Essay by rabeh223High School, 12th gradeB+, August 2008

download word file, 17 pages 0.0

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Faculty: SCIENCE


Task Number: Three

Task Description: Open Ended Investigation

Weighting: 15%

Component/Mode: The Acidic Environment



Did you know? That there are natural substances that can be used as acid, base and neutral indicators, but firstly what are indicators. Indicators are substances those have specific colours to which they change depending on the pH.

For example anthocyanin is an indicator found in red cabbage that changes from purple to red in acid and purple to blue-violet in base. These indicators can be used to classify if a substance is an acid/base or neutral. Most indicators have been developed using these natural substances, except those that have been made synthetically such as universal indicator.

Natural substances such as red cabbage, red grapes, beetroot, skin of lemon, Hydrangea flower etc contain pigments of anthocyanin or curcumin and many others that change colour too the pH range. Anthocyanins are water-soluble pigments that may appear red, purple, or blue according to the pH.

The chemistry part of how these pigments change colour is that when you add acid to anthocyanin, the acid gives the anthocyanin an extra hydrogen atom that makes the molecule a little bigger and changes the colour to pink. When bases are added it removes the hydrogen atom making the anthocyanin smaller and changing its colour to blue-green. We also know that acids are proton (H+) donors and bases are proton acceptors. Therefore pH indicators are themselves acids and bases!

Anthocyanin is extracted from these natural substances by boiling it in distilled water. This causes the anthocyanin to dissociate as separate ions and become dissolved in the distilled water. This pigment that is dissociated into the distilled water now can be used to distinguish between and classify acids...