Student Worksheet

Make Your Own pH Indicator - Activity Instructions

Adults, especially dentists, are always telling us “don’t drink that, don’t eat that - it’ll rot your teeth”! In this experiment, you’re going to make your own pH indicator using everyday ingredients. With your indicator, you’ll test out some common substances around the home (including your favourite fizzy drinks) to find out if they’re as bad for you as the dentists say they are.

Safety advice: A knife or a blender is needed to chop the cabbage. Be careful with sharp blades and be sure to keep fingers out of the way when chopping!

Boiling water can be used to soak the chopped cabbage. Handle hot water and any container of hot water with a lot of care and attention. If any water is spilt, mop it up as quickly as possible to avoid slipping. Cold water can be used in place of boiling water, but it will just take longer for the colour to seep out of the cabbage.

For any burns, allow cold water to run over the affected area and seek medical attention if necessary.

If possible, eye protection should be worn to prevent any liquids from getting into the eyes. Rinse eyes with water should any contact occur.

This is a good activity for kids to complete independently, though they’ll need to get some parental help when it comes to choosing substances to test later on - some acids and alkalis are no joke!

Tooth decay is caused by the bacteria (found in all our mouths) digesting the sugar we eat and producing acid (lactic acid, specifically) as a result of their digestive process. When we consume a lot of sugary foods, the acid builds up, removes the minerals from our teeth and causes cavities. Many of these sugary food and drinks are also acidic, too, meaning we’re putting even more acid in our mouth to cause cavities. This is the main reason dentists tell us we should limit how much sugary food and drinks we consume, as well as telling us to brush our teeth regularly and drink lots of water to help remove the buildup of sugars and replace the minerals lost from our teeth. As you’ll soon find out, toothpaste also plays a special role in fighting the tooth decay caused by acids and not just by helping to replace the lost minerals from our teeth.

The pH of different substances can be tested using a pH indicator. The pH is a measure of how acidic or alkali a substance is; a low pH (pH 1-6) is classed as acidic, a high pH (pH 8-14) is classed as alkaline, and a pH of 7 is classed as neutral (neither acid or alkali).

The chemicals that give red cabbage its colour, as well as many other strongly coloured plants, can be used to measure pH. The chemicals found in red cabbage leaves can be extracted by smashing open the cabbage leaf cells to release colourful colour-changing chemicals. Very acidic solutions turn red, alkaline solutions turn greenish-yellow and close-to-neutral substances are a purplish-blue colour.

Part A: Making Your Indicator

Step 1.

Chop the cabbage up into little pieces until you have about two cups of cabbage. If you have a blender, you could use this to chop the cabbage and for Step 2.

Step 2.

Place the cabbage into a large glass container and add boiling water until the cabbage is completely covered. Leave for at least 10 minutes to let the colour seep out of the cabbage and for the water to cool somewhat.

Step 3.

Filter out the cabbage by pouring the contents of your container through filter paper and into another suitably sized container or bowl. If you don’t have filter paper, and depending on how finely the cabbage was chopped, a sieve could be used to filter out the cabbage. The liquid you have collected is your new pH indicator fluid.

Step 4.

Use a measuring cup to pour 50-100 mL of your indicator into six smaller containers (if you have enough indicator left over, you can use more containers and test more substances). You’re all set to start testing pH!

Part B: Testing The pH of Household Substances

Step 1.

Find six substances around the house to test, one for each container of cabbage indicator you have. Here are some easy suggestions:

Water (this should be neutral and can be used to determine what almost neutral pH looks like for comparison)
Milk
Soft drinks*
Fruit/fruit juice*
Dishwashing detergent
Toothpaste*
Coffee
Tea
A dissolved sugar cube/loose sugar (mix the sugar with hot water for it to dissolve quickly)
Antacid tablets or liquids
Vinegar

*We highly recommend testing these substances.

If the substance you want to test is solid, you could try crushing it and mixing with hot water (as you did with the cabbage) so it can more easily change the pH of the water (this will have mixed effectiveness depending on how easily it can dissolve).

Step 2.

For each of the substances you collect, fill in the table below with your prediction of whether it will turn out to be an acid, alkali or neutral.

Step 3.

For each substance, add a small volume of it to the cabbage indicator liquid. Note the colour the indicator turns and complete the table below, using the following chart to help. Use a different container for each substance, otherwise, your results won’t be accurate.

Acidic

Neutral

Alkaline

Source: Wikimedia Commons

Colour	Red	Purple	Violet	Purplish-blue	Blue	Blue-green	Greenish-yellow
pH	2	4	6	7	8	10	12
Acidic				Neutral	Alkaline

Questions

• How accurate were your predictions? Were there any that surprised you, e.g. was it more neutral than you thought it would be?

• Which substances were acidic? Was this what you expected?

• What do you notice about the sort of substances that are acids?

• What do you notice about the sorts of substances that are alkaline?

• Knowing what you now know about which foods are acidic, and that this can speed up the process of tooth decay, will you think differently about what you eat or drink?

• What can you do to prevent tooth decay, even if you like the occasional fizzy drink?

Taking It Further

What happens when you mix an acidic substance with an alkaline substance? What colour does the indicator turn? Tip: start off by adding a little of an alkali substance to the acidic substance and keep adding a little at a time, taking note of any colour changes along the way. What do you think is happening?

Research chemical neutralisation and use your research to explain what you see when you mix acid and alkali substances together. From looking at your results, why do you think toothpaste is so important in helping to fight tooth decay?

Research what heartburn is and its causes. Use your results from the experiment (what you saw when mixing acids and alkali substances and what you learned about neutralisation) to explain how antacids can be used to treat heartburn.

Make Your Own pH Indicator

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