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9.3 The acidic environment: 1. Indicators
| Syllabus reference (October 2002 version) | ||
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1. Indicators were identified with the observation
that the colour of some flowers depends on soil
composition
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Students learn to:
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Students:
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Extract from Chemistry Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit: 27Jun 08]
Background information:
Human beings have evolved with four main taste
sensors on their tongues: sweet, salt, sour and bitter. These
sensors enabled our ancestors to detect suitable foods. Sweet
sensors detected sugars and other substances providing
energy. Salt sensors detected minerals required in small
amounts for essential chemical reactions. Sour sensors
detected the presence of acid—often a sign that the
food had started to decay as micro-organisms changed sugars
to acids. Bitter sensors, detecting basic or alkaline
substances, provided even greater protection—many
bitter substances in plants are poisonous and so our
ancestors learnt not to persist in eating bitter food.
Sweet and salty foods are usually neutral, sour foods are acidic and bitter foods are basic. With time, some cultures came to value foods kept in acid substances, such as vinegar, because the acid preserved the food. Other cultures learnt to value basic substances, such as quinine from the bark of a South American tree, that provided protection against malaria.
Chemicals called indicators can be used to test substances for acidity or basicity by colour change.
classify common substances as acidic, basic or neutral
In working through this module, you will come across examples of acidic, basic and neutral substances. Keep a record of the examples you encounter in a table like the following one.
Table of examples of acidic, basic and neutral substances
| acidic | neutral | basic |
|---|---|---|
perform a first-hand investigation to prepare and test a natural indicator
- Perform an investigation like the one described below. You may need to make modifications to your planned procedure as you go. Identify and use safe work practices during your investigation, being particularly careful with heating equipment and any hot solutions.
Making indicators
- Collect some coloured plant material such as flowers or a few leaves of red cabbage. Select the coloured parts and cut into small pieces.
- Place them in a small saucepan or microwavable dish and add enough water to just cover the pieces.
- Boil gently in the saucepan for 10 minutes (or microwave in the dish for 5 minutes).
- Cool the mixture.
- Pour off the liquid, leaving the solid behind.
- Record the colour of the solution.
- Add some of the solution to an equal volume of vinegar, shake and record the natural indicator colour in an acidic substance.
- Add some of the solution to sodium bicarbonate (baking soda), shake and record the natural indicator colour in a basic substance.
- If you obtained different colours for a solution with the acidic substance and with the basic substance, then you have prepared an acid-base indicator solution. This solution will give one colour in contact with an acidic substance and another colour with a basic substance.
Keep the unused solution for later activities.
The following web site is useful provided you have a colour monitor with your computer.
Colour images of mixture indicators at different pHs 
Learning Materials Production, Centre for Learning Inovation,
NSW Department of Education and Training. (This web site last
checked on 27 Jun 2008)
identify data and choose resources to gather information about the colour changes of a range of indicators
- Identify the data that will be useful about indicators.
You will find the concept of a pH scale useful in collating
information about colour changes of indicators. The pH
scale ranging typically from 0 (extremely acid) through 7
(neutral) to 14 (extremely basic) allows quantitative
comparison of indicators.
- When choosing resources to conduct
tests with indicators, carry out a risk assessment, using
available safety information so that you can systematically
address potential hazards.
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You could see a number of colour changes if you use an
indicator solution containing more than one substance
that changes colour. Red cabbage solution and universal
indicator solution are both mixtures of indicators. In
going from pH 0 to 14 with these solutions, you will see
at least five colour changes. In the case of universal
indicator, the sequence of colours from acid to base is
the same general sequence as in the visible light
spectrum: red, orange, yellow, green, blue and violet.
Most indicator solutions used in a laboratory undergo a single colour change. The solution changes colour through a narrow pH range of one to three units of pH. These solutions contain only one indicator substance.
- Use a reference book or chemical data book to gather information about the colour changes of the indicators litmus, phenolphthalein, methyl orange and bromothymol blue. Record as a summary in an accessible form, such as in a table.
identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that the range is identified by change in indicator colour
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The following table shows the colour changes and change
ranges for four indicators:
- In changing from yellow to blue, the first indicator goes through green between pH 6.0 and 7.6, the second indicator goes through purple from pH 5.0 to 8.0 while the third goes through orange from pH 3.1 to 4.4. The fourth indicator goes from colourless through pink to red from pH 8.3 to 10.0.
solve problems by applying information about the colour changes of indicators to classify common substances as acidic, neutral or basic
- Before you begin to solve problems, identify and explain the nature of the problem. Consider if the use of indicators will allow classification of all household substances available.
When conducting any tests for the investigation:
- wear protective eyewear—either glasses or safety goggles—when you test household substances.
- read the label and heed the safety warnings and suggestions of every substance that you use.
- use substances from the kitchen, laundry, bathroom or cleaning solutions.
- do not use any substance marked CAUSTIC or POISON.
identify and describe some everyday uses of indicators including the testing of soil acidity/basicity
Indicators are used as solutions or can be adsorbed onto the surface of paper such as filter paper.
Acid-base indicators and pH testing kits are used in a range of everyday situations, including in:
- the soil testing part of a nursery or hardware store
- a swimming pool store or the pool chemicals in a large supermarket
- the aquarium supplies section of a pet shop.
In schools, it is recommended that indicators are used to determine if chemical solutions are neutral before disposal down drains. For further information, refer to the Chemical Safety in Schools (CSIS) package, Appendix G, Disposal.
Domestic waste water and waste water from light industries is often tested to ensure that waste water is not acidic so that it will not corrode sinks, drains and sewerage pipes.
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