The Acid-Base Solutions simulation from PhET lets you compare strong and weak acids and bases, and measure pH with pH paper or a probe and conductivity with electrodes. You can discover how concentration and strength affect pH, and even find a weak acid with the same pH as a strong acid. 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 chapters in Tamil Nadu State Board (Samacheer Kalvi) Chemistry.
What is the Acid-Base Solutions simulation?
This interactive simulation lets you study how acids and bases behave in water. You can change the concentration and the strength of a solution, then use pH paper, a pH probe or conductivity electrodes to see what is happening. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
High school to university
Acids, bases, pH, strength and concentration
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to relate the strength of an acid or base to how much it dissociates in water, to identify the molecules and ions in a solution, to compare strong and weak solutions, to tell concentrated from dilute solutions, and to explain how a pH meter, conductivity and pH paper show whether a solution is acidic or basic, strong or weak.
Launch the Acid-Base Solutions 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 Acid-Base Solutions simulation, step by step
- Predict first. Before you test anything, say whether you expect the solution to be acidic or basic, and strong or weak.
- Pick a solution. Choose a strong or weak acid or base and look at the molecules and ions in the water.
- Measure it. Use the pH probe or pH paper to read the pH, and use the conductivity electrodes to see how well it conducts.
- Change the concentration. Make the solution more concentrated or more dilute and watch how the pH and conductivity change.
- Compare strong and weak. Try to find a weak and a strong solution that give the same pH, and explain how they differ.
Tip for students: compare solutions of the same concentration first. That is how you find out what strength really changes.
The idea behind the simulation: pH = −log[H⁺]
pH = −log₁₀[H⁺]
Here [H⁺] is the concentration of hydronium ions in mol/L. At 25 °C, pOH = −log₁₀[OH⁻] and pH + pOH = 14. A strong acid dissociates almost completely, so [H⁺] is close to its concentration. A weak acid dissociates only a little, so [H⁺] is much smaller, and you find it from the acid constant Ka.
Worked example: A 0.1 mol/L strong acid such as HCl gives [H⁺] = 0.1, so pH = 1. A 0.1 mol/L weak acid with Ka = 1.8 × 10⁻⁵ gives [H⁺] ≈ 1.3 × 10⁻³ mol/L, so pH ≈ 2.88. Same concentration, but the weak acid has a much higher pH.
Acid and base key terms explained
These are the words you will meet while using the Acid-Base Solutions simulation, with simple meanings.
Acid
A substance that gives hydronium ions (H₃O⁺) in water.
Base
A substance that gives hydroxide ions (OH⁻) in water.
pH
A scale that shows how acidic or basic a solution is. Below 7 is acidic, 7 is neutral and above 7 is basic.
Strong acid or base
One that dissociates almost completely in water.
Weak acid or base
One that dissociates only partly, so most of its molecules stay whole.
Concentration
How much solute is in each litre of solution, in mol/L.
Dissociation
The splitting of a substance into ions when it dissolves in water.
Conductivity
How well a solution carries electric current. More ions give higher conductivity.
Equilibrium
The balance in a weak solution where molecules and ions form and recombine at the same rate.
Common mistakes: Mixing up strength with concentration, thinking a weak acid must have a high pH, and forgetting that a dilute strong acid can be less acidic than a concentrated weak one.
Try the pH calculator (practice what you explore)
Choose a type of solution and a concentration to see the pH. For weak solutions, enter Ka or Kb (acetic acid and ammonia are both about 1.8 × 10⁻⁵). Then tap a “what if” button.
What happens when you change a variable
| Change | Effect |
|---|---|
| Acid made more concentrated | pH goes down |
| Base made more concentrated | pH goes up |
| Weak acid swapped for strong acid (same concentration) | pH goes down, conductivity goes up |
| Solution diluted | pH moves closer to 7 |
| Strong acid diluted 10 times | pH rises by about 1 |
Classroom activities for teachers
The Acid-Base Solutions simulation fits a single 40-minute period.
- Predict and test (10 min): students predict the pH and conductivity of a solution, then check them.
- Same concentration (10 min): compare a strong and a weak acid at the same concentration and explain the ions present.
- Same pH challenge (10 min): find a weak and a strong solution with matching pH and explain how their concentrations differ.
- Exit ticket (10 min): students explain in two sentences the difference between strong and concentrated.
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 acids and bases
Score: 0 / 5
Acid-Base Solutions simulation: frequently asked questions
Is the Acid-Base Solutions 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 is the difference between a strong and a weak acid?
A strong acid dissociates almost completely in water, while a weak acid dissociates only partly. Strength is about dissociation, not about how much acid you add.
Can a weak acid have the same pH as a strong acid?
Yes. A concentrated weak acid can give the same pH as a more dilute strong acid, because pH depends on both strength and concentration.
The Acid-Base Solutions simulation turns invisible ions into something you can measure, compare and see. Explore it, take the quiz above, and keep experimenting until predicting the pH feels easy.