Lemon juice is a sour solution of a weak well known acid. It contains a weak acid called citric acid. Our indicator should be able to detect acids by changing colour.
What colour change occurs with our indicator when an acid is present?
Our indicator can also identify bases. These are substances that can react with acids to neutralise them. Bi-Carb of soda, the exact same substance we use in dough to make it rise, is a weak base.
What colour change occurs with our indicator when a weak base is present?
Notice how the colour of the indicator changes when Bi-Carb of soda is added to vinegar, in other words when a base is added to an acid.
Remember, a purple colour is the natural colour of red-cabbage and it indicates a neutral solution, neither an acid nor a base. Click to see how the acid and the base react.
From your observation, describe the colour change of the indicator as Bi-Carb of soda is added to the vinegar.
Was the acid neutralised? Explain
Was the B-Carb of soda neutralised? How do you know?
Detergent is a strong base. It is used to clean up stains in the kitchen. Watch what happens when a small amount of washing powder is placed in the indicator.
Coffee is a very popular drink. It tastes bitter. View the video on the right and decide what type of chemical is coffee.
We can use our indicator to test the acidity of the gas we breath out. Yes carbon dioxide. Using a straw bubble the gas exhaled though the indicator and watch the colour change. Is carbon dioxide an acid, base or neither. If it is neither an acid or a base we say it is neutral.
The image on the right shows the colour of the indicator before gas was bubbled through it. Click to see the colour change after the exhaled air was bubbled through it.
Test 8 items around the kitchen and create a table to identify them as an acid, base or neutral.
A few items you can start with:
- vinegar
- dish-washing liquid
- lemonade
- coke
- carbonated water
