What is the pH scale?
Quick answer
The pH scale measures how acidic or alkaline (basic) a solution is, usually from 0 to 14. A pH of 7 is neutral, like pure water. Below 7 is acidic (like lemon juice), and above 7 is alkaline (like baking soda). Each step is 10 times stronger than the one before.
The pH of everyday things
| pH | Example | Type |
|---|---|---|
| 0–1 | Battery acid | Strongly acidic |
| ~2 | Lemon juice, stomach acid (1.5–3.5) | Acidic |
| ~2.5–3 | Vinegar, cola | Acidic |
| ~5 | Black coffee | Mildly acidic |
| ~5.6 | Normal rain | Slightly acidic |
| 7 | Pure water | Neutral |
| ~7.4 | Human blood | Slightly alkaline |
| ~8 | Seawater | Mildly alkaline |
| ~9 | Baking soda solution | Alkaline |
| ~11–12 | Ammonia cleaner | Strongly alkaline |
| ~12.5–13 | Bleach | Strongly alkaline |
Why each step is 10 times bigger
pH is a logarithmic scale. A solution with pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5. Small changes in pH can mean big chemical differences.
What does pH measure?
pH measures the concentration of hydrogen ions (H⁺) in water. More hydrogen ions mean a more acidic solution. The scale was invented by Danish chemist Søren Sørensen in 1909. What “p” stands for is debated; it’s often explained as “power” or “potential” of hydrogen.
How to measure pH
- Litmus paper: turns red in acid and blue in alkaline.
- Universal indicator: changes through a rainbow of colors.
- Digital pH meters: give precise readings.
- Red cabbage juice works as a homemade indicator.
Why pH matters
- Soil pH affects which plants grow well; blueberries love acidic soil.
- Swimming pools are kept around 7.2–7.8.
- Oceans are slowly becoming more acidic as they absorb carbon dioxide; see why the ocean is important.
- Your body keeps blood pH in a tight range around 7.4.
For more chemistry, see the most abundant element in the universe.
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