Resources | Subject Notes | Chemistry
Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange
Indicators are substances that change colour depending on the pH of a solution. Acids affect indicators in specific ways due to their ability to donate hydrogen ions (H+).
Litmus is a natural indicator derived from lichens. It has two forms: red litmus paper and blue litmus paper.
This is because acids increase the concentration of hydrogen ions, which disrupt the structure of the litmus dye, causing a colour change.
Thymolphthalein is an artificial pH indicator that is typically pink in alkaline solutions and colourless in acidic solutions.
As the concentration of hydrogen ions increases (more acidic solution), the thymolphthalein molecule rearranges its structure, leading to a colour change from pink to colourless.
| Indicator | Colour in Acidic Solution | Colour in Alkaline Solution |
|---|---|---|
| Thymolphthalein | Colourless | Pink |
Methyl orange is another artificial pH indicator that is red in acidic solutions and yellow in alkaline solutions.
In acidic solutions, the methyl orange molecule is protonated, resulting in a red colour. As the solution becomes more alkaline, the hydrogen ions are removed, and the molecule changes to its deprotonated form, which is yellow.
| Indicator | Colour in Acidic Solution | Colour in Alkaline Solution |
|---|---|---|
| Methyl Orange | Red | Yellow |
In summary, acids cause a characteristic colour change in these indicators, with blue litmus turning red, thymolphthalein turning colourless, and methyl orange turning yellow. This is a direct consequence of the increased hydrogen ion concentration in acidic solutions.