. But what is a low pH and why is this a problem?