Thursday, 4 October 2007

The Acid Test

In this experiment we are going to be testing for acids and alkalis using nothing more than a red cabbage. Thanks to Aston for helping out with this one.

We need a home-made way of testing for acids and alkalis - a bit of kit otherwise known as a litmus indicator, or pH indicator. Have you used litmus paper at school? In acids the paper turns red and in alkalis it turns blue. Our home-made tester is slightly different - it turns pink in an acid and a bluey-green colour in an alkali.


The first step is to rip apart a red cabbage and put all the bits of leaves into a large saucepan. Pour boiling water into the saucepan over the leaves and leave it to steep (like a pot of tea) for a few minutes until the water is nice and purple.

Now pour off the purple cabbage water into separate glasses and we are now ready to test for acids and alkalis.

Finding the ingredients...

Acids are usually used in cooking: vinegar is an easy acid to get hold of. Lemon juice is also acidic. We tried lemon juice and vinegar. We tried putting the same amount of lemon juice in the indicator as vinegar. Our experiment seemed to show that lemon juice is more acidic than vinegar... and raspberry juice...

...which was very mildly acidic - which means it could well give you a poorly tummy if you drank too much of it - and also shop-bought coleslaw, which also contains a lot of vinegar but strangely didn't turn our indicator pink. We decided that perhaps the added "acidity regulator" ingredient had kept the acid levels down somehow.

Alkalis are usually used in cleaning (the old fashioned name for "alkali" is caustic, as in caustic soda). We tried baking soda (used in cooking but also a very weak alkali - the weakest alkali amongst the sodium compounds, if you didn't know), a Rennie (an indigestion tablet), bleach and a dishwasher tablet. All turned the indicator a murky blue to varying degrees, but the dishwasher tablet had the most marked effect - turning the indicator a yellow/green colour. It was difficult to tell if the change in colour is due to the fact that the detergent in the tablet is caustic or there is some other sort of chemical reaction going on...

So why would you want to test for acids and alkalis?

Okay, generally you probably wouldn't, but one place where the test is used quite a lot is in gardening. Certain plants like an acid soil while others love an alkali soil. A litmus test tells you if you have got your soil just right for your plant. My grandparents used to have a hydrangea in their front garden that they used to change the colour of the flowers of by mixing different things into the soil - an acid soil makes them pink and a neutral soil (mixing lime into the soil takes away its acidity) makes them purple. Don't laugh: that's the sort of thing that passed as entertainment when I was growing up in the 1970s.

Have a play with other chemicals but remember: cleaning chemicals can be very dangerous so don't let them splash into your face or get mixed up with anything you are planning on eating later.

Happy experimenting!


LizzieD said...

This is a wonderful blog! I'm educating my 10 yr old son at home (not far from you, in Shropshire!) and as I have a lowly Arts background, I need all the help I can get with the Maths, and sciences. (I came across your blog from the Maths forum). I love the idea of being shown how to do "dangerous" experiments at home - all the science stuff we've come across seem to stick to bicarb of soda and vinegar - gets a bit boring after a while!!
Please keep up the good work!

Inhibit said...

The yellow is a reaction you get from a very strong alkali. Something like lye with a PH value at the top of the chart.

YourMathsTutor said...

Hi inhibit,

Thanks for your comment. I wasn't sure if the tablet was so very strongly alkaline that something else was happening.