Colours of Chemistry: The pH Scale


What do you need?

Spring is coming soon. Thus, I thought it was time to do a colorful experiment. Below is the equipment you will need to replicate the experiment. For the alkaline cleaner, we recommend adult supervision.

What is the experiment about?

What happens?

Red cabbage contains anthocyanins which act as coloring agents. Heat from cooking causes these molecules to disperse in water. In the presence of an acid or a base, the structure of the anthocyanin is changed slightly. Because of this, the molecule’s ability to absorb light is also changed.

The strength of an acid refers to its ability or tendency to lose hydrogen ions (H+). In a strong acid, the molecules are completely detached in water. In contrast, a strong base is a basic chemical that can remove hydrogen ions (H+) from a molecule of a very weak acid in an acid-base reaction.

An example of a weak acid is shown in the figure below. Here acetic acid donates one hydrogen ion (green) to water in an equilibrium reaction. This results in an acetate ion and hydronium ion.

Acid reaction example

CH3COOH + H2O is in equilibrium with CH3COO− + H3O+ Credit: Ben Mills

The pH scale

In chemistry, the pH scale is used to define how acidic or basic a solution is. In this experiment, the colour is a direct indicator of the pH level. Touch the interactive figure below to view the red cabbage scale.

By setting up this experiment you will now have your very own pH meter. You can test other foods in the kitchen such as tomatoes, cucumbers or even vinegar – yuk 🙂

Does the pH level change if you add sugar to vinegar?

Where is the pH scale used?

From swimming pools to satellite monitoring of the Earth’s oceans. As the climate changes, so does the chemistry of seawater. If the water becomes too acidic or too basic it can be harmful to various life forms. Similarly, this is why the pH level of a swimming pool is carefully monitored.

pH scale swimming pool

Photo: eak_kkk / Pixabay

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