Dyeing 4
Using dyes to test for acidity
Aim
To understand that dyes can be used to measure the acidity of substances.
Many plant dyes will change colour when mixed with either an acid (substances such as vinegar or lemon juice) or a base (substances such as baking soda or oven cleaner). We call these dyes indicators (substances that change colour when an acid or base is added). Tanekaha dye changes from brown to reddish brown when a base is added. Raurekau dye will turn bright yellow when an acid is added and will turn orange when a base is added.
Acids react with many substances including raw egg and milk. Your stomach makes acid that helps break down your food. Bases are the opposite of acids. For stomach-ache, tablets are made from bases. When you take these tablets, it helps get rid of too much acid in your stomach. When an acid and a base are mixed together, they will cancel each other out by changing into water and a new substance called a salt. Substances such as salt and water are neither acid nor a base: they are neutral.
The following two activities use juice from petals to test substances from the kitchen to see if they are acids or bases.