Resources | Subject Notes | Chemistry
This section focuses on describing the characteristic properties of alkalis (bases) in terms of their effect on common indicators: litmus, thymolphthalein, and methyl orange. Understanding these properties is crucial for identifying and differentiating between acids, bases, and neutral solutions.
Alkalis are bases that are typically ionic compounds formed from metals and hydroxide ions ($OH^-$). Common examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH).
Litmus is a natural indicator derived from lichens. It has two forms: red litmus paper and blue litmus paper.
Effect on Litmus: Alkalis turn red litmus paper blue. This is because the hydroxide ions in the alkali react with the acidic components in the red litmus dye, changing its color.
Thymolphthalein is an artificial pH indicator that is colorless in acidic and neutral solutions but turns pink in alkaline and basic solutions.
Effect on Thymolphthalein: Alkalis turn thymolphthalein pink. The hydroxide ions in the alkali deprotonate the thymolphthalein molecule, causing a change in its electronic structure and resulting in a pink color.
Methyl orange is another artificial pH indicator that is red in acidic solutions, orange in slightly acidic solutions, and yellow in alkaline and basic solutions.
Effect on Methyl Orange: Alkalis turn methyl orange yellow. The hydroxide ions in the alkali react with the acidic proton in the methyl orange molecule, leading to a shift in its absorption spectrum and a change to yellow.
| Indicator | Color in Acidic Solution | Color in Neutral Solution | Color in Alkaline Solution |
|---|---|---|---|
| Litmus (Red) | Red | No change | Blue |
| Litmus (Blue) | No change | Blue | Blue |
| Thymolphthalein | Colorless | Colorless | Pink |
| Methyl Orange | Red | Orange | Yellow |