Take a red cabbage and dice it. Put it in some water in a saucepan, bring it to the boil and keep it there for half an hour. Check on it periodically to make sure it doesn't boil dry. Then strain it and put the liquid into a container. It will be a purplish colour.
Now take some white unpatterned saucers and put the liquid into each of them. Add various substances to them, such as spirit vinegar (which is clear), citric acid powder, bicarbonate of soda, cola (which is brown, so this won't work quite as well), chalk, toothpaste, lime juice (more acidic than lemon juice), urine and mixtures of them. Also, very carefully, you might want to try things like caustic soda, caustic potash, bleach, sulphuric acid and so on, but if you do, add them carefully because the liquid can sputter and the chemicals are dangerous, and don't try to mix the stronger chemicals.
The liquids will change colour, indicating the acidity and alkalinity of the different substances.
That's the basic idea. If you want more info, here it is:
This is the image of Science In The Park doing the activity in autumn 2013, in Vicky Park used as the banner for the FB group you're probably in if you're reading this:
There are substances of different pH (acidity and alkalinity) in this photo, in hummus tubs, including water. I can't remember what I used. There's also a measuring cylinder, further to the discussion I had with someone at Stoneygate today about equipment for science stuff.
pH is a measure of the concentration of hydrogen ions and hydroxyl ions in a water solution of the substance concerned. A hydrogen ion is, in this case, a hydrogen atom without an electron, although that's not really what happens, because a hydrogen ion without an electron is usually in fact a proton because a hydrogen atom is usually a single proton with a single electron in its single orbital, like this:
The more hydrogen ions there are in a water solution, the more acidic the liquid is. Water is dihydrogen monoxide:
Therefore an acid can be thought of as something which causes hydrogen in water to float around separately. An alkali, on the other hand, is a substance which causes hydroxyl groups - the "OH" bits of the H2O - to do the same. An alkali which hasn't dissolved in water is a base. There are substances which do both, such as some amino acids, which are both acid and alkaline. However, the hydrogen and hydroxyl ions needn't be from the water itself, but from the substances.
The pH scale ranges from zero to fourteen, with acids at the bottom and alkalis at the top. Caustic soda, or sodium hydroxide, is one of the most powerful alkalis known with a pH of fourteen. The scale is logarithmic - each step is ten times the previous one - and it's a measurement of concentration of hydrogen and hydroxyl ions.
Organic acids, which are often found in living things, are generally weaker than inorganic, also known as mineral, acids, although they can be quite strong. Examples of organic acids include ethanoic (acetic) acid, which is in vinegar and is the end of a process where life gets energy out of sugar, then ethanol (common alcohol) and finally acetic acid and is also used for defence by spider-like animals called vinegaroons (which turn up in one of the Harry Potter films):
|"Whip scorpion" by Glenn Bartolotti - Own work. Licensed under CC BY-SA 4.0 via Commons - https://commons.wikimedia.org/wiki/File:Whip_scorpion.jpg#/media/File:Whip_scorpion.jpg|
Other organic acids are citric acid, which is in all living things but associated with lemons and other citrus fruits, formic acid, which ants use to defend themselves and makes them taste lemony (I speak from experience - I used to eat ants before I went veggie) and is also in nettle stings (along with lots of other things such as histamine), oxalic acid, which is in rhubarb leaves and is a corrosive poison and also makes up the kidney stones you get when you overdose on vitamin C, tartaric acid, which is in sherbert, and ascorbic acid, which is part of vitamin C. Although organic acids are acidic, they can become alkaline after the body processes them, so you can eat a whole load of acidic foods and they can reduce the acidity of your body. Often things which are described as acids are only technically acids and their acidity may not be harmful or relevant, such as DNA (deoxyribonucleic acid) and amino acids.
Inorganic acids are sometimes in organic life forms, such as the hydrochloric acid in the stomach. This was first discovered after a Canadian, Alexis St Martin, suffered a gunshot wound and it was found that his stomach produced a liquid able to dissolve a pig's rib completely within ten hours. Phosphoric acid is the acid put into colas which stops them from causing people who drink them from being sick due to the enormous amount of sugar they have in them. My version of cola uses citric acid, or usually lime juice in fact. Onions make sulphuric acid in the eyes when they're cut which is why the eyes water when that happens.
Inorganic acids can be stronger than organic ones because they are often smaller with respect to the number of hydrogen ions they contain.
Alkalis are less well-known than acids but they include caustic soda - sodium hydroxide - which is good for making soap, calcium carbonate, which is limestone, calcite and chalk and can be used as an antacid in the stomach, and sodium bicarbonate, which is used for baking scones but of course also for doing the carbon dioxide thing.
As for the indicators, i.e. the substances which change colour according to how acidic or alkaline the substance they're dipped into is, the really good ones are used in litmus paper. Litmus uses pigments from lichen, which is this stuff:
|"N2 Lichen" by Roantrum - Flickr. Licensed under CC BY 2.0 via Commons - https://commons.wikimedia.org/wiki/File:N2_Lichen.jpg#/media/File:N2_Lichen.jpg|
The stuff in red cabbage water which indicates pH is anthocyanin, which is also found in a lot of other plants. Coloured petals are often that colour because the stuff in them is an anthocyanin and the pH in them is at a certain level.
Finally, if you ever want to eat blue food, make scones with red cabbage water and they will come out turquoise!