Students know lemons are sour and soap is slippery, but the pH scale often stays an abstract number line. The free PhET simulation pH Scale lets them test everyday liquids, add water and watch the pH change. This PhET pH Scale lab gives you a ready lesson for grades 7–11.
In this guide8 sections
Lab at a glance
- Grade level: 7–11
- Time: 45–50 minutes
- Objectives: classify solutions as acidic, basic or neutral; describe how dilution affects pH; relate pH to hydronium and hydroxide ions.
Know the simulation
- Macro screen: choose an everyday solution, measure its pH with a probe, add water or drain liquid.
- Micro screen: see relative amounts of hydronium (H₃O⁺) and hydroxide (OH⁻) ions.
- My Solution screen: adjust the pH directly and see how ion concentrations change.
Step-by-step lesson plan
- Hook (5 min). Students rank five household liquids from most acidic to most basic before testing.
- Macro testing (12 min). Measure each solution and record pH in a table. Compare with predictions.
- Dilution (10 min). Pick one acid and one base. Add water in steps and record pH each time.
- Micro screen (10 min). Compare ion amounts for an acid, a base and water.
- Discussion (5 min). Why does adding water move pH toward 7 but never past it?
- Exit ticket (3 min). Classify three new solutions and explain one.
Worksheet questions
- Which tested solutions were acidic, which were basic and which were close to neutral?
- What happened to the pH of an acid as you added water? Of a base?
- Can adding water turn an acid into a base? Explain.
- On the Micro screen, which ion is more common in an acid? In a base?
- Why is the pH scale described as logarithmic?
Answer guide
| Question | Expected answer |
|---|---|
| 1 | Drinks like soda, juice and coffee are acidic; soap and drain cleaner are basic; water and blood are near neutral (blood is slightly basic). |
| 2 | Adding water moves both acids and bases closer to pH 7. |
| 3 | No. Dilution moves pH toward 7 but does not cross it. |
| 4 | Acids have more H₃O⁺; bases have more OH⁻. |
| 5 | Each step of 1 pH unit is a tenfold change in hydronium concentration. |
Common misconceptions
- “Neutral means no ions.” Pure water contains small, equal amounts of H₃O⁺ and OH⁻.
- “pH 4 is twice as acidic as pH 2.” It is actually 100 times less acidic.
- “All acids are dangerous.” Many foods are acidic; strength and concentration matter.
Extensions
- Test real household liquids with red cabbage indicator (with safety guidance).
- Connect to chemical reactions in our Balancing Chemical Equations lab.
- Review molecule structure in our Build a Molecule lab.
Frequently asked questions
What does the PhET pH Scale simulation show?
It shows the pH of everyday solutions, how dilution changes pH and the relative amounts of hydronium and hydroxide ions.
Does adding water change pH?
Yes. Adding water moves the pH of acids and bases closer to 7.
Is the pH Scale simulation free?
Yes, it is free to use online from PhET at the University of Colorado Boulder.
pH Scale turns a number line into an experiment. For more ready-to-teach simulations, browse our science lab guides.
