Question

prepare red cabbage juice by boiling a price of red cabbage in water. use it as an indicator and test acidic and basic solutions with it. write your observations in the form of taste
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Answer 1

Answer:

Indicators are chemical compounds that can be added to a solution to determine whether it is acidic or alkaline.  

The indicator will change color depending on whether an acid or an alkali is added.  

The color 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 weak alkaline 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 a strong acid called hydrochloric acid and oven cleaner contains a strong alkali called ammonia.

So we could use our indicator to tell the difference between strong and weak acids and alkalis.  

We also tested some bleach, which contains the strong alkali sodium hydroxide. The sodium hydroxide in the bleach turned the indicator pale green at first, but then (as the bleach bleached the colour out of the indicator) it went pale yellow!  

   

Now we tested some other substances, these are some of the other things we tried:  

• orange juice  

• washing powder  

• cream cleaner  

• soap  

• lemon juice  

• cream cleaner  

 

We put a few drops of each substance into our little beakers of indicator and stirred them carefully, then watched for the colour changes. Here are some photos of us doing the experiment.  

We found that the acids turned the indicator pink or red, and alkalis turned it blue, green or yellow depending on how strong the alkali was.  

We also tried testing sugar and salt. Neither of them made the indicator change colour - so they were not acid or alkaline. Sugar and salt solution are neutral (like water).