This week, some colour-changing chemistry. We're going to make an indicator solution, which changes colour when it is mixed with an acid or a base.
Warning: The sharp knife can cut you, the hot water may burn and many household chemicals, such as borax, may be toxic. Have an adult assist you with this activity.
You will need
Fresh red cabbage (which is purple)
a sharp knife
a cutting board
hot tap water
2 large plastic bottles
a strainer
a funnel
disposable plastic cups
an acid, such as lemon juice or vinegar
a base, such as dishwashing detergent or borax.
Not all dishwashing detergents will work well for this activity. I found "Morning Fresh" didn't work at all, but "Dawn" was very effective. You may need to experiment a bit to find one that works well.
Other household chemicals, such as cleaners.
an adult
What to do
To make your indicator solution:
Finely slice 3-4 red cabbage leaves.
Place the sliced cabbage in a plastic bottle.
I found it helped to use a rolled piece of paper to funnel the cabbage into the bottle.
Half fill the bottle with hot water and screw the lid on tightly.
Swirl the bottle around and then unscrew the lid just enough to let the pressure out (you have just heated the air and made it expand. Tighten the lid again.
Shake the bottle for a few minutes until the water becomes a deep purple colour.
Leave the solution to cool.
Now strain the solution into the other bottle and add enough water to the solution to make about one litre.
To use your indicator
Place a small amount of lemon juice in a cup.
Place a small amount of dishwashing detergent in another cup.
Add the indicator solution to each cup until they are half full.
Observe the colour of the solution. You should find the cup with the lemon juice turns red, while the cup with the detergent may turn blue.
Try testing other substances around the home.
As well as liquids, you can also test solids that dissolve in water.
Remember that some chemicals, especially cleaners, can be toxic. Make sure to read any safety instructions carefully.
What's happening?
You have made an acid-base indicator.
Acids and bases are chemical compounds. To understand the difference, first we need to know that an ion is a molecule or atom that has an electrical charge (normally there are equal numbers of positive and negatively charged particles in an atom or molecule, so they cancel out and there is no overall charge). There are several ways to define an acid or base, but one of the most common is:
An acid is a compound that can give up a positively-charged hydrogen ion in a chemical reaction.
A base is a compound that will join onto a hydrogen ion in a chemical reaction.
Acids are often found in living things. The "A" in DNA stands for "acid". Bases are often found in cleaning compounds.
We describe whether things are acidic, basic or neutral by using a scale called the pH scale. Pure water has a pH of 7 and is regarded as neutral. The pH scale ranges from: 0 for a very strong acid to 3 - 5 for weak acids to 8 - 9 for weak bases to 13 - 14 very strong bases.
If you mix an acid and a base together in water, they will neutralise each other. Normally, they react with each other to produce water and another chemical, called a salt. Try adding some lemon juice to the cup that contains the detergent and cabbage solution. If you add enough, the base in the detergent will be neutralized and the solution will change colour.
Red cabbage contains a chemical called flavin, which reacts with acid and bases to produce different colours. In low pH (acid), flavin turns red. In high pH (base), it turns greenish yellow. If the pH is near the middle of the scale, it turns a blue colour.
Many plants contain pigments that can be used as acid-base indicators. The most well-known is a lichen called Litmus, which changes from blue to red depending on the pH. The outside of a rhubarb stem can be used to make an indicator the same way we did with the cabbage leaves. You can even use a strong cup of tea, which will become lighter coloured if you add lemon juice (you can then add some sugar and ice, refrigerate it for a while and have a refreshing drink).
Applications
Soil can be acidic or basic, depending on what chemicals are in it. Many plants will only grow well in a particular pH range, so farmers and gardeners often have to add chemicals to their soil to produce the right pH for their plants. Acid-base indicators like our cabbage solution are often used to work out how acidic or basic the soil is.
Finely chop some cabbage
Place the cabbage in a bottle, add water and shake until the water turns purple. Take care with the hot water.
Strain the cabbage from the liquid
The indicator solution, mixed with lemon juice (left), on it own (middle) and mixed with detergent (right).
