why natural indicators can test for acid and alkali?

Acid-alkali indicators are substances whose solutions change color due to changes in pH. Each acid-alkali indicator is usually a weak acid or a weak alkali, and the acidic form and the alkaline form have different colours.

For example, consider an acid-alkali indicator which is a weak acid HA(aq) and can be slightly ionized in water to give H+(aq) and A-(aq) ions. In acidic solutions, the indicator exists mainly as HA(aq) molecules and thus the solution shows the colour of HA(aq) molecules. But in alkali solutions, the indicator exists mainly as A-(aq) ions and thus the solution shows the colour of A-(aq) ions.

Based on the sources of indicators, there are two types of acid-alkali indicators: natural indicators (which are extracted from plants) and synthetic indicators (which are made in synthesis by human).

Some acid-base indicators are natural indicators, which are extracted from plants. For example, red cabbage juice and tea pigments show different colors when the pH is different. The color of tea darkens in an alkaline solution, but the color becomes lighter when lemon juice is put into a tea. Red cabbage juice turns blue in an alkaline solution, but it shows a distinct red color in an acidic solution.

Natural indicators exist in many plants. Some fruits change their colours (usually from green to yellow) when they ripe. This is because the pH values of these fruits change when they ripe, and thus the natural indicators in the fruits change their colours due to the changes in pH.

Natural indicators are just a type of acid-alkali indicators. The basic principle of natural indicators is just the same as acid-alkali indicators (refer to paragraph 2).
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