The pH scale
An aquarium shop pins up a card. Keep the water between pH 6.5 and 7.5, it says.
pH is a number that shows how acidic or basic something is.
- A line from 0 to 14.
- Colours fill in: red, yellow, green, blue, violet.
- 7 is in the middle: neutral.
- A few things take their approximate place on the line.
Slide the pH to where each thing belongs. Then press Place it.
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The rule
Usually, pH runs from 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is basic. A lower pH means more H⁺.
Universal indicator is a mixture of dyes. Its colour shows the pH. Paper with the indicator on it is often used. The colours are only a rough guide, and a shade can differ.
| Reading, to the nearest whole number | Band | A universal indicator reads |
|---|---|---|
| 0 to 3 | strongly acidic | red |
| 4 to 6 | weakly acidic | yellow or orange |
| 7 | neutral | green |
| 8 to 11 | weakly basic | blue |
| 12 to 14 | strongly basic | violet |
| Thing | Band |
|---|---|
| Lemon juice | strongly acidic |
| Stomach acid | strongly acidic |
| Curd | weakly acidic |
| Pure water | neutral |
| Soap in water | weakly basic |
| Sodium hydroxide solution | strongly basic |
Compare equal amounts of acid in equal water. A strong acid frees many H⁺ ions. A weak acid frees fewer. Adding water to an acid spreads out its H⁺ ions. So its pH rises toward 7.
A teacher adds acid to water, never water to acid. The mixing gives out a lot of heat.
The aquarium card asks for water close to neutral, pH 7.