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Home›Chemistry

Acids and bases

Lemon juice, stomach acid, baking soda and drain cleaner sit at different points on one scale. Slide along the pH scale and watch four indicators change colour, compare strong and weak acids molecule by molecule, and run a titration to its equivalence point.

Progress

About this course

In pure water at 25 °C only about two molecules in every billion have split into ions, and that tiny amount is where acids and bases begin. From there, lemon juice, stomach acid, baking soda and drain cleaner all sit at different points on one scale. Vinegar at pH 2.4 has about 40,000 times more hydronium ions per litre than pure water at pH 7.

The five units start with what acids and bases are and the typical reactions of an acid with a metal, a carbonate and a base. The pH scale follows, then indicators: phenolphthalein stays colourless in any solution below pH 8.3 and turns pink above it. Strong and weak acids come next. At 0.100 M, hydrochloric acid is fully ionized with pH 1.00, while acetic acid has given up only 1.3% of its protons and has pH 2.87. The last unit is neutralization and titration, where adding 0.100 M sodium hydroxide to 25.00 mL of 0.100 M hydrochloric acid makes the pH jump from 3.70 to 10.30 between 24.9 mL and 25.1 mL. The lab slides along the pH scale with common substances and four indicator tubes that change colour. It draws one hundred acid molecules at your chosen concentration, with ionized ones shown as separate A⁻ and H₃O⁺, and it runs the titration the way OpenStax Table 14.2 does.

Before you start

Acids and bases are a family of reactions, so Chemical reactions is a good first step, especially its units on balancing and types of reaction. The pH scale is logarithmic, and questions ask for pOH, pKa and molar concentrations, so a calculator with a log key is useful.

When you finish

You will be able to place lemon juice, baking soda and drain cleaner on the pH scale, convert between pH and pOH, and choose an indicator such as phenolphthalein for a given range. You can explain why 0.100 M hydrochloric acid has a lower pH than acetic acid at the same strength, and use a titration result to find an acid's concentration.

  1. 1What are acids and bases?In pure water at 25 °C only about two molecules in every billion have split into ions, and that tiny amount is where acids and bases begin.
  2. 2The pH scaleVinegar at pH 2.4 has about 40,000 times more hydronium ions per litre than pure water at pH 7.
  3. 3Acid-base indicatorsPhenolphthalein stays colourless in any solution below pH 8.3 and turns pink above it.
  4. 4Strong and weak acidsAt 0.100 M, hydrochloric acid is fully ionized and has pH 1.00, while acetic acid has given up only 1.3% of its protons and has pH 2.87.
  5. 5Neutralization and titrationAdd 0.100 M sodium hydroxide to 25.00 mL of 0.100 M hydrochloric acid and the pH jumps from 3.70 to 10.30 between 24.9 mL and 25.1 mL.

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Sources for this course (10)
  • OpenStax Chemistry 2e (openstax.org/books/chemistry-2e): 4.2 neutralization and antacids (stomach roughly 0.03 M HCl, milk of magnesia Mg(OH)2, Al(OH)3)
  • 14.1 Brønsted-Lowry acids and bases (Boyle 1680, Davy 1815, Arrhenius 1884, Brønsted and Lowry 1923, autoionization about two in a billion molecules, Kw = 1.0 × 10⁻¹⁴ at 25 °C, about 5.6 × 10⁻¹³ at 100 °C)
  • 14.2 pH and pOH (pH = −log[H3O+], stomach acid 1.2 × 10⁻³ M gives 2.92, vinegar about 4 × 10⁻³ M gives 2.4, blood 7.3, KOH 0.0125 M gives 12.10, neutral pH 6.31 at 80 °C, rainwater 5 to 6, acid rain below 5, pH meter resolution 0.001, Figure 14.2: 1 M HCl 0, gastric juice about 1.5, lime juice about 2, wine about 3.5, coffee about 5, milk of magnesia about 10.5)
  • 14.3 relative strengths (six strong acids and six strong bases, Ka, percent ionization, 0.100 M acetic acid 1.3%)
  • 14.6 buffers (normal blood pH about 7.4)
  • 14.7 titrations (25.00 mL of 0.100 M HCl or acetic acid with 0.100 M NaOH, Table 14.2, equivalence pH 7.00 and 8.72, half-equivalence pH = pKa, Ka acetic acid 1.8 × 10⁻⁵, methyl orange Ka 4.0 × 10⁻⁴ red HIn and yellow In⁻, interval about pKa ± 1, phenolphthalein colourless below pH 8.3)
  • LibreTexts Acid-Base Indicators (litmus about 5 to 8, methyl orange 3.1 to 4.4, phenolphthalein 8.3 to 10.0; chem.libretexts.org) and bromothymol blue 6.0 to 7.6 yellow to blue (Prairie South indicator table via LibreTexts Chemistry 30)
  • PubChem acetic acid pKa 4.76 at 25 °C, sodium bicarbonate 0.1 M pH 8.3, ammonia 0.1 N pH 11.1, sodium hydroxide (pubchem.ncbi.nlm.nih.gov)
  • NOAA Ocean acidification (noaa.gov: surface ocean pH has fallen by 0.1 since the industrial revolution, about a 30 percent increase in acidity)
  • IUPAC CIAAW abridged standard atomic weights.

Checked: 2026-10-17