Chemistry
Acids & bases
From lemon juice to soap: the pH scale gives sour and soapy a number.
What you need first
Bite into a lemon and your face scrunches up: sour! Let soapy water run over your fingers and it feels slippery. Both are chemical properties you already know. Chemistry gives them names and even a measuring scale.
The pH scale from 0 to 14
How acidic or soapy a solution is gets measured with the pH value. The pH scale runs from 0 to 14. Values below 7 mean acidic, and the smaller the number, the stronger the acid. Values above 7 mean basic. Solutions like that are called bases, and they are the ones that feel soapy. Right in the middle, at pH 7, a solution is neutral, just like pure water. One thing matters here: a single step on the scale already means a tenfold difference. A solution at pH 3 is ten times as acidic as one at pH 4.
Indicators show their colours
You cannot see the pH value, but make it visible: these are dyes that change colour depending on the pH. The most famous one is in red cabbage! Its juice turns red in acids, stays purple when neutral, and turns green to yellow in bases. A vegetable becomes a measuring device that is often used in class.
From stomach acid to drain cleaner
Your stomach acid has a pH of about 1.5 and helps you digest. Lemon juice sits at about pH 2, vinegar at pH 3, pure water at pH 7. On the basic side come soapy water at about pH 9 and drain cleaner at about pH 13. Careful: strong acids and strong bases are corrosive and can badly injure skin and eyes. Never experiment with such substances on your own, only with adults or in class wearing safety goggles. And you never identify an unknown substance by tasting or touching it, always use an indicator instead.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What is typical of acids in everyday life?
At what pH value is a solution neutral?
Up to what value does the pH scale go?
Order these liquids by their pH value, from the strongest acid to the strongest base.
- 1Stomach acid (about pH 1.5)
- 2Soapy water (about pH 9)
- 3Drain cleaner (about pH 13)
- 4Pure water (pH 7)
- 5Vinegar (about pH 3)
You add red cabbage juice to a glass of vinegar and it turns red. What does that tell you?
Red cabbage juice is an indicator. Match each colour to the matching solution.
Where this leads