Teaching Sequence

Work through this resource material in the following sequence:

20 minutes – Part A: Why Do Cavities Happen
20 minutes – Part B: Educating on Electrons
15 minutes – Part C: Reflecting on Teaching

Part A: Why Do Cavities Happen?

Step 1.

Begin by asking students why cavities/tooth decay occurs. You might like to write up student responses on the board, or to ask students to brainstorm in groups and present their best guesses.

You might also want to direct or encourage students as they get closer to the answer. That is, the correct answer is that tooth decay forms due to acid production by bacteria in the mouth after consuming sugary foods. Acid is produced when bacteria in the mouth ferment sugar, forming the by-product lactic acid. But without knowing about lactic acid per se, students may still be able to approximate valuable answers. They might know that high-sugar diets cause cavities, for instance, or that cavities have some cause-effect relationship with the things that people eat, or even that tooth decay occurs when people don’t brush their teeth. You can help students to develop these different components towards a broader understanding of why tooth decay occurs. For instance, you could ask leading questions about different components, such as:

What are cavities/tooth decay? (Holes in your teeth)
What helps prevent cavities? (Brushing your teeth, low sugar diet, fluoride toothpaste, drinking fluoridated water)
What kinds of foods are worse for causing cavities/tooth decay? (High-sugar foods – especially those that are sticky)

Step 2.

Once students have identified acid as the key element in causing tooth decay, ask students why acids cause tooth decay.

Responses will probably involve a whole lot of synonyms, as students try to move around the idea of acids dissolving things without actually explaining the chemical process. For instance, students might respond that acids cause cavities because acids melt or eat things. Push students on the actual mechanism here: we know that acids dissolve things, but why does that happen? What makes acids good at dissolving things?

Students probably will not be able to answer this question if they have not done any previous work with acids and bases. Ideally, a string of unsatisfying and rejected answers here will help to prime their curiosity, such that the explanation will make more of an impression.

Step 3.

Explain to students that acids dissolve things because they have a high concentration of hydrogen ions. Hydrogen ions are extremely reactive, and will essentially pull other atoms out of their usual structure. If enough of the molecules that make up our teeth react with the hydrogen in acids, we end up with gaps where those molecules were.

This explanation is obviously a bit more informal and narrative-focused: more scientific, technical explanations may also be appropriate, depending on the class’s background knowledge and ability. The next part of this lesson will delve into the details of the electron shell and some low-level chemical reaction material. You might also want to look further into how bacteria in the mouth change sugar into acid - for that, we recommend the second lesson in this unit, Where's the Acid?

Step 4.

Introduce the pH scale to students by explaining that we can rank liquids by the amount of hydrogen ions that they contain.

Liquids with high numbers of hydrogen ions are considered very acidic, while liquids with low numbers are called bases.

Also introduce students to the numbering system we use to represent pH, with low numbers representing stronger acids and higher numbers representing stronger bases.

Note: Approaching pH in terms of hydrogen is often difficult, as higher numbers of hydrogen ions mean a lower pH, rather than a higher one. It’s worth emphasising this reversal with students, as it can be difficult to remember.

Step 5.

Help students to develop their understanding of the pH scale by adding common products at different points along the scale.

We have listed some of these common products below:

Stomach acid (gastric acid) - pH 1
Lemons - pH 2
Apples - pH 3 - 4
Water - pH 7
Baking soda - pH 9
Hand soap - pH 10
Bleach - pH 12

Please note that all of these products are approximates - the strength of any given acid or base will depend partly on its concentration.

To help students gain a more personal understanding of this scale, ask them to guess where different products are positioned.

Part B: Educating on Electrons

In this part, we take the idea of hydrogen ions as extremely reactive, and dig into some of the science around why that happens. We’ll give you the information here as a brief overview, and then move on to suggestions around ways of teaching it.

Briefly, then, atoms like to have full valence shells. In their un-ionized form, they have a neutral electrical charge, as they have the same number of protons (which are positively charged) and electrons (which are negatively charged). As these atoms gain or lose electrons, filling or emptying their valence shells, their charges change accordingly. Ions will further bond with each other, as ions with negative and positive charges are attracted to each other.

For example, sodium (Na) has one electron in its outer shell. Once it loses that electron, it takes on a net positive charge (Na+). Chlorine (Cl), on the other hand, has seven electrons in its outer shell. Upon gaining a final electron, it again fills its outer shell and takes on a net negative charge (Cl—). The positive and negative charges of the sodium and chlorine ions then attract each other, and the two bond to form NaCl, which of course is more commonly known as table salt.

Step 1.

Begin by showing students this video, which covers the same basic information as above.

Concept of Valency - Introduction - CBSE 9 (https://www.youtube.com/watch?v=lVSF2lP4oBA)

Step 2.

Guide a discussion with the class, covering all of the key points, especially:

that atoms have a set number of electrons in their valence shell; and
that atoms like to have a full set of electrons in their valence shell.

Step 3.

In their workbooks or on note paper, students should write out what they think are the key facts around ions and electron exchange. They should then come up with a metaphor or example for explaining the process to younger audiences.

For example, in the video the speaker talks about how everybody wants as much chocolate as they can get. What other metaphors or examples could we use to explain this process?

Step 4.

Once students have come up with their metaphor or example, they should get into pairs and share their examples with each other, along with their reasoning as to why the example would be appropriate for a younger audience.

This learning ties into Communicating the Science of Tooth Decay, the final lesson in this unit, which gives students the opportunity to practise their science communication skills with a younger class.

Part C: Reflecting on Teaching

In this part, students reflect on the teaching in this lesson and think about how they could utilise different techniques for teaching and communicating science to other people. If you are doing Communicating the Science of Tooth Decay, this reflection can serve as some of the background thinking for when students design their own teaching material.

Step 1.

Students should begin by listing the components of the lesson so far. You might want to prompt them through some of this work, or draw their attention to aspects that they might otherwise overlook.

Step 2.

Once they have listed the components of the lesson, students should describe their reactions to the different parts. There is space for this work on the worksheet. Also provide students with these guiding questions:

Which parts of the lesson do you feel you learnt the most from? Why?
Which parts of the lesson do you feel you learnt the least from? Why?
Which activities do you think would be the best for younger students?
How would you start a lesson to grab the interest of younger students?

To support students in their thinking, you might like to share some of your own teaching strategies and experience, particularly around good attention-grabbing ways to start lessons. At the very least, spend some time exploring the idea of an initial hook and its value in a teaching context. You might like to link the idea of a hook to other relevant contexts, such as marketing or advertising.

Step 3.

Ask students to get into pairs and explain their answers to each other. Encourage them to reflect critically on their experiences and the differences or similarities between their personal responses and the wider responses of the wider class. Science communication is often about communicating with broad groups of people, and one person’s individual preferences might not reflect the best approach for a wider audience.

Step 4 (optional).

If your students are doing Communicating the Science of Tooth Decay, make sure that they retain their learnings and thoughts from this exercise - for instance, by putting their worksheets into a folder or binder somewhere. These learnings will be useful when students come to design their own teaching material in that lesson.

Teacher Reflection

Take this opportunity to reflect on your own teaching:

What did you learn about your teaching today?
What worked well?
What didn't work so well?
What would you share?
Where to next?
How are you going to get there?

SugarByHalf - The Power of pH

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