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Introduction:

Acids and bases are important classes of substances that we encounter in many different

places every day of our lives. Acids can be found in our stomach, where they help digest food,

and can be found in the food itself, where they are responsible for sour tastes. Bases (also

known as alkali) can be found in our small intestine, where they neutralise stomach acid, and

when present in food bases create bitter tastes. You may be familiar with other examples

such as sulfuric acid in car batteries or sodium bicarbonate in baking soda.

We classify acids as substances that can ‘release’ hydrogen ions (H+). In contrast, bases are

substances that can ‘absorb’ H+. Chemists use the pH scale to show how acidic or how basic a

substance is. A pH below 7 is acidic and a pH above 7 is basic . A pH of 7 is neutral (neither

acid nor base). Shown below are some examples of common substances and where they fall

on the pH scale.

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A substance that can release a lot of H+ is called a strong acid and has a low pH, whereas

something that can absorb a lot of H+ is called a strong base and has a high pH. We can say

that vinegar (pH 3) is a weaker acid that hydrochloric acid (pH 0). Similarly, baking soda

(pH 9) is a weaker base than sodium hydroxide (pH 14).

One way chemists determine pH is with an indicator – a dye that changes colour depending

on how acidic or basic a substance is. The guide contains instructions for making your own

indicator and a guide showing you how to use it.

Equipment and consumables:

For red cabbage indicator:

Red cabbage

Knife

Water

Cooking pot

Heating element (stove-top, hotplate or similar) or kettle

Funnel

Plastic Bottle

Sieve or large filter paper

For investigations:

Clear plastic cups

Plastic spoons/popsicle sticks for stirring

A range of household substances to test (see below)

Straws

Small soft drink bottles

Paper towels and rubbish bags for clean-up

Bucket (if no sink is available)

Test samples:

Household substances that make good test examples include colourless, water-based

products such as:

milk, lemons/lemon juice, water, white vinegar, carbonated drinks (soda water, tonic,

lemonade) etc.

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Cleaning products can be interesting examples; however, many of these items are poisonous

and may not be suitable for younger students. Examples:

bleach and window cleaners

Solid substances and thick liquids can be prepared by dissolving/diluting 1 teaspoon of

powder in approximately half a cup of water. Examples:

baking soda (sodium bicarbonate), antacids, laundry powder, dish-washing detergent,

liquid soap

Preparing the Red-Cabbage Indicator:

1. Dice 200 g (approximately six large leaves) of red cabbage using a knife.

2. Place the diced leaves in a pot and add 500 mL (two cups) of water.

3. Using a heating element*, bring to a moderate heat for 15 minutes – boiling the

cabbage is not necessary (although don’t worry if the water gets too hot as this will

not damage the indicator).

4. Stir the mixture every few minutes to help extract the indicator.

5. Let the mixture cool, and then remove the leaves from the liquid using a sieve or filter

paper. Using a funnel and a plastic bottle, collect the liquid – this is your pH

indicator.

*If a hotplate is not available, a similar result can be achieved by pouring two cups of

boiling water over the diced cabbage and waiting for 15 minutes.

The indicator you have collected may be purple or blue, depending on the pH of the water

you added in step 2. Keep the indicator in the fridge when not in use; it can last for some

time without spoiling.

Using the indicator:

1. Add some of the red-cabbage pH indicator to a clear plastic cup.

2. To the same cup, add an equal volume of the substance to test.

3. Mix well. The colour of the indicator should change.

4. Compare the colour of your indicator with the chart below – what is the pH? Does

this mean the substance is an acid or base, or is it neutral?

5. When finished, you can dispose of all liquids can by pouring them down the sink with

plenty of water.

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Other indicators: you can buy (see website below) paper strips which have indicators

soaked into them. They change colour depending on pH. Acid turns blue litmus paper red,

and base turns red litmus paper blue. Universal indicator gives a spectrum of colours (like

red cabbage indicator but the colour at each pH is different).

Some ideas for investigations, using the cabbage (or paper strip)

indicators:

Measure the pH of common household substances.

Most food items and drinks will be acidic (although water may vary), while many

cleaners, soaps and detergents will be basic. Compare the pH you find with the examples

below and with the items shown in the picture on the previous page.

pH Malt Vinegar 2.6 Coca-Cola 2.7 Sprite 3.5 Pepsi 3.2 Milk 6.8 Lemon Juice 2.4 Tap Water 8.2 Sea Water 7.8 Rain Water 6.0 Laundry Powder 10.9 Liquid Soap 6.7 Orange Juice 4.2 Coffee 5.5 Baking Soda 8.5

Mix acidic substance with bases – what happens to the indicator?

A strong acid can be made less acidic by adding a base. The reverse is also true; a strong

base can be made less basic by adding acid. This is known as neutralisation.

Upset stomachs (indigestion/acid reflux) have too much acid. Antacids are bases so

neutralise the excess acid. Most antacid tablets are slow release so need to be ground-up

before adding to an acid, and even then might be slow to react. Adding baking soda to an

acid will demonstrate the same effect.

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Compare the pH of water from a variety of sources. Tap water, bottled water,

sea/river water, water from kitchen filters etc.

Completely pure water has a pH of 7, in practise however, the pH of your water will vary

based on the substances dissolved in the sample which, in turn, is determined by its

source.

Try making indicators from other dark red food items – Will beetroot, blackcurrants,

or blueberries work?

Blackcurrants, blackberries, blueberries, and raspberries contain the same indicator as

red cabbage, although in lower concentration. The fruit juices found in supermarkets

(such as Ribena) can be used as indicators provided they don’t contain added artificial

dyes.

Investigate how carbon dioxide gas can change the pH of water.

Carbon dioxide (CO2) is a colourless, odourless gas that is produced by humans when we

breathe out and when wood, gas, coal, petrol, diesel etc are burnt. On its own, carbon

dioxide is neutral but when dissolved in water it forms an acid (carbonic acid). Students

can see the effect of carbon dioxide on pH by using a straw to blow (for quite a while) into

a small amount of cabbage indicator in the bottom of a bottle. (Note: whether this works

may depend on the initial pH of water in your district.)

Fizzy (carbonated) drinks contain carbon dioxide. The effects of carbon dioxide on pH

can be seen by comparing the pH of water, soda water and ‘flat’ soda water. The fizzy

sample should be about 1 pH unit more acidic than the flat sample, although this

difference may be difficult to judge using your indicator.

The acidic effect of carbon dioxide can be linked with the problems of climate change.

The carbon dioxide released from burning fuels and from cars and trucks enters the air,

and from there it can be absorbed into the ocean, slowly causing seawater to become

more acidic. This is a problem because many sea creatures, such as pipis, tuatuas, clams,

molluscs, crayfish, and coral rely on shells made of tightly packed calcium carbonate; if

the oceans become too acidic these shells will dissolve. Students can investigate this by

by adding a piece of chalk (calcium carbonate) to an acid such as vinegar – the chalk

should fizz and dissolve. A seashell left to soak for a day in vinegar should show signs of

acid attack. If the shell is smashed into small pieces, it will react more quickly. The ‘fizz’

or gas bubbles that form on the shell are carbon dioxide: adding acid to calcium

carbonate gives carbon dioxide as a product.

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How Indicators Work:

The red-cabbage indicator you made in this experiment is actually a mixture of several

different types of molecules known as anthocyanins which plants use to attract animals and

insects. Each type of anthocyanin acts as a natural indicator by having a lower pH form and a

higher pH form, each of which has a different colour. One type of anthocyanin might switch

between a blue form to a pink form at pH 7 – we could use this to immediately see whether a

sample is an acid (pH < 7) or a base (pH > 7). Other anthocyanins might switch between a

blue and green form at pH 9, for example. As we vary the pH, we observe a gradual colour

change rather than an abrupt switch because the red-cabbage extract has a mixture of

different indicators, each of which changes colour at a different pH value.

Indicators can be made from other molecules as well (not anthocyanins). Different indicators

give different colours across the pH range but most commonly, red/pink indicates acid and

blue/green indicates base.

Useful websites:

http://simple.wikipedia.org/wiki/Acid and http://simple.wikipedia.org/wiki/Base_(chemistry) (from the

simple-English version of Wikipedia)

http://www.all-about-ph.com/ (a lot of information at about the right level of complexity)

http://www.webinnate.co.uk/science/week1.htm (shows the red cabbage experiment being

demonstrated to a class, with good pictures)

http://www.teachengineering.org/view_activity.php?url=collection/wst_/activities/wst_environmental/w

st_environmental_lesson02_activity3.xml (Might be useful for as it includes learning objectives and

where the experiment fits in the (American) curriculum)

http://www.thescienceshop.co.nz/shop/Science+Equipment+for+Primary+%26+Middle+School/Chemi

cals.html (sells red and blue litmus and universal indicator paper)