First, we need to understand the chemistry of purple litmus. The main ingredient of purple litmus is a natural pigment called "litmus", which has the properties of an acid-base indicator. In an acidic environment, the carboxyl groups in litmus are protonated, forming a red carboxyl cation, giving purple litmus a red color. In an alkaline environment, the carboxyl group in the litmus strips its protons to form a blue carboxyl anion, giving the purple litmus a blue color.
Next, we can experimentally verify the color change of purple litmus when it encounters an acid-base. First of all, we can prepare some common acids and alkalis, such as hydrochloric acid, hydrochloric acid, dilute hydrochloric acid, sodium hydroxide solution, etc. Then, we put purple litmus paper into each of these acids and bases to observe the change in color.
First, we put the purple litmus paper in dilute hydrochloric acid. Dilute hydrochloric acid is an acidic substance, so purple litmus paper will appear red. As the concentration of dilute hydrochloric acid increases, the color of the test strip gradually deepens, and finally it appears dark red.
Next, we put the purple litmus paper into the sodium hydroxide solution. The sodium hydroxide solution is an alkaline substance, so the purple litmus paper will appear blue. As the concentration of sodium hydroxide increases, the color of the test strip gradually deepens, eventually taking on a dark blue color.
With this experiment, we can clearly see that purple litmus will appear red when it encounters an acid and blue when it encounters a base. Therefore, purple litmus is a very useful acid-base indicator that can be used to detect the acidity and alkalinity of a solution.
In addition to being used to detect the acidity or alkalinity of a solution, purple litmus can also be used to detect the acidity or alkalinity of a gas. For example, we can detect the acidity and alkalinity of carbon dioxide gas by placing a moistened purple litmus paper in carbon dioxide gas and observing the color change of the test paper. If the test strip turns red, the carbon dioxide gas is acidic;If the test strip turns blue, the carbon dioxide gas is alkaline.
In conclusion, purple litmus is a very useful acid-base indicator that can be used to detect the acidity and alkalinity of solutions and gases. By looking at the color change of purple litmus, we can easily judge the acidity and alkalinity of a solution or gas. Hopefully, this simple experiment will help you better understand the properties and uses of purple litmus.