The pH scale provides a way of expressing the concentration of hydrogen ions, H⁺, in aqueous solution. Because the scale is logarithmic, a change of one pH unit represents a tenfold change in hydrogen-ion concentration.
This interactive PhET simulation allows you to investigate the pH of different substances, explore the relationship between pH, H⁺ concentration and OH⁻ concentration, and see what happens when solutions are diluted with water.
Make predictions before using the measuring tools. Compare substances with different pH values and pay particular attention to what a difference of one, two or three pH units really means in terms of ion concentration.
pH Scale is provided by PhET Interactive Simulations, University of Colorado Boulder, and is used under the CC BY-NC 4.0 licence.
1. Explore familiar substances
Test several of the substances provided. Before measuring each one, predict whether it will be acidic, neutral or alkaline.
2. Compare pH with H⁺ concentration
Choose two solutions whose pH values differ by one unit. Compare their H⁺ concentrations. What factor separates them?
3. Increase the difference
Now compare solutions differing by two pH units, and then three. What factors separate their H⁺ concentrations? Use this to explain why the pH scale is described as logarithmic.
4. Compare H⁺ and OH⁻
Examine the concentrations of H⁺ and OH⁻ in acidic, neutral and alkaline solutions. What relationship do you observe?
5. Investigate neutrality
Explore a solution at pH 7. Compare the concentrations of H⁺ and OH⁻. What is special about them?
6. Dilute an acidic solution
Choose an acidic solution and add water. Predict the direction in which its pH will change. Does dilution make the solution alkaline?
7. Dilute an alkaline solution
Repeat with an alkaline solution. In which direction does the pH move? What value does it approach as the solution becomes increasingly dilute?
Keep asking:
What does a change in pH actually mean in terms of hydrogen-ion concentration?
Remember that the pH scale is logarithmic: a change of one pH unit corresponds to a tenfold change in H⁺ concentration.