What are acids and bases?
Let's learn a little chemistry
Acids and bases are everyday household items like aspirin (acetylsalicylic acid), baking soda (sodium bicarbonate), vinegar (acetic acid), and milk of magnesia (magnesium hydroxide). Yet, most of us do not know what makes an acid an acid or a base a base. Chemically, an acid is a substance that gives up a hydrogen ion (H+) when dissolved in water. Commonly, we think of acids as having a sour taste and reacting with bases to give off bubbles of carbon dioxide gas. A base is defined chemically as a substance that gives up a hydroxide ion (OH-) when dissolved in water. Most people associate bases with a bitter taste and a slippery feel, like soaps which contain bases.
Photo credit: Jamesongravity on Flickr
We measure how acidic or basic something is with the pH scale. The scale goes from 0 to 14. A substance is neutral with a pH of 7, just like pure water. If a solution has a pH less than 7, it is acidic. The more acidic the lower the pH. For example, lemon juice has a pH of 2 and vinegar is about 3. Because the scale is logarithmic, a pH of 2 is 10 times more acidic than a pH of 3, which, in turn is 10 times more acidic than a pH of 4. If a solution has a pH greater than 7, it is basic, and the higher the number the more basic it is. Baking soda has a pH of 9 and ammonia has a pH of 11, much more basic.
What's an indicator?
Acid base indicators have distinctly different colors when they are added to acids and bases. A common indicator used in chemistry classes is phenophthalein, which is colorless in acids, but turns bright pink in basic solutions.
Most people don't have a bottle of phenophthalein in their kitchen cabinets, so how do you easily demonstrate acid base chemsitry to elementary and middle school students? By using purple cabbage and tea, both of which are fine acid base indicators.
Purple cabbage extract
An easy, but smelly, acid base indicator
Photo credit: brittgow on Flickr
What you need:
- a pot
- distilled water
- 1/4 head of purple cabbage, chopped
- a bowl
- a colander
- a coffee filter
- some common acids: vinegar, lemon juice, orange juice, soda, black coffee
- some common bases: baking soda, milk of magnesia, ammonia, bleach
- clear containers (beakers, small cups or glasses, test tubes, etc.)
What you do:
- Put the cabbage in the pot and cover with distilled water. Bring to a boil, simmer for 10 minutes. Let it cool down.
- Strain the liquid, using a colander lined with a coffee filter, into a bowl.
- Pour some of the purple cabbage extract into your clear containers, leaving room for the other substances.
- Make solutions of powders (baking soda) by mixing with distilled water. Make dilutions of dark (coffee) or opaque liquids (orange juice and milk of magnesia) by diluting with distilled water.
- Add a few drops of an acid to the purple cabbage extract and watch what happens. Try this with as many acids and bases as you have. The bases should turn the cabbage extract green and perhaps even yellow if it is a strong enough base (high pH). The acids should turn the extract violet or red (the lower the pH the redder).
A much more pleasant acid base indicator
Photo credit: timlewisnm on Flickr
What you need:
- a small pot of weakly brewed black tea (not too dark)
- several clear containers (beakers, test tubes, small plastic cups or glasses)
- common acids (see above)
- common bases (see above)
What you do:
- Put approximately the same amount of tea in each container
- Add a few drops of each of your acids and bases to the containers. Watch what happens. Acids will make the tea yellow orange - the stronger the acid (low pH) the lighter the tea looks. Bases will make the tea brown or even black, perhaps even murky looking.