Acids, bases and indicators – titration drop by drop
Integrated ScienceChemical ReactionsAges 13–14Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Chemical Reactions
A beaker holds an acid or base solution with an indicator already added (litmus, phenolphthalein, or universal pH paper). Students add HCl or NaOH one mL at a time from two burettes (button or tap on the beaker), watch the indicator change color, the pH scale needle move, and the pH-vs-volume curve grow. The amounts of H⁺ and OH⁻ and the neutralization equation update continuously so students can pinpoint the neutralization point.
Lesson: Acids; bases and the pH scale; neutralization reactions
What it shows
Acids release H⁺ ions in water and bases release OH⁻ ions; in neutralization they combine, H⁺ + OH⁻ → H₂O, forming water and a salt. The simulation calculates pH from the moles of H⁺ and OH⁻ left in the beaker, using Ka for the weak acid CH₃COOH, and colors the solution with the chosen indicator. Because pH is logarithmic, it changes slowly for most of a titration and then jumps sharply near the equivalence point, where n(H⁺) = n(OH⁻). With a weak acid the equivalence point lies above pH 7, because the sodium acetate formed is basic.
How to use
Choose the solution in the beaker, its concentration and volume, and an indicator. Press + 1 mL NaOH or + 1 mL HCl one step at a time and ask students to predict the color before each addition. Record the volume at which the indicator changes and compare it with n(H⁺) and n(OH⁻). Use Add continuously to build the whole pH curve, then press Reset and repeat with another indicator.
Parameters you can change
- Solution in the beaker Hydrochloric acid HCl, Acetic acid CH₃COOH (weak acid), Sodium hydroxide NaOH, Calcium hydroxide Ca(OH)₂, Sodium chloride NaCl (neutral), Distilled water
- Molar concentration (beaker and burette) 0.01–1 mol/L
- Volume of solution in the beaker 5–60 mL
- Indicator Litmus, Phenolphthalein, Universal pH paper
Questions to explore
- Why does sodium chloride solution not change the color of litmus?
- How many mL of NaOH neutralize 20 mL of HCl of equal concentration, and how much HCl does 20 mL of Ca(OH)₂ need?
- Why does a single mL of base change the pH so much near the equivalence point?
Teaching ideas
All guides- Titration Virtual Lab: Reacting Volumes and pH CurvesA titration virtual lab: find the end point with phenolphthalein, calculate an acid concentration, then compare strong and weak acid pH curves and read pKa.
- Titration Unknowns Virtual Lab: Vinegar, Juice and H₂O₂A titration virtual lab with hidden unknowns: find the acid in vinegar and fruit juice with NaOH, and the % H₂O₂ with KMnO₄, from concordant titres.
