Tooth Decay and Toothpaste

Introduction

Tooth enamel is the hardest substance in the human body, harder than bone. Yet it can be worn away by something as ordinary as a sugary drink. The culprit is acid, made by bacteria living on our teeth. Every time you eat sugar, the pH in your mouth drops and your enamel is attacked. Toothpaste, saliva and good habits all help to fight back, and much of the chemistry involved is neutralisation.

Core explanation

What enamel is made of. Enamel is made mainly of a mineral called hydroxyapatite, a form of calcium phosphate. Like many basic minerals, it reacts with acids and slowly dissolves when the surrounding liquid becomes too acidic.

Where the acid comes from. A sticky film called plaque builds up on teeth. Plaque contains millions of bacteria. When we eat or drink sugars, these bacteria feed on them and produce acids, mainly lactic acid, as waste products. The pH at the surface of the tooth can fall from about 7 to below 5 within minutes.

The critical pH. Enamel begins to dissolve when the pH at the tooth surface falls below about 5.5 . This is called the critical pH. Each time we have sugary food or drink, the pH stays below 5.5 for perhaps 20 to 30 minutes before saliva brings it back up. Over time, repeated acid attacks create tiny holes, which grow into cavities.

Saliva as a natural defence. Saliva is slightly alkaline to neutral and contains hydrogencarbonate ions, which neutralise acids. It also contains calcium and phosphate ions that can help repair early damage to enamel.

How toothpaste helps. Most toothpastes are mildly alkaline, often with a pH of around 7 to 10. They contain mild bases and abrasives, such as calcium carbonate or sodium hydrogencarbonate, which neutralise acids left on the teeth. Brushing also removes plaque, so fewer bacteria remain to make acid. Most toothpastes also contain fluoride , which makes enamel more resistant to acid attack.

Acidic drinks. Some drinks, such as colas and fruit juices, are acidic themselves, with a pH that can be around 2.5 to 4. They can dissolve enamel directly, even without bacteria. This is called dental erosion.

Step-by-step reasoning

To explain how a sugary snack leads to tooth decay:

1. Bacteria in plaque feed on the sugar. 2. They produce acids as waste. 3. The pH at the tooth surface falls below 5.5. 4. Acid reacts with enamel, dissolving some of it. 5. Repeated attacks form a cavity.

Visual explanation

Imagine a graph of mouth pH against time. It starts at about 7, drops steeply after a sugary snack to around 5, stays below a dashed line at 5.5 for about half an hour, then slowly rises back to 7 as saliva neutralises the acid. Frequent snacks make the line dip again and again.

Real-world analogy

Enamel under acid attack is like a sandcastle facing waves. One wave washes away only a little sand, but wave after wave, all day long, wears the castle down. Saliva and toothpaste are like rebuilding the walls between waves.

Real-world example

Dentists advise people to limit sugary snacks to mealtimes rather than grazing through the day. Eating sugar five times a day produces five separate acid attacks, while eating the same sugar in one meal produces just one. Chewing sugar-free gum after a meal increases saliva flow and helps neutralise the acid.

Why?

Why is toothpaste made alkaline rather than neutral? After eating, the mouth contains acids made by bacteria. A mildly alkaline toothpaste can neutralise these acids, helping raise the pH above the critical value of 5.5 so enamel stops dissolving.

Common misconception

"Sugar itself dissolves teeth." Sugar does not attack enamel directly. Bacteria in plaque turn sugar into acid, and it is the acid that dissolves enamel. This is why removing plaque by brushing is so important.

Worked example

Question: A drink has a pH of 3.2. Explain whether it is likely to damage teeth and suggest one way to reduce the risk.

Reasoning: pH 3.2 is well below the critical pH of 5.5, so the drink itself is acidic enough to dissolve enamel. Reducing contact time with teeth reduces damage.

Answer: Yes, it can dissolve enamel because its pH is below 5.5. Drinking it quickly with a meal, or through a straw, and rinsing with water afterwards reduces the risk.

Quick check

1. What is the approximate critical pH below which enamel dissolves? Answer: About 5.5.

Exam focus

Be able to link each step of tooth decay: sugar, bacteria, acid, low pH, enamel dissolving. Explain toothpaste as a mild base that neutralises acid, and do not confuse the role of fluoride (strengthening enamel) with neutralisation.

Advanced insight

Fluoride ions can replace some hydroxide ions in hydroxyapatite, forming fluorapatite. Fluorapatite is less soluble in acid than hydroxyapatite, so its critical pH is lower, about 4.5. This explains why fluoride in toothpaste and drinking water has greatly reduced tooth decay in many countries.

Summary

Tooth enamel is a basic calcium phosphate mineral that dissolves in acid below a pH of about 5.5. Bacteria in plaque turn sugars into acids, lowering the pH at the tooth surface after each sugary snack. Saliva and mildly alkaline toothpaste neutralise these acids, brushing removes plaque, and fluoride makes enamel more resistant to attack. Acidic drinks can also erode enamel directly.

Practice questions

1. What is plaque? Answer: A sticky film of bacteria and food particles that builds up on teeth. 2. Name the main type of acid produced by bacteria in plaque. Answer: Lactic acid. 3. Explain why toothpaste is mildly alkaline. Answer: So that it can neutralise the acids in the mouth, raising the pH above the level at which enamel dissolves. 4. Why is snacking on sweets throughout the day worse for teeth than eating the same sweets at one time? Answer: Each snack causes a separate acid attack, so the teeth spend more time below the critical pH.