Diluting Acids and Bases – How Does the pH Change?
ChemistryAcids, Bases & TitrationAges 16–17Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Acids, Bases & Titration
A virtual experiment with a pH meter: start from an HCl, CH₃COOH, NaOH or NH₃ solution of concentration C; at each dilution step keep one portion and add distilled water up to the mark (10-fold or 100-fold). The table of pH after each step and the graph of pH against log(dilution factor) show that a strong acid rises by 1 pH unit for every 10-fold dilution, a weak acid by only about 0.5, and that the pH never crosses 7 because water itself ionizes.
Lesson: pH of solutions – diluting strong and weak acids and bases
What it shows
Diluting a solution lowers the concentration of the solute, but the pH responds differently for strong and weak electrolytes. HCl and NaOH dissociate completely, so each 10-fold dilution changes the pH by one unit. CH₃COOH and NH₃ are only partly ionized; dilution increases their degree of dissociation, so the pH shifts by only about half a unit per 10-fold step. At very low concentrations the ions from the self-ionization of water dominate, and the pH approaches 7 without crossing it.
How to use
Choose a Solute and an Initial concentration, and set Each dilution step to 10-fold or 100-fold. Press Dilute Once to watch a portion being kept and topped up with distilled water, or Run (keep diluting) for a series of steps. Follow the pH meter, the results table and the graph of pH against the dilution factor on a log scale. Turn on Curves for all 4 to compare strong and weak acids and bases.
Parameters you can change
- Solute HCl (strong acid), CH₃COOH (weak acid, Ka = 1.75×10⁻⁵), NaOH (strong base), NH₃ (weak base, Kb = 1.8×10⁻⁵)
- Initial concentration 0.001–1 M
- Each dilution step 10-fold (10 mL → 100 mL), 100-fold (1 mL → 100 mL)
- Show the theoretical curves of all four substances
Questions to explore
- How many times must 0.1 M HCl be diluted to reach pH 3?
- Why does a 10-fold dilution of CH₃COOH raise the pH by only about 0.5?
- If an HCl solution is diluted again and again, can its pH ever rise above 7? Why?
Teaching ideas
All guides- pH Scale Lesson Plan with an Interactive SimulationA pH scale lesson plan with a free simulation: dilution data with expected pH values, ion concentrations on a log scale, and a five-question set to attach.
- How to Embed a Simulation in Schoology and BrightspaceEmbed a simulation in Schoology and Brightspace (D2L): paste the embed code into a page, set the frame height, add questions and plan the lesson page.
