The pH Scale simulation from PhET lets you measure the pH of everyday liquids such as coffee, saliva and soap, and decide whether each one is acidic, basic or neutral. You can see the hydronium and hydroxide ions, switch between a logarithmic and a linear scale, test what dilution does, and even design your own liquid. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for the acids, bases and pH lessons in Tamil Nadu State Board (Samacheer Kalvi) Chemistry.
What is the pH Scale simulation?
This interactive simulation lets you test the pH of common liquids and see what is going on at the molecular level. You can compare the number of hydronium and hydroxide ions, try the effect of adding water, and build your own solution. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
pH, acids, bases, dilution and concentration
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to decide whether a liquid is acidic, basic or neutral, to put acids and bases in order, to describe how water’s equilibrium changes with pH, to find the hydroxide, hydronium and water concentrations at a given pH, to relate a liquid’s color to its pH, and to predict how dilution and volume affect pH.
Launch the pH Scale simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the pH Scale simulation, step by step
- Predict first. Before you test a liquid, say whether you think it is acidic, basic or neutral.
- Measure the pH. Test different liquids and note the pH of each one.
- Look at the ions. Compare the number of hydronium (H₃O⁺) and hydroxide (OH⁻) ions in each liquid.
- Try dilution and volume. Add water or change the amount of liquid and see whether the pH changes.
- Design your own liquid. Make a custom solution and place it on the scale.
Tip for students: pH is a log scale. Each step of 1 on the pH scale is a 10 times change in the concentration of ions.
The idea behind the simulation: pH = −log[H₃O⁺]
pH = −log₁₀[H₃O⁺]
Here [H₃O⁺] is the hydronium ion concentration in mol/L. At 25 °C, [H₃O⁺] × [OH⁻] = 10⁻¹⁴, so pH + pOH = 14. Pure water has a pH of 7 and is neutral. A pH below 7 is acidic and a pH above 7 is basic.
Worked example: A liquid has a pH of 3. Then [H₃O⁺] = 10⁻³ mol/L and [OH⁻] = 10⁻¹¹ mol/L. Pure water has [H₃O⁺] = 10⁻⁷ mol/L, so this liquid has 10,000 times more hydronium ions than pure water. It is acidic.
pH key terms explained
These are the words you will meet while using the pH Scale simulation, with simple meanings.
pH
A number that shows how acidic or basic a liquid is.
Acid
A liquid with a pH below 7. It has more hydronium ions than hydroxide ions.
Base
A liquid with a pH above 7. It has more hydroxide ions than hydronium ions.
Neutral
A liquid with a pH of 7, such as pure water, with equal hydronium and hydroxide ions.
Hydronium (H₃O⁺)
The ion that makes a solution acidic.
Hydroxide (OH⁻)
The ion that makes a solution basic.
Dilution
Adding water to make a solution less concentrated.
Logarithmic scale
A scale where each step is a 10 times change.
Common mistakes: Thinking that pH 4 is only a little more acidic than pH 5 when it is 10 times more acidic, believing that diluting an acid can make it basic, and mixing up which side of 7 is acidic.
Try the pH to concentration calculator (practice what you explore)
Choose a liquid or type a pH from 0 to 14 to find the ion concentrations. Then tap a “what if” button.
What happens when you change a variable
| Change | Effect |
|---|---|
| pH goes down by 1 | Hydronium ions go up 10 times |
| pH goes up by 1 | Hydroxide ions go up 10 times |
| Add water to an acid | The pH rises toward 7 |
| Add water to a base | The pH falls toward 7 |
| Change the volume only (same liquid) | The pH stays the same |
Classroom activities for teachers
The pH Scale simulation fits a single 40-minute period.
- Predict and test (10 min): students sort liquids as acidic, basic or neutral, then check with the pH probe.
- Order the liquids (10 min): students put the liquids in order from most acidic to most basic.
- Dilution test (10 min): students dilute an acid and record how the pH and ion counts change.
- Exit ticket (10 min): students explain in two sentences why pH 3 is much more acidic than pH 5.
Looking for the matching chapters? Browse the Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the acids and bases lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of pH
Score: 0 / 5
pH Scale simulation: frequently asked questions
Is the pH Scale simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
What does a pH of 7 mean?
A pH of 7 is neutral. The hydronium and hydroxide ion concentrations are equal, as in pure water at 25 °C.
Does adding water change the pH?
Yes. Adding water to an acid raises its pH toward 7, and adding water to a base lowers its pH toward 7. Changing only the volume of the same liquid does not change the pH.
The pH Scale simulation turns invisible ions into something you can measure, compare and see. Explore it, take the quiz above, and keep experimenting until reading the pH scale feels easy.