A few weeks before sowing or planting out cabbage I apply garden lime to the vegetable beds to increase the pH of the soil. Members of the cabbage family find it much easier to grow in base-rich or alkaline soils. I wouldn’t dream of doing this to my blueberries though, they are acid lovers! But what do we mean by acids and bases? And how can we tell whether a substance is acidic or basic? Well interestingly, cabbage could help us to discover some answers!
In this kids science experiment you can learn how to make your own red cabbage litmus paper – a useful indicator of whether a substance is acidic or basic. In addition we have some great ideas for exploring the properties of acids and bases in greater depth for older kids.
Safety Note: Adult supervision is required at all times during this experiment. Children need to be aware that handling strong acids and bases can be very dangerous. The chemicals suggested here are only mildly acidic or basic, nevertheless it is never too early to get into the habit of wearing protective clothing and goggles!
For This Experiment You Will Need The Following:
- Half a red cabbage
- A pan to boil the cabbage in (an adult job!)
- A bowl to strain your cabbage water into.
- Strips of cartridge paper cut into lengths about 15cm long by 2cm wide.
- A cooling rack placed on top of a tray is a useful place to dry your litmus strips ready for use.
- A collection of clean glass jars.
You will also need to gather together some household items to test for acidity or basicity. Here are our suggestions:
- Lemon juice
- Orange juice
- Coke or lemonade
- Bicarbonate of soda (baking soda) in a solution of water
- Toothpaste mixed into some water
- Indigestion tablets dissolved in water
- Soapy dishwater
- Whisked eggs
What to do:
Chop the cabbage up into small chunks, place it in the pan and just cover with water. Bring the pan of cabbage and water to the boil and continue to boil for ten minutes. Allow the pan to cool and then use a colander to strain the liquid into a bowl. Now soak your strips of cartridge paper in the bowl of liquid for a minute, they should turn a bluish-purplish colour. Just do a few strips at a time. You now need to wait for your strips to dry out (we left ours overnight) before you can use them in your experiment. We think the safest thing to do is just lay the strips out on a cooling rack positioned above a tray to catch the drips.
Fill your clean, glass jars with different substances to test. Don’t forget to label them! Dip the end of a strip of litmus paper into each jar. Note what happens. Does the paper stay pretty much the same colour? In which case you have either a neutral solution or just a very mild base. If it your paper turns violet, pink or red you definitely have tested an acid. In stronger base solutions your paper will turn turquoise, green or even yellow. (We got yellow when we used a strong solution of washing soda – but this is an irritant and careful handling is required!) Don’t forget to record what happens.
Take It Further:
So we have found a way to test whether a substance is an acid or a base, but this doesn’t explain what exactly an acid or a base is. Most people know that acids have a sour taste while bases are bitter and tend to be slippery in consistency (like soap.) For centuries people were aware of these properties and exploited them in various ways, but it took scientists a long time to work out why an acid is an acid or why a base is a base.
Acids and bases are measured on the pH scale (This stands for potential of hydrogen -see below for further explantion). This ranges from 0 to 14 with zero being the most strongly acidic and fourteen being the most strongly basic. A pH of 7 is considered neutral. Pure distilled water is neutral. Why not use your paper to do further experiments? Try distilled water (you can buy this cheaply from auto-spares stores), water from your rain barrel and some posh mineral water! Can you see any differences in the results?
If you were to mix a base and an acid together they would try to neutralize each other. A chemical reaction takes place. You can see this happening if you mix some of your bicarbonate of soda (baking soda) with the vinegar, it will create a really pleasing ‘fizz’. What is going on? Here is a useful way to help your Little Sparks visualize the processes at work, cue the M&Ms!
Older children will be aware that atoms are made up of positively charged protons, (also possibly neutrons which carry no charge) and negatively charged electrons. When acids and bases are in solution with water (our blue circles) they breakdown or ‘dissociate‘. When this happens they start ‘swapping’ their hydrogen protons (which are written as H+ and here represented by red M&Ms!). Acids ‘donate’ hydrogen protons to the solution. Bases ‘take-up’ hydrogen protons meaning the solution has a greater number of negative ions (these are hydroxide ions written as OH- and represented here by blue M&Ms!). Both strong acids and strong bases are highly charged, this makes them great conductors and very useful in batteries. Our Little Sparks found this visual representation really useful in understanding acids and bases, and were able to play with different strengths of acids and bases as well as represent the process of neutralization.
After you have eaten the M&Ms you will need to neutralize the sugar acid attack on your teeth by using toothpaste, which you now know from your litmus test is a base!