DIY Indicators

DIY Indicators

Acids and bases (alkalis) form solutions that have a pH of <7 or >7 respectively. The pH is a measure of acidity and is defined as -log10[H+]. Indicators are substances which exhibit different colours in acid and base(alkaline) solutions. They are often coloured organic substances such as litmus, which is extracted from lichen. Litmus turns red in acids and blue in alkalis. Indicators are normally weak acids:

HInd(aq) ? H+(aq) + Ind-(aq)

Where HInd is a weak acid and Ind- is its conjugate base; both have different colours. The change in colour results from a change of structure as a H+ is removed from or attached to the molecule. In acid solution, HInd predominates; in alkali solution, the H+ ions are removed from the equilibrium, more HInd ionises and the conjugate base Ind- predominates.


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Experiments you can do at home

Welcome to Dr Hal's Backyard Science, experiments you can try at home. These are experiments that you can try in your garden or house with parental permission and supervision. Each of them comes with some health and safety instructions that an adult must read. The text is reproduced by kind permission of Education in Chemistry a journal of the Royal Society of Chemistry.

The Experiments:


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