Teacher sheet · Ages 15–16 · about 45–50 minutes
The chemistry of tooth decay
Lesson page: echoesof.science/school/teeth/ages-15-16/ · Pupil worksheet: echoesof.science/school/teeth/ages-15-16/print/
Learning aims
- Use the hydroxyapatite equilibrium and Le Châtelier's principle to explain demineralisation.
- Use the logarithmic pH scale in calculations.
- Explain how fluoride and saliva help remineralisation.
- Carry out a quantitative investigation with a control and evaluate uncertainty and the model.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: which is more acidic, pH 3 or pH 5, and by what factor? |
| 10 min | Read the text; work through the equilibrium and the critical pH. |
| 20 min | Weigh shell pieces and apply the toothpaste treatment (acid step and reweighing next lesson). |
| 5 min | ppm and pH calculations. |
| 5 min | Quiz or sorting game. |
Materials for the experiment
- Eggshell pieces from cleaned, dried eggshells (about 2 cm × 2 cm, 6–10 pieces)
- Fluoride toothpaste (check the ppm on the tube)
- White vinegar (about 4–5% acetic acid)
- Beakers or labelled cups
- Top-pan balance (0.01 g)
- Tweezers, paper towels, timer
- Safety glasses
Safety: Wear safety glasses: vinegar irritates eyes. Wash hands after handling eggshell (raw egg can carry bacteria); use shells that were washed and dried, or boiled. Do not taste anything. Wipe up spills. Check for egg allergies.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Enamel only dissolves in very strong acid. | Below about pH 5.5 enamel tends to dissolve; many drinks and plaque acids reach that. |
| pH 4 is just a bit more acidic than pH 5. | It has ten times the hydronium-ion concentration. |
| Fluoride kills all the bacteria. | Its main effect is on the enamel surface (fluoroapatite); it slows bacteria but does not remove them. |
Worksheet answers
- Ca₁₀(PO₄)₆(OH)₂(s) ⇄ 10Ca²⁺ + 6PO₄³⁻ + 2OH⁻. H⁺ reacts with OH⁻ (and phosphate), lowering their concentration; the equilibrium shifts right, so more solid dissolves.
- Difference of 3 pH units → 10³ = 1,000 times the [H₃O⁺].
- (1.84 − 1.52) / 1.84 × 100 = 17.4%. Each reading ±0.01 g, so the mass loss 0.32 g is ±0.02 g (about ±6% of the loss): small compared with the change, but important when comparing groups whose means are close.
- Eggshell is calcium carbonate, not hydroxyapatite; there is no saliva, plaque or repair cycle; the acid exposure is continuous and stronger than in the mouth (any two).
Quiz answers
- What is the formula of calcium hydroxyapatite? Ca₁₀(PO₄)₆(OH)₂. Hydroxyapatite contains calcium, phosphate and hydroxide.[7]
- A drink at pH 4 has how many times the [H₃O⁺] of a drink at pH 6? 100 times. Two pH units = 10 × 10 = 100 times.[8]
- Why does adding acid make more hydroxyapatite dissolve? H⁺ removes OH⁻, so the equilibrium shifts right. Removing a product shifts the equilibrium towards the products (Le Châtelier).[7]
- The critical pH for enamel is about… 5.5, and it varies between people. About 5.5, but it is not a fixed value.[7]
- How does fluoride make enamel more resistant? Fluoroapatite is less soluble in acid. Fluoroapatite dissolves less easily and lowers the critical pH.[7]
- A toothpaste contains 1,450 ppm fluoride. How much fluoride is in 1 g of it? 1.45 mg. 1,450 mg per kg ÷ 1,000 = 1.45 mg per g.[1]
Going further
Calculate the theoretical mass of CO₂ released when 1.00 g of pure CaCO₃ reacts completely (M(CaCO₃) = 100 g/mol, M(CO₂) = 44 g/mol), and compare with your measured mass loss.
Sources used in this lesson
- [1] Taking care of children's teeth (NHS (UK National Health Service)): reliability High
- [2] The Tooth Decay Process: How to Reverse It and Avoid a Cavity (National Institute of Dental and Craniofacial Research (NIH, USA)): reliability High
- [3] The Tooth Decay Process (children's oral health) (National Institute of Dental and Craniofacial Research (NIH, USA)): reliability High
- [6] Community Water Fluoridation: Frequently Asked Questions (Centers for Disease Control and Prevention (USA)): reliability High
- [7] Critical pH and Le Châtelier: how everyday substances and habits can dissolve your teeth away (Centre for Advanced Science Education, University of Waterloo (Canada)): reliability Medium-high
- [8] Chemistry 2e, 14.2 pH and pOH (OpenStax, Rice University): reliability High
- [9] Chemistry 2e, 18.6 Occurrence, Preparation, and Properties of Carbonates (OpenStax, Rice University): reliability High
- [10] ChemShorts for Kids: eggs, soda and tooth enamel (Chicago Section of the American Chemical Society (ChemShorts series by Paul Brandt)): reliability Medium-high
- [11] Dental health and acid science experiment (teeth.org.au (Australian dental health education site; the PDF does not name its author)): reliability Medium
Full list with links and a note on each source's reliability: echoesof.science/school/teeth/sources/
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