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Red Cabbage Juice

Written by : Abdul Wahab

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Red Cabbage Juice.

As we have seen, acids and bases have the property ofmodifying the color of certain substances. This is thecase with the juice of the red cabbage. This liquid has ablue-violet color, but when it comes in contact withacidic substances it becomes red, while in contact withbasic substances it becomes green and even yellow.Let's see how it is possible to use the juice of the redcabbage to measure the pH of various substances.

Red cabbage. Cut the cabbage intoslices and put them in a pot.

Add water enough to cover

the slices and boil for half an hour.

Pour the juice

in a low container.

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During winter and spring, it is easy to find a redcabbage at the produce market or greengrocer. It is a

cabbage which has a red-violet color. Buy one of themand cut it in little slices . Put them in a pot and pour

enough water to cover them. Boil for half an hour, thenturn off the heat and let the temperature come down.Pour the blue-violet liquid you have obtained into alarge, low container. The boiled cabbage slices areedible and you can use them in a recipe.

Use of the red cabbage juice as an indicator in theliquid state.Pour some drops of this liquid on a white surface and

observe its changes of color when it is mixed withvinegar or with baking soda. You will see that this liquidbecomes red in contact with vinegar or lemon juice,while it becomes green in contact with baking soda.

This behavior is unusual, and later on we will try toexplain it.

Pour one centimeter of the red cabbage juice indicatorliquid into a transparent glass. Add water up to half of

the glass. Now, pour vinegar into the glass and observethe color changes of the liquid. Repeat the experimentby adding, this time, a little baking soda instead of thevinegar. Also in this case, you will see color changes.

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Preparing red cabbage pH papers.

Cut some porous white paper or card sheets intorectangles and soak them in the juice so they well

absorb it. After about half an hour, remove the cardsand put them away to dry. To do it quicker, you canalso dry them with a hairdryer. Cut the cards in strips.Put away the red cabbage cards which are not usedimmediately: they will last some months. If you storethem in a closed envelope to reduce their oxidization,they will last longer. Put the remaining juice in a bottle.After a few days, this juice goes bad and you have tothrow it away. To keep it longer, store it in a

refrigerator.

Soaking cards with red cabbage Drying the cards.

Red cabbage paper in solution oflemon juice.

Red cabbage paper in solution oflemon juice.

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Red cabbage card in a saturatedsolution of baking soda.

DETERMINE THE pH OF SOME SUBSTANCES

In this paragraph, we will measure the pH of somecommon substances. We will do this with litmus paperand with the pH meter (if available). We will use alsothe red cabbage paper we prepared.

In the following table, write the values of pH you

determine for each substance, using litmus paper andthe pH meter. By means of colored pencils, draw thecorresponding color of the red cabbage paper.

TABLE 1 - MEASURING THE pH OF SOME SUBSTANCES

SUBSTANCES

pH(litmus

p.)

pH(pH

meter)

COLOR(red

cabbagepaper)

distilled water

tap water

rain water

soda water

vinegar

lemon juice

wine

beer

Coca-Cola

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milk

baking soda (saturatedsolution)

ammonia (20% sol. about,to be diluted again with 3

parts of water). To be keptin a closed bottle.

normal aspirin (1 tb in 20 mlof distilled water)

buffered aspirin (1 tb in 20ml of distilled water)

shampoo

soap

other substances commonly

used at home and not

dangerous. Do not usestrong acids or basesbecause they are dangerous.

Do not use bleach.

EQUIPMENTS

pH meter if available

litmus paper a roll

red cabbage paper strips

beakers or glass pots 20

glass or plastic rods 1

At the end of the measurements, compare the valuessupplied by the litmus paper and by the pH meter.Observe the corresponding colors of the red cabbagepaper.Highlight the two more acidic substances.

Highlight the two more basic substances.

Observations:

Vinegar, lemon and Coca-Cola are acidic substances.Baking soda and ammonia are basic substances.Distilled water has a neutral pH.

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THE COLOR SCALE OF RED CABBAGE

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