Red Cabbage Indicator
Red Cabbage Indicator
Indicators are chemical compounds that can be added to a solution to determine whether it is acidicor alkaline.
The indicator will change colour depending on whether an acid or an alkali is added.
The colour in red cabbage (it is a pigment called an anthocyanin) makes a very good indicator.
Acids and alkalis
Acids have a sour taste, like vinegar (which contains ethanoic acid) and lemons (which contain citric acid).
Alkalis are substances that react with acids and neutralise them. Soap and washing powder are alkaline.
Acids and alkalis are found in a surprising number of places. Some are edible and are found in foods. Others are very strong and can be harmful, such as the acid in car batteries and the alkali in oven cleaners.
Our experiment
First we had to extract the purple dye. To do this we tore our red cabbage leaves into very tiny pieces.
You could also chop up the leaves using a sharp knife or a blender, or grind them in a pestle and mortar.
We put the chopped cabbage into a jug, poured hot water onto it and left it for a few minutes. The water turned a bright purplish-blue colour. Then we used a fine sieve over a funnel to strain the pieces of cabbage out.
Each of us had a pipette and a small clear beaker. We pipetted a small amount of indicator solution into our beakers.
To begin with the indicator was purple. This is because water alone is not acidic or alkaline - we say it is neutral.
Then we added a different test compound to each small beaker.
We started by testing vinegar, which is weakly acidic, and sodium bicarbonate, which makes a weakalkaline solution in water.
The acid turned the indicator pink, and the alkali turned it blue.
The limescale remover turned the indicator red and the oven cleaner turned it green.
Limescale remover contains astrong acid called hydrochloric acid and oven cleaner contains astrong alkali called ammonia.
So we could use our indicator to tell the difference between strongand weak acids and
Indicators are chemical compounds that can be added to a solution to determine whether it is acidicor alkaline.
The indicator will change colour depending on whether an acid or an alkali is added.
The colour in red cabbage (it is a pigment called an anthocyanin) makes a very good indicator.
Acids and alkalis
Acids have a sour taste, like vinegar (which contains ethanoic acid) and lemons (which contain citric acid).
Alkalis are substances that react with acids and neutralise them. Soap and washing powder are alkaline.
Acids and alkalis are found in a surprising number of places. Some are edible and are found in foods. Others are very strong and can be harmful, such as the acid in car batteries and the alkali in oven cleaners.
Our experiment
First we had to extract the purple dye. To do this we tore our red cabbage leaves into very tiny pieces.
You could also chop up the leaves using a sharp knife or a blender, or grind them in a pestle and mortar.
We put the chopped cabbage into a jug, poured hot water onto it and left it for a few minutes. The water turned a bright purplish-blue colour. Then we used a fine sieve over a funnel to strain the pieces of cabbage out.
Each of us had a pipette and a small clear beaker. We pipetted a small amount of indicator solution into our beakers.
To begin with the indicator was purple. This is because water alone is not acidic or alkaline - we say it is neutral.
Then we added a different test compound to each small beaker.
We started by testing vinegar, which is weakly acidic, and sodium bicarbonate, which makes a weakalkaline solution in water.
The acid turned the indicator pink, and the alkali turned it blue.
The limescale remover turned the indicator red and the oven cleaner turned it green.
Limescale remover contains astrong acid called hydrochloric acid and oven cleaner contains astrong alkali called ammonia.
So we could use our indicator to tell the difference between strongand weak acids and