no longer an exclusively vicarious one.

Friday, January 28, 2005

Chemistry: Acidic environment: 1

1. Indicators were identified with the observation that the colour of some flowers depends on soil composition.

Classify common substances as acidic, basic or neutral
Properties of acids:
- have a sour taste
- change the colour of natural dyes
- react with active metals, eg. Zn or Mg, to produce H2 gas and a salt
- react with marble/ limestone to release CO2 gas
- react with bases to produce water and salts
- sting/ burn the skin
- conduct electricity
- eg. Nitric acid, battery acid (sulfuric acid), vinegar (acetic acid), stomach acid (hydrochloric acid)
Properties of bases:
- have a bitter taste
- feel slippery of greasy
- change red litmus to blue
- react with acids to produce salts
- eg. Limewater (calcium hydroxide), bleach (sodium hypochlorite), bicarbonate of soda (sodium hydrogen carbonate)
Properties of neutral substances:
- not acidic nor basic
- pH of around 7
- eg. Water

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 in indicator colour
An indicator is an organic or synthetic dye that shows a colour change depending on how acidic or basic a solution is. A small volume of the indicator should show a constant, definite colour in either an acidic or basic solution. It has one colour in an acidic solution, and another colour in a more basic solution. Different indicators change colour in different concentrations of acid or base. Thus they produce a range of colours. Indicators themselves are acids or bases.

Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity
Acid-base indicators and pH testing kits are used in a range of everyday situations:
- acidity levels in fish tanks and swimming pools can be checked with indicators, otherwise algae and bacteria may begin to grow, or the water in a pool can irritate eyes and skin
- to determine soil acidity, a white, unreactive powder such as calcium sulfate is mixed with the soil, then a few drops of indicator added and the resulting colour is recorded. It is important to know the acidity of soil for gardening, because some plants like azaleas need acidic soil, while others, like most annual flowers need alkaline soil to be able to absorb nutrients
- in schools, it is recommended that indicators are used to determine if chemical solutions are neutral before disposal down drains
- domestic waste water and waste water from light industry is often tested to ensure that the waste water is not acidic so that it will not corrode sinks, drains and sewerage pipes

Perform a first-hand investigation to prepare and test a natural indicator
1. Collect some coloured plant material such as flowers or a few leaves of red cabbage. Select the coloured parts and cut into small pieces.
2. Place them in a small saucepan or microwavable dish and add enough water to just cover the pieces.
3. Boil gently in the saucepan for 10 minutes (or microwave in the dish for 5 minutes).
4. Cool the mixture.
5. Pour off the liquid, leaving the solid behind.
6. Record the colour of the solution.
7. Add some of the solution to an equal volume of vinegar, shake and record the natural indicator colour in an acidic substance.
8. 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.

Identify data and choose resources to gather information about the colour changes of a range of indicators
- research pH and pH scales
- 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.
Method:
Measure 10mL of 1M HCl solution into a test tube and label.
Using a pipette, transfer 1mL of this solution in to a measuring cylinder, and fill to 10mL with distilled water. Transfer this solution to another test tube, calculating the concentration and label.
Repeat steps 1 and 2 five times.
Repeat steps 1 to 3 using 1M NaOH solution
Draw up a table listing the solutions and their concentrations, and distilled water and test 1mL of each with 5 drops of each of the five indicators, recording the colour observed.
Results:

Universal
Methyl orange
Methyl red
Bromethyl blue
Litmus
pH
Acid






1
Purple
Orange
Yellow
Blue
Purple
0
0.1
Purple
Orange
Yellow
Blue
Purple
1
0.01
Purple
Yellow
Light yellow
Light blue
Purple
2
0.001
Purple
Yellow
Light yellow
Light blue
Purple
3
0.0001
Light purple
Yellow
Light yellow
Light blue
No change
4
0.00001
Green
Yellow
Light yellow
Light blue
No change
5
0.000001
Blue
Yellow
Light yellow
Light blue
No change
6
Base





7 (neutral)
1
Red
Red
Pink
Yellow
Pink
14
0.1
Red
Red
Pink
Yellow
Pink
13
0.01
Red
Coral
Pink
Yellow
Pink
12
0.001
Orange/Red
Orange/Red
Pink
Yellow
Pink
11
0.0001
Yellow/Orange
Yellow/Orange
Yellow/Orange
Apple
Pink
10
0.00001
Apple
Yellow
Light yellow
Blue/Green
Pink
9
0.000001
Green
Yellow
Light yellow
Blue
Pink
8

Solve problems by applying information about the colour changes of indicators to classify some household substances as acidic, neutral basic
Method:
Pour 20mL of one substance into each 6 test tubes (or equal to the number of indicators used).
Add one drop of a different indicator to each of the 6 test tubes. Mix and record the observed colour.
Repeat for each substance.
Results:
Substance
Colour in indicator
Acid/Base

Litmus
Phenolphthalein
Methyl Orange
Bromethyl Blue

Listerine
Pink
No change
Light green
Green
Acid
Shampoo
Pink
Clear
Light orange
Yellow
Acid
Vinegar
Pink
No change
Red
Yellow
Acid
Egg white
Blue
Pink
Light orange
Deep blue
Base
Clearasil
Pink
No change
Yellow-green
Bright green
Acid
Bicarb soda
Blue
Magenta
Ochre
Grey blue
Base
Borax
Blue
Pink
Yellow
Blue
Base
7-Up
Pink
Clear
Orange
Yellow/orange
Acid
Ammonia
Blue
Pink
Yellow
Blue
Base
Conditioner
-
-
-
Sea green
Acid
Water
Blue
Clear
Yellow/orange
Yellow
-NB: The colour of the original indicator and the original colour of the substance must be taken into account

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