Theory
Acids and bases are among the most familiar substances in chemistry. They are found in food, cleaning products, the human body, and the laboratory. This introductory lab builds the core ideas: what makes something acidic or basic, how the pH scale measures it, and the difference between strong and weak acids and bases.
1. What are acids and bases?
An acid is a substance that produces hydrogen ions (H⁺) in water; examples include hydrochloric acid, vinegar (acetic acid), and citric acid. A base produces hydroxide ions (OH⁻) in water; examples include sodium hydroxide, ammonia, and baking soda. Acids taste sour and turn blue litmus red; bases taste bitter, feel slippery, and turn red litmus blue.
2. The pH scale
The pH scale runs from 0 to 14 and measures how acidic or basic a solution is. A pH below 7 is acidic, a pH of 7 is neutral (pure water), and a pH above 7 is basic. The scale is logarithmic: each whole-number change in pH means a tenfold change in acidity. pH is defined as pH = −log₁₀[H⁺].
3. pH, pOH, and the ion product of water
In any aqueous solution at 25 °C, the hydrogen-ion and hydroxide-ion concentrations are linked by [H⁺][OH⁻] = 1 × 10⁻¹⁴. Taking negative logarithms gives the simple relationship pH + pOH = 14. So if you know the pH, you can find the pOH, and from either you can find the ion concentrations.
4. Strong and weak acids and bases
A strong acid or base dissociates completely in water, releasing all of its H⁺ or OH⁻ ions; hydrochloric acid and sodium hydroxide are strong. A weak acid or base only partly dissociates, so only a fraction of its molecules release ions; acetic acid and ammonia are weak. The extent of dissociation of a weak acid is described by its acid dissociation constant, Ka (and Kb for a base): a larger Ka means a stronger weak acid.
5. Indicators
An indicator is a dye that changes colour depending on pH. Litmus is red in acid and blue in base. Universal indicator shows a range of colours across the whole pH scale, from red in strong acid through green at neutral to purple in strong base, letting you estimate pH by colour.
6. Buffer solutions
A buffer solution resists changes in pH when a small amount of acid or base is added. A buffer is made of a weak acid together with its conjugate base (its salt), for example acetic acid and sodium acetate, or a weak base together with its conjugate acid, for example ammonia and ammonium chloride. A mixture of a strong acid and its salt (such as HCl and NaCl) is not a buffer, because the strong acid is already fully dissociated. The pH of a buffer can be estimated with the Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), where [A⁻] is the concentration of the conjugate base and [HA] that of the weak acid. When the two are equal, the log term is zero and the pH equals the pKa.
Apparatus and Reagents
The equipment and materials a real introductory acid-base experiment uses. In this simulation they are modelled for you, but the observations correspond to what each test would actually show.
- A pH meter or pH probe, and universal indicator solution or paper.
- Red and blue litmus paper.
- Common acids: dilute hydrochloric acid (strong) and vinegar or acetic acid (weak).
- Common bases: dilute sodium hydroxide (strong) and ammonia solution (weak).
- Everyday samples for testing: lemon juice, soap solution, baking-soda solution, pure water.
- Test tubes, a test-tube rack, a stirring rod, and a colour chart for universal indicator.
Instructions
This is a predict-and-check lab. In each tab you decide the answer yourself first, choose it, and only then does the simulation confirm the correct result so you can compare. Record every result in your worksheet.
Simulation — Exploring Acids and Bases
Team Questions
Work these out with your team, type each answer, and check it.
Example Lab Report
A worked example showing the expected format and the predict-and-check workflow.
Introduction to Acids, Bases, and the pH Scale
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Objective — To classify solutions as acidic, neutral, or basic using the pH scale, to relate pH, pOH, and ion concentrations, and to distinguish strong from weak acids and bases, comparing every prediction with the simulation.
Part A — The pH Scale (worked example)
Lemon juice at pH 2 was predicted to be acidic (pH below 7); pure water at pH 7 was predicted neutral; soap solution at pH 10 was predicted basic (pH above 7). All three matched the simulation.
Part B — pH, pOH, and Ions (worked example)
For a solution of pH 3, the pOH was found from pH + pOH = 14, giving pOH = 11, and [H⁺] = 10⁻³ mol/L. For a basic solution of pH 10, the pOH was 4. Each matched the simulation.
Part C — Strong, Weak, and Indicators (worked example)
Hydrochloric acid was identified as a strong acid (fully dissociated) and acetic acid as a weak acid (partly dissociated); sodium hydroxide as a strong base and ammonia as a weak base. Blue litmus turned red in acid, and universal indicator was green at neutral. A larger Ka was recognised as the stronger weak acid.
Part D — Buffers (worked example)
A buffer was identified as a weak acid with its conjugate base (acetic acid and sodium acetate), while HCl and NaCl was correctly rejected as not a buffer. Using pH = pKa + log([A⁻]/[HA]) with pKa = 4.7: an equal mixture gave pH 4.7, a 10:1 ratio gave pH 5.7, and a 1:10 ratio gave pH 3.7. All matched the simulation.
Discussion and Conclusion — Every prediction agreed with the simulation. The pH scale classified solutions by their hydrogen-ion concentration; pH and pOH summed to 14; and strong acids and bases dissociated completely while weak ones only partly did, with Ka measuring the strength of a weak acid.
Practice Quiz
For extra practice on this topic, use the interactive Acids and Bases quiz in the Quizzes section of the site.